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Plasma amino acid ratios related to brain serotonin synthesis in response to food intake in bulimia nervosa
Plasma Amino Acid Ratios Related to Brain Serotonin Synthesis in Response to Food Intake in Bulimia Nervosa Hanno Pijl, Adam F. Cohen, Robbert J. Verkes, Hans P.F. Koppeschaar, Jolein A. Iestra, Hendrik C. Schoemaker, Marijke Fr61ich, Willem Onkenhout, and A. Edo Meinders
Fifteen bulimic women (BN) and 19 healthy Ji'male controls (CO) were studied. The subjects were cross-over treated with either.fluoxetine (FXT) or placebo during 4 days. They received, in randomized order, a breakfast containing pure carbohydrate (CLIO) or a protein-rich (PROT) breakfast following day 3 and 4 of each treatment period. Twenty-nine different food items ,'ere offered fi)r lunch. The .lasting serum glucose and insulin concentrations and the fasting plasma tr3'ptophan (Trp)/large neutral amino acid (LNAA) ratio were slightly higher in BN. The changes of these metabolic parameters in res'ponse to a CHO or PROT breakfast were similar in both groups. Across breakfast O'pe, the plasma (Trp)/(LNAA) ratio at 120 min q[~er breakfast was higher in BN. Total caloric intake at lunchtime was less in BN. In CO, less carbohydrate wa.s selected at lunchtime following the CHO breakfast, an effect that was abolished by FXT Breal~(ast type or FXT did not have any apparent effect on .fbod intake at lunchtime in BN. This might indicate that bulimic subjects are less sensitive to serotoninergic stimuli than control subject,v.
Key Words:
Amino acids, serotonin, carbohydrate, food preferences, bulimia nervosa
BIOL PSYCHIATRY 1995:38:659-668
Introduction Serotonin (5-hydroxytryptamine [5-HT]) mediated neurotransmission in several brain nuclei plays an important
From the Departments of General Internal Medicine iHP, AEM). Dietetics and Nutrition (JAil, Clinical Chemistr5 IMFI, Pediatricx (WO), and Psychiatr_~ (RJV), University Hospital Leiden. the Netherlands: Centre for Human Drug Research, Leiden, the NetherLands (AFC, HCS) and Deparlment of Endocr3 nology, University Hospital Utrecht. the NetherLands (HPFK). Address repnnt requests to Dr Elanno Pijh Deparlment of General Internal Medicine. University Hospital Leiden. P.O. Box 9600. Bldg. 1. CI R3. 2301) RC Leiden, Netherlands
© 1995 Society o! Biological Ps~chiatr~
role in the regulation of food intake (Blundell 1992). Stimulation of postsynaptic 5-HT receptors appears to inhibit specifically carbohydrate consumption in rats (Leibowitz et al 1990). More vigorous stimulation reduces total caloric intake as well (Leibowitz et al 1988). In humans, serotoninergic drugs exert a similar influence on food intake (Goodall et al 1992). Brain 5-HT is produced locally in serotoninergic neurons (Green et al 1966). The production rate is dependent on the availability of the precursor amino acid tryptophan (Wurtman et al 1980). Since tryptophan (Trp) competes with other so-called 0006-3223/95/$09.50 SSDI 0006-3223(95)00043-G
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large neutral amino acids (LNAA; e.g., valine, isoleucine. leucine, phenylalanine, and tyrosine) for transport across the blood brain barrier (BBB), the (Trp)/(LNAA) ratio in plasma determines the amount of Trp that reaches the brain: a high ratio allows for enhanced transport into brain tissue (Fernstrom and Wurtman 1972). The consumption of carbohydrates induces an increase of the plasma (Trp)/ (LNAA) ratio through an insulin-mediated mechanism (Fernstrom and Wurtman 1971). The consumption of protein affects the plasma (Trp)/(LNAA) ratio by directly contributing the dietary amino acids to the plasma compartment. Thus the change of the ratio is likely to depend on the amino acid composition of the ingested proteins. Since Trp is the least abundant amino acid in many proteins and because it is metabolized by the liver after transport through the portal vein, the plasma (Trpt/ (LNAA) usually decreases in response to protein intake (Pijl et al 1993). Consequently, brain 5-HT concentration (Fernstrom and Wurtman 1971 : Teff and Young 1988) and synaptic release (Schwartz et al 1989) decrease after protein ingestion and increase after the consumption of carbohydrate. Bulimia nervosa (BN) is an eating disorder characterized by recurrent episodes of binge eating, combined with self-induced vomiting, purging, or other methods to prevent weight gain (American Psychological Association 1987). Several lines of evidence support the notion that brain 5-HT is implicated in the pathogenesis of BN. Since the role of 5-HT in the regulation of eating behavior appears to be predominantly inhibitory, a defect of 5-HT function could theoretically contribute to the binge behavior in BN. Indeed, frequent bingeing is associated with low levels of 5-HT metabolites in cerebrospinal fluid (Jimerson et al 1992); bulimic patients exhibit a reduced prolactin response to an endogenous serotoninergic challenge with D,L-fenfluramine (McBride et al 1991 ); drugs increasing 5-HT synaptic release and/or blocking 5-HT reuptake. thereby enhancing postsynaptic activation of 5-HT receptors, appear to be useful tools for the pharmacological treatment of BN (Walsh 1991; Blouin et al 1988). In view of what has been discussed previously, a defect in brain 5-HT neurotransmission could be brought about by changes of peripheral amino acid metabolism. Delivery ol tryptophan to brain tissue might be hampered in bulimic patients as a result of alterations of the (Trp)/(LNAA) ratio in basal conditions or in response to food intake, thereb~ reducing brain 5-HT synthesis and synaptic release. The resultant changes in postsynaptic 5-HT action could lead to aberrant eating patterns. The present study was designed to compare changes in plasma (Trp)/(LNAA) ratio induced by carbohydrate or
protein consumption for breakfast in bulimic subjects with those in normal controls; to study associated differences in caloric content and macronutrient composition of subsequent food choice at lunchtime between the two groups; and to investigate the effects of manipulating the central serotoninergic system by fluoxetine on food choice in both groups.
Subjects and Method Healthy bulimic women with a regular menstrual cycle, who did not use oral contraceptives or any other medication, were recruited through a mailing among members of the Dutch Association of Bulimia and Anorexia Nervosa patients. After physical examination, electrocardiogram (EKG) and screening for pregnancy, all bulimic subjects attended a psychiatrist for confirmation of the diagnosis. Normal-weight control women were recruited through a local newspaper advertisement. Fifteen bulimic (age 27.7 _+ 5.7 years [mean _+ standard deviation], range 19-44 years, body mass index [BMI] 22.0 -4- 2.2 kg/m2), and 19 control subjects (age 32.3 - 8.7 years, range 2 0 - 4 6 years; BMI 22.5 -+ 1.9 kg/m 2, range 20.0-25.0 kg/m 2) were enrolled in the study. The bulimic subjects met Diagnostic and Statistical Manual of Mental Disorders, 3rd ed, rev. (DSM-III-R) criteria for bulimia nervosa (American Psychological Association, 1987). There was no history of (psychoactive) substance abuse, bipolar or psychotic disorder, or major depression during the 6 months prior to the study. The average number of binges per week amounted to 8. l -+ 6.7. All but one (using laxatives) of the bulimic subjects vomited in order to prevent weight gain. To characterize our subjects further we used the Eating Disorder Inventory (Garner et al 1983). Comparing the data of our patients with Dutch norm data we concluded that our group was comprised of typical bulimia nervosa patients with relatively few symptoms of anorexia nervosa (Parker Brody and Van Furth 1994). All patients were seen by a single psychiatrist. Control subjects did not have a history of eating disorders or any other major psychiatric disease. Written informed consent was obtained from all women. The study was approved by the ethics committee of the University Hospital Leiden. The present study was conducted within the framework of a larger experiment investigating the effects of nutrients, plasma amino acid levels, and serotoninergic drugs on fi)od choice in different patient populations. The results of another part of the experiment, concerning obese subjects, have been published elsewhere (Pijl et al 1993). The subjects used as controls in this study were the same as those in the experiment described in the previous
P l a s m a A m i n o Acids in Bulimia Nerw~sa
publication. The design of the present study parallels that of the former. It had a double-blind, crossover, 2 × 2 factorial design with drug/placebo and type of breakfast as factors. The subjects received either fluoxetine (FXT, 60 mg orally in one daily dose) or placebo (PLAC) in randomized order during 4 days, starting within 3 days after the onset of menstrual bleeding. FXT is a serotoninergic drug, blocking 5-HT reuptake from the synaptic cleft, thereby enhancing postsynaptic 5-HT action (Fuller and Wong 1989). At 2 consecutive days following day 3 and day 4 of treatment the subjects were admitted to the study center at 8.30 AM for testing. All tests were performed in the midfollicular phase of the menstrual cycle, which is of importance since tood choice is influenced by the phase of the menstrual cycle (Lissner et al 1988). Smoking, eating, and drinking anything but water and tea without sugar and milk were prohibited for 12 hours prior to admission. An intravenous (i.v.) cannula was inserted into an antecubital vein. Subsequently, at 9.00 AM on the first test day the subjects received either a breakfast containing pure carbohydrate (80 g maltodextrin) or a comparable (as far as taste and appearance were concerned) protein-rich breakfast (60% protein: 30 g milk protein, and 40% lactose, sucrose, maltodextrin). The next morning the alternative breakfast was offered. Both types of breakfast were fluid, isovolemic (300 ml), and isocaloric (300 kcal). Blood samples for measurement of serum glucose and insulin concentrations were collected at 0, 2, 4, 6, 8, 10, 15, 30, 60, 120, and 150 rain following breakfast. Plasma levels of valine, isoleucine, leucine. phenylalanine, tyrosine, and tryptophan were measured at times 0 and 120 min after breakfast. The reason for measuring these amino acids at this particular time was that the (Trp)/(LNAA) was observed to change maximally at 120 min after an acute food challenge in normal men (Ashley et al 1985, 1982). The subjects were not allowed to eat or drink anything until lunch at 180 min after breakfast. This time was chosen because we expect the brain 5-HT concentration to change about 1 hour after a change in plasma amino acid levels (Fernstrom and Wurtman 1972). Twenty-nine different food items of known macronutrient content and weight were offered (Table 1). All items were available commercially and known to the subjects. The subjects were asked to compose their own lunch from these items. During the meal the subjects were allowed to take additional portions of any product. They were instructed to eat to satiety. Remaining food was weighed and subtracted from the records. The subjects received the alternative drug condition in the same period of the next menstrual cycle. Test procedures were performed according to the protocol as de-
BIOL PSYCHIATRY 1995;38:659-668
Table
1. F o o d I t e m s O f f e r e d
Macronutrient
for Lunch
661
and Their
Composition
Food
W e i g h t (g) C a r b o h y d r a t e (g) Protein (g) Fat (g)
Currant bun Curd cheese
50 90
25 15
4 7
2 1
A p p l e treacle G i n g e r cake Powdered chocolate
15 20 10
9 14 6
-1 1
--2
Jelly
15
9
--
--
Apple Banana
150 200
17 44
-2
---
O r a n g e juice
150
12
--
--
5
5
--
--
Sugar Cheese Cummin cheese C h e e s e spread
20 20 15
----
5 5 3
6 6 3
Liverwurst Smoked beef Ham Boiled e g g Peanut butter
15 15 20 50 15
----2
2 5 3 7 3
4 1 4 6 9
Tomato W h o l e w h e a t bread
75 35
2 14
1 2
-1
Rye bread W h i t e bread Diet m a r g a r i n e Margarine Butter L o w - f a t milk
45 35 10 10 10 250
18 16 --10
2 3 ---8
-1 4 8 8 3
Buttermilk Yogurt
250 150
12 6
8 5
5 6
8
1
1
1
Coffee creamer
scribed. Consequently, each subject received all four possible treatment modalities (drug × breakfast type). If a protein-rich (PROT) breakfast was offered at the first day of testing of the previous study period, a carbohydrate (CHO) breakfast was offered the first day of the second testing and vice versa. This was done to avoid subjects learning the test conditions, which might bias the results on food choices.
Assay Techniques Blood samples for measurement of serum glucose and insulin and plasma tyrosine, tryptophan, phenylalanine, isoleucine, leucine, and valine were centrifuged and immediately frozen ( - 4 0 ° C ) until assay. Glucose was measured enzymatically. Insulin was measured using a radioimmunoassay (Roelfsema and Frolich 1985). Blood for amino acid measurements was collected in tubes containing etheylene diamine tetraacetic acid (EDTA). Plasma was deproteinized using an equal volume of 5% (w/v) sulfosalicylic acid in water and analyzed for amino acid concentrations by ion exchange chromatography and nin-
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hydrin derivatization on an LKB 4151 Alpha Plus automated amino acid analyzer using standard conditions. The sum of albumin-bound and free plasma concentrations of tryptophan was determined using a standard addition calibration curve. In the circulation tryptophan is largely (75-80%) bound to albumin, which does not cross the BBB (McMenamy and Oncley 1985). Only free plasma tryptophan is available for uptake by brain tissue. The ratio of free to bound plasma tryptophan reflects on equilibrium state. Tryptophan uptake by the BBB carrier mechanism results in a shift in the equilibrium such that bound tryptophan is released. Studies suggest that a significant amount of tryptophan dissociates from albumin and is transported across the BBB during passage through the cerebral capillary plexus (Yuwiler et al 1977: Wurtman and Pardridge 1979). The total plasma concentration of tryptophan (i.e., albumin bound plus free) is therefore likely to be a better approximation of the amount of tryptophan available for access to the brain than the measured free tryptophan concentration (Yuwiler et al 1977; Pardridge 1979). Data Analysis Calculations were carried out using SPSS/PC+ V4.0.1 (SPSS Inc, Chicago, IL). Insulin resistance was calculated by the formula described by Matthews et al (1985): R //(22.5xe-mC;), where R = insulin resistance, / - fasting serum insulin concentration, and G - fasting serum glucose concentration. Time-integrated glucose and insulin responses to the different types of breakfast were calculated as area under the response curve (AUC) employing the trapezoidal rule. Zero was used as baseline. The plasma tryptophan concentration was divided by all other LNAA to calculate the plasma (Trp)/(LNAA) ratio. The macronutrient composition in grams of each lood item offered for lunch was known prior to the experiment. The total amount of each macronutrient in lunch food in grams was calculated by adding up the amount of macronutrient in all consumed food items. The kJ were subsequentl~ calculated by multiplying the number of grams by 16.7 (for carbohydrate and protein) and 38.0 (for fat). Within-group data on glucose, insulin, amino acid ratios, and tk)od intake during placebo treatment were analyzed using paired Student's t test. The effect of FXT on each of the plasma parameters and on diet composition in both groups separately was analyzed using repeated measures analysis of variance (ANOVA) with as within subject factors treatment (PLAC/FXT) and breakfast-type (CHO/PROT). There was no reason to group measurements according to breakfast for the comparison of lhsting values during placebo treatment in both groups. Therefore.
these comparisons were carried out using a nested ANOVA with the two measurements in fasting conditions before the treatment modalities PLAC/CHO-breakfast and PLAC/PROT-breakfast nested within subjects. Breakfastinduced parameters were compared using repeated measures A N O V A with between subjects factor subject type (bulimic/control) and within subjects factor breakfast type (CHO/PROT). For food consumption at lunch a three-way A N O V A was carried out with treatment (PLAC/FXT) added as another within subjects factor. Subjects with missing data were eliminated entirely (one subject in the control group). Results are shown as mean ± standard deviation.
Results Bulimia Nervosa METABOLIC PARAMETERS. Placebo treatment. The time-integrated serum glucose and insulin concentrations in response to the CHO breakfast were significantly higher than in response to the PROT breakfast (AUC of glucose 939 _+_ 174 [CHO] vs. 781 + 71 [PROT] raM-150 min 1, p = I).(X)2 and AUC of insulin 6386 -+ 3426 [CHO] vs. 3807 _+ 1362 [PROT] mU.L 1"150 rain -1, p = 0.002). Plasma amino acid concentrations decreased in response to CHO consumption and increased in response to PROT consumption (Table 2). This pattern was equally apparent in all individual subjects. A N O V A revealed a highly significant effect of breakfast type on the change of individual amino acids (p < 0.001 for every single amino acid), The plasma (Trp)/(LNAA) ratio increased (27%) in response to CHO consumption and decreased (29%) in response to PROT consumption (Table 3). Thus, at 120 rain after breakfast, the ratio was 87% higher in response to CHO consumption than in response to PROT consumption. This effect of breakfast type on the height of the ITrp)/(LNAA) ratio was a highly significant (p < 0.001). t=luoxetine treatment. Analysis of variance revealed a significant effect of FXT on glucose response. The drug increased the time-integrated serum glucose concentration in response to both types of breakfast (1058 ± 172 [CHO-FXT] vs. 939 _+ 174 [CHO-PLAC] and 870 + 165 [PROT-FXT] vs. 782 _+ 71 [PROT-PLAC], p = 0.002). The insulin response was not significantly affected by FXT, although the time-integrated insulin concentrations in response to both types of breakfast were slightly higher during treatment with the drug (6583 --- 3225 [CHO-FXT] vs. 6385 +- 3426 [CHO-PLAC] and 4547 + 2571 [PROTFXT} vs. 1362 ± 1755 [PROT-PLAC], p = 0.13). FXT treatment did not influence the plasma amino acids
Plasma Amino Acids in Bulimia Nervosa
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Table 2. Amino Acid Concentrations (ptM) during Placebo in Control and Bulimic Subjects Fasting
C h a n g e a f t e r 1211 m i n CHO
A m i n o acid Isoleucine Leucine
CO
PROT
BN
CO
BN
53,9 + 13.1
49,3 + 7.5
25,5 ~ 4.8
24.4 _+ 9,5
60.6 ± 32.9
CO
72.3 ,+ 23.5
BN
131.0 _+ 48.7
109.8 _+ 28,7
96.0" .+ 16.1
49,5 + 8.8
- 39,6 + 17,3
115.7 + 62.6
Phenylalanine
50.7 .+ 7.6
48.0 + 6.8
14,4 :+ 3.8
13.5 _+ 6.3
26.1 +- 14.4
33.1 .+ 15.5
Tyrosine
58,8 + 17.7
52.5 + 10.1
21.5 :+: 4.9
16.9 .+ 8,2
62.8 + 32.2
70.8 + 31.2
196.3 + 42,3
176.4 + 27.5
55.2 =+: 14,3
42.7 ,+ 17.3
149.7 + 74.8
177.3 + 62.1
43.1 + 13.8
47.7 + 10.4
- 8 , 8 :~ 4,0
- 7 , 9 .+ 14.7
14.1 + I t
Valine Tryptophan
2 4 . 6 + 14.2
Notes. Fasting plasma concentrations oI the individual LNAA I~IMI and their change alter 121! mm in response to a carbohydrate (CHO) and protein-rich (PROT) breakfast during placebo treatment in control IC()) and bulimic (BNI subjects, Data are mean ~ S D "p = l).050 compared to normal-weight subjects.
either in basal conditions or in response to breakfast (Table 4). CONSUMPTION AT LUNCH TIME. Placebo treatThe total weight and calorie content of food consumed at lunch time were not affected by breakfast type (Table 5). The weight and energy percentage of the separate macronutrients were not affected by breakfast type either. FOOD
ment,
Fluoxetine treatment. FXT treatment (Table 5) did not affect total weight or calorie content of lunchtime food consumed by bulimic subjects. The weight and energy percentage of consumed macronutrients were not affected by drug treatment either. There was no significant drug × breakfast type interaction with respect to their effects on food intake.
Comparison between Bulimic and Control Subjects METABOt,IC PARAMETERS. Placebo treatment. The bulimic subjects were slightly more resistant to insulin than the control subjects as calculated by the Matthews formula (R = 1.2 -+ 0.1 vs. R = 0.7 -+ 1.2, p = 0.008). The fasting serum glucose and insulin concentrations were significantly higher in bulimic subjects as well (glucose 5.0 _+ 0.4 [BN] vs. 4.7 + 0.5 [CO] raM, p = 0.03 and insulin 5.3 + 2.1 [BN] vs. 1.6 _+ 0.9 [CO] mU/L, p < 0.001). The serum glucose concentrations remained below l0 mM after the CHO load in both groups. The timeintegrated serum glucose and insulin concentrations in response to either type of breakfast were not different in both groups (AUC of glucose after the CHO-breakfast 939 :t 174 [BN] vs. 920 + 179 [CO] raM-150 min -1 and insulin AUC in response to the CHO breakfast 6385 +
Table 3. Plasma Tryptophan Ratio during Placebo Treatment CO Breakfast type
0 rain
1211 rain
Carbohydrate
0 . 0 9 4 + 0,031
11, I 1 7 " ± 0.043
Protein
0.093 + 0.025
1/.065 - I).017
BN
A iTrpt/ILNAA)
0 rain
120 m i n
0,024" ~ ().014
0 , 1 1 6 -+ 0.034
0 . 1 4 7 " _+ 0 . 0 3 8
0.027 + 0.016
0,112 ~ 0.021
0.079 ± 0.012
,5 ( T r p ) / ( L N A A ) 0 . 0 3 i f ' -+ 0 . 0 4 7 -0,033
-+ 0.018
Plasma ratio of tryptophan to other large neutral amino acids before and 12[) mm after a carboh~drale-rich or protein-rich breakfast during placebo treatment in control (CO) and bulimic IBN) subjects. Data are mean :- S D "p < 0.fX)l compared to protein-rich breakfast AN()VA re'~ealed signifikanl d~tlelcnces between both fasting and postabsorptive ratios in CO and BN (p < 0.05).
Table 4. Plasma Tryptophan Ratio during Fluoxetine Treatment CO
BN
0 rain
1211 m i n
A ('I'rpl/(LNAA)
0 rain
120 m i n
Carbohydrate
0.095 -+ 0.1125
0.118" -+ 0 . 0 4 0
0.023" : : 0 . 0 2 0
0.103 -+ 0.023
0.142" = 0.033
Protein
1/,089 ~. 0,025
0,061 ± I).016
0.029 -_ 0 . 0 3 0
0.112 + 0.021
0 . 0 8 0 +_ 0 . 0 1 6
B r e a k f a s t type
,5 ( T r p ) / ( L N A A ) 0 . 0 3 9 " -+ 0.028 -0.032
-+ 0.013
Plasma ratio of tryptophan to other large neutral amino acids before and 1211 nun after a carbohydrate-rich or protein-rich breakfast during fluoxetine treatment in control (COl and bulimic (BN) subjects. Data are mean - S D "p < 0.001 compared to protein-rich breakfast. Both tasting and postabsorptix c ratios were higher in BN compared to CO (p < 0.051. A N O V A did not revea] a significant difference from data obtained during placebo Ireatmenl cTable 3)
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Table 5. Food Selection
at
Lunchtime in Bulimic Subjects Placebo CHO
Total e n e r g y
Fluoxetine PROT
CHO
PROT
1468 + 621
1587 ÷ 955
1364 + 537
1459 -+ 524
(302 + 18(t) 50 ~ 16
~337 + 218) 48 + 1 I
(268 ± 1421 49 ± 15
(288 -+ 119) 47 -+ 8
!47 * 28~ :~5 * 15
146 ' 28) 36 - 9
(41 ± 22) 34 + I I
(41 ± 17) 35 -+ 8
(g) E n e r g y p e r c e n t a g e protein
112 6) 17 t 4
{15 +_ 111 17 ~ 4
(12 -+ 6) 17 -+ 6
(13 + 6) 19 _+ 5
(g)
~14 " 7)
115 ~ %
(14 + 7)
(16 + 6)
(g) Energy percentage carbohydrate (g/ E n e r g y p e r c e n t a g e fat
Notes, Food intake !kJ. gl and macronutricnt composition !energy percentage, gi al lunchtime alter carbohydrate (CHO) and protein (PROT) rich breakfasts in bulimic subjects treated with placebo or l]uoxcliiic Data are given a~ mean + SD.
3426 [BN] vs. 5821 -+ 3167 ICOI mU-L J.150 min data after PROT not shown). All fasting individual plasma amino acid concentrations (Table 2) were slightly lower in the bulimic subjects than in the controls, except for plasma tryptophan levels, which were somewhat higher. The differences did not reach statistical significance except for values for leucine (p 0.05). However, as a result the fasting plasma (Trp)/ (LNAA) ratio was significantly higher (34%) in BN (p 0.03, Table 3). The changes in plasma amino acids in response to breakfast were not significantly different between the two groups. The plasma (Trp)/(LNAA) ratios at 120 rain after breakfast were significantly higher (24%) in BN (p = 0.03). Fluoxetine treatment. FXT treatment did not affect any of the metabolic parameters in control subjects, in contrast to the significant effect of the drug on the glucose response in BN. Analysis of variance did not reveal a significant drug × subject type interaction with respect to metabolic parameters.
FOOD
CONSUMPTION
AT LUNCHTIME.
Placebo
treat-
ment. The total weight and calorie content of food con-
sumed at lunchtime were significantly lower in bulimic subjects (p < 0.001 and p = 0.007, respectively) (Tables 5,6). The weight of each individual macronutrient was lower in BN as well (p < 0.001). However, the macronutrient composition of lunchtime food was comparable in both groups, witness the absence of significant differences in energy percentage of protein, fat, and carbohydrate between bulimic and control subjects. Under PLAC treatment, the control subjects tended to consume less carbohydrate and more fat (both in g and as energy percentage) after the CHO breakfast. In bulimic subjects breakfast type did not have any detectable influence on subsequent food consumption. However, A N O V A failed to reveal a significanl breakfast type × subject type interaction with respect to their effects upon food intake. Fluoxetine treatment. Although FXT treatment (Tables 4,5) tended to decrease total food intake in both groups (total caloric intake was 10% less during FXT for
Table 6. Food Selection at Lunchmne in Control Subjects Placebo ('H()
Fluoxetine PROT
CHO
PROT
Total e n e r g y
2747 - 776
2~77 * 852
2502 _+ 1059
2315 + 1051
(g) Energy percentage carbohydrate
c473 1871 "~1 ~ 12
~510 - 2211 56" ! 13
(443 +_ 208) 54 ± 12
(432 _4- 1831 54 + 12
(g) E n e r g y p e r c e n t a g e fat
(81 : 191 ~,5 ~ 10
/85 : 25) ~1 ~' + 9
(79 + 261 30 + 12
(71 ± 22) 31 ± 9
(g) E n e r g y p e r c e n t a g e protein
1 2 0 : 2 121 t5 +_ 4
! 2 3 " " 121 15 ~ 4
(22 + 16) 15 + 3
(21 - 15) 15 _+ 5
(g)
~25 ~ 101
i24 : I I )
(22 ± 111
(21 ± 14)
Notes. Fnod intake (k J, g) and macronutrient ct)nl[)OSlllOtl (energy percentage, g~ at lunchtime after carbohydrate iCHO) and protein (PROT) rich breakfasts in control subjects treated with placebo or fluoxetine Data are gt,.en a> nlean ~ SD. "p - 0.058 compared to CHO breakfast J'p < 0.05 compared to CHO breakfas~ [hese data haxc heeta published prevlOLl,I3 'Ptl] Lt a[, I t)9~l
Plasma Amino Acids in Bulimia Nervo,~a
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both groups taken together), there was no statistically significant drug effect on caloric intake. FXT abolished the effect of breakfast type on lunch food in control subjects. However, there were no significant drug × subject type interactions with respect to their effects on food intake.
Discussion The results of this experiment suggest that patients with BN might be slightly more resistant to insulin with respect to its effects on glucose metabolism than healthy control subjects. Bulimic patients had significantly elevated fasting serum glucose and insulin concentrations and a higher index of insulin resistance as calculated according to the formula of Matthews (Matthews et al 1985) compared to control subjects. Moreover, both serum glucose and insulin concentrations in response to either type of breakfast were slightly (although not significantly) higher in bulimic than in control subjects. Although all differences were small and the fasting values were within the normal range for our laboratory, the resultant chronic "hyperinsulinemia" might have metabolic consequences with respect to plasma amino acid concentrations. It has recently been shown that insulin resistance has differential effects on glucose and amino acid metabolism: glucose disposal is reduced in insulin-resistant subjects, while the influence of insulin on amino acid uptake in skeletal muscle remains unchanged (Pijl et al 1994; Luzi et al 1990; Caballero and Wurtman 1991). Since insulin induces a decline in plasma amino acid concentrations by promoting their uptake into skeletal muscle tissue, hyperinsulinemia (induced by insulin resistance) theoretically leads to (chronic) hypoaminoacidemia. Indeed, all basal plasma amino acid levels. except for the concentration of tryptophan, were lower (although not significantly) in "'hyperinsulinemic" BN patients. As a consequence, the plasma (Trp)/(LNAAI ratio in fasting conditions was (significantly) higher in the bulimic group. However, the difference was rather small (24%). On the basis of results obtained in animal experiments, it was estimated that an acute rise in the (Trp)/ (LNAA) ratio of at least 50~ or an acute fall of at least 30% is necessary to affect brain 5-HT synthesis (Ashley et al 1985). It is not known to what extent smaller but chronically present differences are able to alter brain 5-HT-mediated neurotransmission. Therefore, it remains unclear whether the 24% higher (Trp)/(LNAA) ratio in bulimic patients is likely to bring about an enhancement or: brain 5-HT synthesis in these patients. Other studies have produced contradictory data on insulin-mediated glucose and amino acid metabolism in BN. In an experiment conducted by Turner et at, the
fasting serum glucose concentration was reported to be comparable in bulimic subjects and normal controls, but the serum glucose concentrations in response to an oral carbohydrate challenge were slightly higher in bulimic patients (Turner et al 1991). Serum insulin concentrations were not mentioned in their report. In a study conducted by Schweiger et al (1987) the serum glucose and insulin concentrations in response to a balanced test meal were higher in bulimic patients. Another study using the glucose clamp technique indicates that bulimic subjects might be more resistant to insulin with respect to its effects on glucose metabolism than patients with anorexia nervosa (Kiriike et al 1990). Normal control subjects were not included in this study. In contrast, Blouin reported that fasting serum glucose and insulin levels as well as their response to an i.v. glucose challenge are comparable in BN and healthy control subjects (Blouin et al 1993). Several other experiments revealed normal glucose metabolism in bulimic patients as well (Hohlstein et al 1986; Mitchell and Bantle, 1983). On the basis of these data, it can be concluded that if glucose metabolism is altered in BN at all, the difference with control subjects is very small and of disputable clinical significance, although a longstanding slight hyperinsulinemia associated with insulin resistance might explain the lower plasma amino acid concentrations in bulimic patients. Relatively few studies have examined amino acid metabolism in BN. Lydiard et al (1988) did not find differences between the fasting (Trp)/(LNAA) ratios in bulimic subjects and normal-weight controls. Blouin et al (1993) did not observe any difference between either the fasting (Trp)/(LNAA) ratio or its response to an i.v. glucose challenge in bulimic subjects and normal-weight controls. Turner et al (1991) did not find differences between the increase of the (Trp)/(LNAA) ratio induced by an oral carbohydrate load in bulimics and normal controls either. In contrast, Kaye et al (1988) suggested that the change in the (Trp)/(LNAA) ratio in response to bingeing might contribute to the generation of satiety in bulimic subjects, since bulimics who developed an increase in the ratio during bingeing had fewer cycles of bingeing than bulimics who did not develop such an increase. It is interesting that three of nine (bulimic) subjects participating in their experiment did not develop an increase of the ratio in response to bingeing. However, since the authors did not register the type of food consumed during each binge, the failure of the (Trp)/(LNAA) ratio to rise in these subjects could have been due to the composition of the consumed binge food rather than to specific metabolic alterations. The results of the present experiment indicate that bulimics and normal controls do not exhibit different metabolic
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responses to either an oral cacbohydrate or an oral protein challenge. As far as subsequent food selection at lunch time was concerned, bulimic subjects consumed significantly fewer calories and grams of food than control subjects. This result has to be interpreted with much caution, since other than metabolic factors, especially the fact that the subjects knew that their food intake would be recorded, might have contributed to the difference. The macronutrient composition of the food selected for lunch was comparable in bulimic and control subjects. The calorie content, total weight, and macronutrient composition of lunch food were not significantly affected by the type of breakfast preload or FXT treatment in bulimic patients. Although fewer calories and grams of food were consumed after either breakfast type during FXT treatment in comparison with PLAC, the differences did not reach statistical significance. This is somewhat in contrast to the results obtained in control subjects, who tended to select less carbohydrate and more fat after the CHO breakfast. We found these effects of a nutritional challenge on subsequent macronutrient selection to be statistically significant in a larger study population comprising nonbulimic subjects (Pijl et al 1993). The effect was abolished by FXT treatment, indicating that 5-HT-mediated neurophysiological mechanisms in the brain could be involved in this phenomenon. The observation that bulimics do not seem to respond to the breakfast preload in a similar way, in spite of the fact that their metabolic response was comparable to that observed in control subjects, might indicate that their brain nuclei are less sensitive to serotoninergic stimuli than the central nuclei of control subjects. Although it is not known whether longstanding differences in plasma amino acid concentrations affect brain 5-HT physiology, one could hypothesize that the elevation of the plasma (Trp)/ (LNAA) ratio in bulimic patients leads to chronically enhanced 5-HT synthesis and postsynaptic 5-HT receptor downregulation in these people. This would disturb 5-HTmediated satiety mechanisms in BN. On the other hand. since feedback mechanisms regulate the synaptic release of 5-HT (Jacobs and Azmitia 1992), it seems unlikely that
small differences in precursor concentration substantially affect synaptic 5-HT concentrations in the brain. Differences between the results of experiments investigating parameters of amino acid (and perhaps glucose) metabolism in women might be partly explained by the fact that tests were performed during different phases of the menstrual cycle. It has been shown that plasma amino acid concentrations vary in different phases of the menstrual cycle, at least during high mental stress (Tuiten 1993). Since it is not known whether small but chronically present alterations in insulin sensitivity and amino acid metabolism have effects on brain physiology, it seems important to take all possible variables influencing these parameters into consideration. In summary, the results of the present experiment combined with data of previous studies indicate that no major disturbance of either glucose or amino acid metabolism as measured in response to various nutritional challenges is apparent in patients with BN. Although bulimics might have a slightly impaired glucose tolerance, the clinical importance of this finding remains questionable. The increased plasma (Trp)/(LNAA) ratios of bulimic subjects might be a metabolic consequence of their (relative) glucose intolerance. There is no scientific basis to suggest a role for increased (Trp)/(LNAA) ratios in the pathogenesis of bingeing behavior, although downregulation of postsynaptic 5-HT receptors might be an ultimate consequence. Thus, it is unlikely that bingeing is brought about by "peripheral" metabolic disturbances hampering brain 5-HT-mediated neurotransmission. However, a large body of data indicates that brain 5-HT is involved in the pathogenesis of BN. Numerous defects other than peripheral metabolic of origin, such as disturbed tryptophan transport across the blood-brain barrier, centrally located defects of 5-HT synthesis, or 5-HT receptor defects, might explain these data.
Yhis studywas supportedby a grant from the NWO Council for Medical Research. H. Pijl is an NWO research fellow.
References American Psychological Association 11987): Diagnostic and Statistical Manual c!f Mental Diseases, 3rd edition, revise& Washington DC: American Psychological Association.
composition influences plasma tryptophan to large neutral amino acid ratios of healthy lean young men. J Neural Transm 63:271-283.
Ashley DV, Barclay DV, Chauffard FA. M6ennoz D, Leathwood PD (1982): Plasma amino acid responses in humans to evening meals of differing nutritional composition. Am J Clin Nutr 36: 143-153. Ashley DV. Liardon R. Leathwood PD (1985): Breakfast meal
Blouin AG, Blouin JH, Perez EL, Bushnik T, Zuro C, Mulder E I1988): Treatment of bulimia with fenfluramine and desipramine. J Clin Psychopharmacol 8:261-269. Blouin AG, Blouin JH, Bushnik T, Braaten J, Goldstein C, Sarwar G (1993): A double blind placebo-controlled glucose
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challenge in bulimia nervosa: Psychological effects. Biol Psychiatry 33:160-168. Blundell JE (1992): Serotonin and the biology of feeding. Am J Clin Nutr 55:155S-159S. Caballero B, Wurtman RJ (1991): Differential effects of insulin resistance on leucine and glucose kinetics in obesity. Metabolism 40:51-58. Fernstrom JD, Wurtman RJ (1971): Brain seromnin content: Increase following ingestion of carbohydrate diet. Science 174: 1023-1025. Fernstrom JD, Wurtman RJ 119721: Brain serotonin content: Physiological regulation b} plasma neutral amino acids. Science 178:414-416. Fuller RW, Wong DT (1989J: Fluoxelme: A serotonergic appc tite suppressant drug. Drug Der Res 17:1 -15. Garner DM, Olmstead MP, Polivy ,I (1983): Development anti validation of a multidimensional eating inventory for anorexia nervosa and bulimia nerw~sa Inl J Eating Disord 2:15-34. Goodall E, Feeney S, McGuirk 1. Sil~erstone T (19921: A comparison of the effects of l)- and L-fenfluramine and D-amphetamine on energy and macronutrient intake in human subjects. Psychopharmacolo~,y (Berl~ 106:221-227. Green H, Sawyer JL 11966); Demonstration. characterization, and assay procedure of tryptophan h~droxylase in rat brain. Anal Biochem 15:53-64. Hohlstein LA, Gwirtsman HE. Whalen F, Enns MP (19861: Oral glucose tolerance in bulimia, lnt ,I Eatin~ Disord 5:157 16(/. Jacobs BL, Azmitia EC (1992i: Structure and function of the brain serotonin system. Phvsio[ Rev 72:165-229. Jimerson DC. Lesem MD, Kaye WH. Brewerton TD (1992i: Low serotonin and dopamine metabolite concentrations m cerebrospinal fluid from bulimic patients with frequent binge episodes. Arch Gen Psychiatry 49:132-138. Kaye WH, Gwirtsman HE, Brewerton TD. George DT, Wurtman RJ (19881: Bingeing behavior and plasma anaino acids: A possible involvement of brain serotonin in bulimia nervosa. Psychiatry Res 23:31-43. Kiriike N, Nishiwaki S, Nagata T. Okunu Y, Yamada J, Tanaka S, Fujii A, Kawakita Y (19901: Insulin sensitivity in patients with anorexia nervosa and bulimia. Acta Psvchiatr Scaml 81:236-239. Leibowitz SF, Weiss GF, Shor Posner (; (1988): Hypothalamic serotonin: Pharmacological. biochemical, and behavioral analyses of its feeding-suppressive action. Clin NeurophaJ~ macol 11 Suppl 1:$51-$71. Leibowitz SF. Weiss GF, Suh JS !1990i: Medial hypothalam,c nuclei mediate serotonin's inhibitory effect on feeding beha~ ior, Pharmacol Biochem Behav 37:735 742.
Lydiard RB, Brady KT, O'Neil PM, Schlesier Carter B, Hamilton S. Rogers Q, Ballenger JC (1988): Precursor amino acid concentrations in normal weight bulimics and normal controls. Prog Neuropsychopharmacol Biol Psychiatry 12:893898. Matthews DR, Hosker JP, Rudenski AS, Naylor BA, Treacher DF, Turner RC (1985): Homeostasis model assessment: Insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man. Diabetologia 28:412-419, McBride PA, Anderson GM, Khait VD, Sunday SR, Halmi KA I 1991 ): Serotonergic responsivity in eating disorders. Psyehopharmacol Bull 27:365-372. McMenamy RH, Oncley JL (19581: The specific binding of L-tryptophan to serum albumin. J Biol Chem 233:14361447. Mitchell JE, Bantle JP (1983): Metabolic and endocrine investigations in women of normal weight with the bulimia syndrome. Biol Psychiat©' 18:355 365. Pardridge WM ( 1979): The role of blood-brain barrier transport of tryptophan and other neutral amino acids in the regulation of substrate-limited pathways of brain amino acid metabolism. J Neural Transm Suppl 15:43-54. Parker Brody AB, Van Furth EF (19941: Een bescbrijvend onderzoek naar kenmerken en comorbiditeit van de patient op de wachtlijst voor de eetstoornis afdeling van de Robert Fleury Stichting. (Unpublished). Pijl H, Koppeschaar HPF, Cohen AF, lestra JA, Schoemaker HC, Frolich M, Onkenhout W, Meinders AE (1993): Evidence for brain serotonin mediated control of carbohydrate consumption in normal weight and obese humans, lnt J Obesi O, 17:513-520. Pijl H, Potter van Loon BJ, Toornvliet AC, Radder JK, Onkenhour W, Frolich M, Meinders AE (1994): Insulin induced decline of plasma amino acid concentrations in obese subjects wilh and without non-insulin dependent diabetes. Metabolism 43 (5):640-646. Roelfsema F, Frolich M (1985): Glucose tolerance and plasma immunoreactive insulin levels in acromegalics before and after selective transsphenoidal surgery. Clin Endocrinol (Oxf) 22:531--537. Schwartz DH, McClane S, Hernandez L, Hoebel BG (19891: Feeding increases extracellular serotonin in the lateral hypothalamus of the rat as measured by microdialysis. Brain Res 479:349 -354. Schweiger U, Poellinger J, Laessle R, Wolfram G, Fichter MM, Pirke KM (1987): Altered insulin response to a balanced test meal m bulimic patients, lnt J Eating Disord 6:455-468. Yeff KL, Young SN (1988): Effects of carbohydrate and protein administration on rat tryptophan and 5-hydroxytryptamine: Differential effects on the brain, intestine, pineal, and pancreas. Can J Physiol Pharmacol 66:683-688.
Lissner L, Stevens J, Levitsky I)A, Rasnmssen KM. Strupp BJ (19881: Variation in energy intake during the menstrual cycle: Implications for food-intake research. Am J Clin Nutr 4~: 956-962.
Tuiten A (I 993): Interactions between mental and bodily mechanisms in anorexia nervosa and premenstrual complaints. Thesis, University of Utrecht.
Luzi L, Castellino P, Simonson DC. Petrides AS, DeFronzo RA (19901: Leucine metabolism in IDDM. Role of insulin and substrate availability. Diabete.s 39:38--48.
Turner MS, Foggo M, Bennie J, Carroll S, Dick H, Goodwin GM !19911: Psychological, hormonal and biochemical changes following carbohydrate bingeing: A placebo controlled study
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in bulimia nervosa and matched controls. Psychol Med 21:123-133.
Wurtman RJ, Hefti F, Melamed E (1980): Precursor control of neurotransmitter synthesis. Pharmacol Rev 32:315-335.
Walsh BT (1991): Psychopharmacologic treatment of bulimia nervosa. J Clin Psychiatr)' 52 Suppl:34-38.
Yuwiler A, Oldendorf WH, Geller E, Braun L (1977): Effect of albumin binding and amino acid competition on tryptophan uptake into brain. J Neurochem 28:1015-1023.
Wurtman RJ, Pardridge WM (1979): Summary: Circulating tryptophan, brain tryptophan, and psychiatric disease. ,I Neural Transm Suppl 15:227 236.
Fifteen bulimic women (BN) and 19 healthy Ji'male controls (CO) were studied. The subjects were cross-over treated with either.fluoxetine (FXT) or placebo during 4 days. They received, in randomized order, a breakfast containing pure carbohydrate (CLIO) or a protein-rich (PROT) breakfast following day 3 and 4 of each treatment period. Twenty-nine different food items ,'ere offered fi)r lunch. The .lasting serum glucose and insulin concentrations and the fasting plasma tr3'ptophan (Trp)/large neutral amino acid (LNAA) ratio were slightly higher in BN. The changes of these metabolic parameters in res'ponse to a CHO or PROT breakfast were similar in both groups. Across breakfast O'pe, the plasma (Trp)/(LNAA) ratio at 120 min q[~er breakfast was higher in BN. Total caloric intake at lunchtime was less in BN. In CO, less carbohydrate wa.s selected at lunchtime following the CHO breakfast, an effect that was abolished by FXT Breal~(ast type or FXT did not have any apparent effect on .fbod intake at lunchtime in BN. This might indicate that bulimic subjects are less sensitive to serotoninergic stimuli than control subject,v.
Key Words:
Amino acids, serotonin, carbohydrate, food preferences, bulimia nervosa
BIOL PSYCHIATRY 1995:38:659-668
Introduction Serotonin (5-hydroxytryptamine [5-HT]) mediated neurotransmission in several brain nuclei plays an important
From the Departments of General Internal Medicine iHP, AEM). Dietetics and Nutrition (JAil, Clinical Chemistr5 IMFI, Pediatricx (WO), and Psychiatr_~ (RJV), University Hospital Leiden. the Netherlands: Centre for Human Drug Research, Leiden, the NetherLands (AFC, HCS) and Deparlment of Endocr3 nology, University Hospital Utrecht. the NetherLands (HPFK). Address repnnt requests to Dr Elanno Pijh Deparlment of General Internal Medicine. University Hospital Leiden. P.O. Box 9600. Bldg. 1. CI R3. 2301) RC Leiden, Netherlands
© 1995 Society o! Biological Ps~chiatr~
role in the regulation of food intake (Blundell 1992). Stimulation of postsynaptic 5-HT receptors appears to inhibit specifically carbohydrate consumption in rats (Leibowitz et al 1990). More vigorous stimulation reduces total caloric intake as well (Leibowitz et al 1988). In humans, serotoninergic drugs exert a similar influence on food intake (Goodall et al 1992). Brain 5-HT is produced locally in serotoninergic neurons (Green et al 1966). The production rate is dependent on the availability of the precursor amino acid tryptophan (Wurtman et al 1980). Since tryptophan (Trp) competes with other so-called 0006-3223/95/$09.50 SSDI 0006-3223(95)00043-G
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large neutral amino acids (LNAA; e.g., valine, isoleucine. leucine, phenylalanine, and tyrosine) for transport across the blood brain barrier (BBB), the (Trp)/(LNAA) ratio in plasma determines the amount of Trp that reaches the brain: a high ratio allows for enhanced transport into brain tissue (Fernstrom and Wurtman 1972). The consumption of carbohydrates induces an increase of the plasma (Trp)/ (LNAA) ratio through an insulin-mediated mechanism (Fernstrom and Wurtman 1971). The consumption of protein affects the plasma (Trp)/(LNAA) ratio by directly contributing the dietary amino acids to the plasma compartment. Thus the change of the ratio is likely to depend on the amino acid composition of the ingested proteins. Since Trp is the least abundant amino acid in many proteins and because it is metabolized by the liver after transport through the portal vein, the plasma (Trpt/ (LNAA) usually decreases in response to protein intake (Pijl et al 1993). Consequently, brain 5-HT concentration (Fernstrom and Wurtman 1971 : Teff and Young 1988) and synaptic release (Schwartz et al 1989) decrease after protein ingestion and increase after the consumption of carbohydrate. Bulimia nervosa (BN) is an eating disorder characterized by recurrent episodes of binge eating, combined with self-induced vomiting, purging, or other methods to prevent weight gain (American Psychological Association 1987). Several lines of evidence support the notion that brain 5-HT is implicated in the pathogenesis of BN. Since the role of 5-HT in the regulation of eating behavior appears to be predominantly inhibitory, a defect of 5-HT function could theoretically contribute to the binge behavior in BN. Indeed, frequent bingeing is associated with low levels of 5-HT metabolites in cerebrospinal fluid (Jimerson et al 1992); bulimic patients exhibit a reduced prolactin response to an endogenous serotoninergic challenge with D,L-fenfluramine (McBride et al 1991 ); drugs increasing 5-HT synaptic release and/or blocking 5-HT reuptake. thereby enhancing postsynaptic activation of 5-HT receptors, appear to be useful tools for the pharmacological treatment of BN (Walsh 1991; Blouin et al 1988). In view of what has been discussed previously, a defect in brain 5-HT neurotransmission could be brought about by changes of peripheral amino acid metabolism. Delivery ol tryptophan to brain tissue might be hampered in bulimic patients as a result of alterations of the (Trp)/(LNAA) ratio in basal conditions or in response to food intake, thereb~ reducing brain 5-HT synthesis and synaptic release. The resultant changes in postsynaptic 5-HT action could lead to aberrant eating patterns. The present study was designed to compare changes in plasma (Trp)/(LNAA) ratio induced by carbohydrate or
protein consumption for breakfast in bulimic subjects with those in normal controls; to study associated differences in caloric content and macronutrient composition of subsequent food choice at lunchtime between the two groups; and to investigate the effects of manipulating the central serotoninergic system by fluoxetine on food choice in both groups.
Subjects and Method Healthy bulimic women with a regular menstrual cycle, who did not use oral contraceptives or any other medication, were recruited through a mailing among members of the Dutch Association of Bulimia and Anorexia Nervosa patients. After physical examination, electrocardiogram (EKG) and screening for pregnancy, all bulimic subjects attended a psychiatrist for confirmation of the diagnosis. Normal-weight control women were recruited through a local newspaper advertisement. Fifteen bulimic (age 27.7 _+ 5.7 years [mean _+ standard deviation], range 19-44 years, body mass index [BMI] 22.0 -4- 2.2 kg/m2), and 19 control subjects (age 32.3 - 8.7 years, range 2 0 - 4 6 years; BMI 22.5 -+ 1.9 kg/m 2, range 20.0-25.0 kg/m 2) were enrolled in the study. The bulimic subjects met Diagnostic and Statistical Manual of Mental Disorders, 3rd ed, rev. (DSM-III-R) criteria for bulimia nervosa (American Psychological Association, 1987). There was no history of (psychoactive) substance abuse, bipolar or psychotic disorder, or major depression during the 6 months prior to the study. The average number of binges per week amounted to 8. l -+ 6.7. All but one (using laxatives) of the bulimic subjects vomited in order to prevent weight gain. To characterize our subjects further we used the Eating Disorder Inventory (Garner et al 1983). Comparing the data of our patients with Dutch norm data we concluded that our group was comprised of typical bulimia nervosa patients with relatively few symptoms of anorexia nervosa (Parker Brody and Van Furth 1994). All patients were seen by a single psychiatrist. Control subjects did not have a history of eating disorders or any other major psychiatric disease. Written informed consent was obtained from all women. The study was approved by the ethics committee of the University Hospital Leiden. The present study was conducted within the framework of a larger experiment investigating the effects of nutrients, plasma amino acid levels, and serotoninergic drugs on fi)od choice in different patient populations. The results of another part of the experiment, concerning obese subjects, have been published elsewhere (Pijl et al 1993). The subjects used as controls in this study were the same as those in the experiment described in the previous
P l a s m a A m i n o Acids in Bulimia Nerw~sa
publication. The design of the present study parallels that of the former. It had a double-blind, crossover, 2 × 2 factorial design with drug/placebo and type of breakfast as factors. The subjects received either fluoxetine (FXT, 60 mg orally in one daily dose) or placebo (PLAC) in randomized order during 4 days, starting within 3 days after the onset of menstrual bleeding. FXT is a serotoninergic drug, blocking 5-HT reuptake from the synaptic cleft, thereby enhancing postsynaptic 5-HT action (Fuller and Wong 1989). At 2 consecutive days following day 3 and day 4 of treatment the subjects were admitted to the study center at 8.30 AM for testing. All tests were performed in the midfollicular phase of the menstrual cycle, which is of importance since tood choice is influenced by the phase of the menstrual cycle (Lissner et al 1988). Smoking, eating, and drinking anything but water and tea without sugar and milk were prohibited for 12 hours prior to admission. An intravenous (i.v.) cannula was inserted into an antecubital vein. Subsequently, at 9.00 AM on the first test day the subjects received either a breakfast containing pure carbohydrate (80 g maltodextrin) or a comparable (as far as taste and appearance were concerned) protein-rich breakfast (60% protein: 30 g milk protein, and 40% lactose, sucrose, maltodextrin). The next morning the alternative breakfast was offered. Both types of breakfast were fluid, isovolemic (300 ml), and isocaloric (300 kcal). Blood samples for measurement of serum glucose and insulin concentrations were collected at 0, 2, 4, 6, 8, 10, 15, 30, 60, 120, and 150 rain following breakfast. Plasma levels of valine, isoleucine, leucine. phenylalanine, tyrosine, and tryptophan were measured at times 0 and 120 min after breakfast. The reason for measuring these amino acids at this particular time was that the (Trp)/(LNAA) was observed to change maximally at 120 min after an acute food challenge in normal men (Ashley et al 1985, 1982). The subjects were not allowed to eat or drink anything until lunch at 180 min after breakfast. This time was chosen because we expect the brain 5-HT concentration to change about 1 hour after a change in plasma amino acid levels (Fernstrom and Wurtman 1972). Twenty-nine different food items of known macronutrient content and weight were offered (Table 1). All items were available commercially and known to the subjects. The subjects were asked to compose their own lunch from these items. During the meal the subjects were allowed to take additional portions of any product. They were instructed to eat to satiety. Remaining food was weighed and subtracted from the records. The subjects received the alternative drug condition in the same period of the next menstrual cycle. Test procedures were performed according to the protocol as de-
BIOL PSYCHIATRY 1995;38:659-668
Table
1. F o o d I t e m s O f f e r e d
Macronutrient
for Lunch
661
and Their
Composition
Food
W e i g h t (g) C a r b o h y d r a t e (g) Protein (g) Fat (g)
Currant bun Curd cheese
50 90
25 15
4 7
2 1
A p p l e treacle G i n g e r cake Powdered chocolate
15 20 10
9 14 6
-1 1
--2
Jelly
15
9
--
--
Apple Banana
150 200
17 44
-2
---
O r a n g e juice
150
12
--
--
5
5
--
--
Sugar Cheese Cummin cheese C h e e s e spread
20 20 15
----
5 5 3
6 6 3
Liverwurst Smoked beef Ham Boiled e g g Peanut butter
15 15 20 50 15
----2
2 5 3 7 3
4 1 4 6 9
Tomato W h o l e w h e a t bread
75 35
2 14
1 2
-1
Rye bread W h i t e bread Diet m a r g a r i n e Margarine Butter L o w - f a t milk
45 35 10 10 10 250
18 16 --10
2 3 ---8
-1 4 8 8 3
Buttermilk Yogurt
250 150
12 6
8 5
5 6
8
1
1
1
Coffee creamer
scribed. Consequently, each subject received all four possible treatment modalities (drug × breakfast type). If a protein-rich (PROT) breakfast was offered at the first day of testing of the previous study period, a carbohydrate (CHO) breakfast was offered the first day of the second testing and vice versa. This was done to avoid subjects learning the test conditions, which might bias the results on food choices.
Assay Techniques Blood samples for measurement of serum glucose and insulin and plasma tyrosine, tryptophan, phenylalanine, isoleucine, leucine, and valine were centrifuged and immediately frozen ( - 4 0 ° C ) until assay. Glucose was measured enzymatically. Insulin was measured using a radioimmunoassay (Roelfsema and Frolich 1985). Blood for amino acid measurements was collected in tubes containing etheylene diamine tetraacetic acid (EDTA). Plasma was deproteinized using an equal volume of 5% (w/v) sulfosalicylic acid in water and analyzed for amino acid concentrations by ion exchange chromatography and nin-
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1995:38:659- 668
hydrin derivatization on an LKB 4151 Alpha Plus automated amino acid analyzer using standard conditions. The sum of albumin-bound and free plasma concentrations of tryptophan was determined using a standard addition calibration curve. In the circulation tryptophan is largely (75-80%) bound to albumin, which does not cross the BBB (McMenamy and Oncley 1985). Only free plasma tryptophan is available for uptake by brain tissue. The ratio of free to bound plasma tryptophan reflects on equilibrium state. Tryptophan uptake by the BBB carrier mechanism results in a shift in the equilibrium such that bound tryptophan is released. Studies suggest that a significant amount of tryptophan dissociates from albumin and is transported across the BBB during passage through the cerebral capillary plexus (Yuwiler et al 1977: Wurtman and Pardridge 1979). The total plasma concentration of tryptophan (i.e., albumin bound plus free) is therefore likely to be a better approximation of the amount of tryptophan available for access to the brain than the measured free tryptophan concentration (Yuwiler et al 1977; Pardridge 1979). Data Analysis Calculations were carried out using SPSS/PC+ V4.0.1 (SPSS Inc, Chicago, IL). Insulin resistance was calculated by the formula described by Matthews et al (1985): R //(22.5xe-mC;), where R = insulin resistance, / - fasting serum insulin concentration, and G - fasting serum glucose concentration. Time-integrated glucose and insulin responses to the different types of breakfast were calculated as area under the response curve (AUC) employing the trapezoidal rule. Zero was used as baseline. The plasma tryptophan concentration was divided by all other LNAA to calculate the plasma (Trp)/(LNAA) ratio. The macronutrient composition in grams of each lood item offered for lunch was known prior to the experiment. The total amount of each macronutrient in lunch food in grams was calculated by adding up the amount of macronutrient in all consumed food items. The kJ were subsequentl~ calculated by multiplying the number of grams by 16.7 (for carbohydrate and protein) and 38.0 (for fat). Within-group data on glucose, insulin, amino acid ratios, and tk)od intake during placebo treatment were analyzed using paired Student's t test. The effect of FXT on each of the plasma parameters and on diet composition in both groups separately was analyzed using repeated measures analysis of variance (ANOVA) with as within subject factors treatment (PLAC/FXT) and breakfast-type (CHO/PROT). There was no reason to group measurements according to breakfast for the comparison of lhsting values during placebo treatment in both groups. Therefore.
these comparisons were carried out using a nested ANOVA with the two measurements in fasting conditions before the treatment modalities PLAC/CHO-breakfast and PLAC/PROT-breakfast nested within subjects. Breakfastinduced parameters were compared using repeated measures A N O V A with between subjects factor subject type (bulimic/control) and within subjects factor breakfast type (CHO/PROT). For food consumption at lunch a three-way A N O V A was carried out with treatment (PLAC/FXT) added as another within subjects factor. Subjects with missing data were eliminated entirely (one subject in the control group). Results are shown as mean ± standard deviation.
Results Bulimia Nervosa METABOLIC PARAMETERS. Placebo treatment. The time-integrated serum glucose and insulin concentrations in response to the CHO breakfast were significantly higher than in response to the PROT breakfast (AUC of glucose 939 _+_ 174 [CHO] vs. 781 + 71 [PROT] raM-150 min 1, p = I).(X)2 and AUC of insulin 6386 -+ 3426 [CHO] vs. 3807 _+ 1362 [PROT] mU.L 1"150 rain -1, p = 0.002). Plasma amino acid concentrations decreased in response to CHO consumption and increased in response to PROT consumption (Table 2). This pattern was equally apparent in all individual subjects. A N O V A revealed a highly significant effect of breakfast type on the change of individual amino acids (p < 0.001 for every single amino acid), The plasma (Trp)/(LNAA) ratio increased (27%) in response to CHO consumption and decreased (29%) in response to PROT consumption (Table 3). Thus, at 120 rain after breakfast, the ratio was 87% higher in response to CHO consumption than in response to PROT consumption. This effect of breakfast type on the height of the ITrp)/(LNAA) ratio was a highly significant (p < 0.001). t=luoxetine treatment. Analysis of variance revealed a significant effect of FXT on glucose response. The drug increased the time-integrated serum glucose concentration in response to both types of breakfast (1058 ± 172 [CHO-FXT] vs. 939 _+ 174 [CHO-PLAC] and 870 + 165 [PROT-FXT] vs. 782 _+ 71 [PROT-PLAC], p = 0.002). The insulin response was not significantly affected by FXT, although the time-integrated insulin concentrations in response to both types of breakfast were slightly higher during treatment with the drug (6583 --- 3225 [CHO-FXT] vs. 6385 +- 3426 [CHO-PLAC] and 4547 + 2571 [PROTFXT} vs. 1362 ± 1755 [PROT-PLAC], p = 0.13). FXT treatment did not influence the plasma amino acids
Plasma Amino Acids in Bulimia Nervosa
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Table 2. Amino Acid Concentrations (ptM) during Placebo in Control and Bulimic Subjects Fasting
C h a n g e a f t e r 1211 m i n CHO
A m i n o acid Isoleucine Leucine
CO
PROT
BN
CO
BN
53,9 + 13.1
49,3 + 7.5
25,5 ~ 4.8
24.4 _+ 9,5
60.6 ± 32.9
CO
72.3 ,+ 23.5
BN
131.0 _+ 48.7
109.8 _+ 28,7
96.0" .+ 16.1
49,5 + 8.8
- 39,6 + 17,3
115.7 + 62.6
Phenylalanine
50.7 .+ 7.6
48.0 + 6.8
14,4 :+ 3.8
13.5 _+ 6.3
26.1 +- 14.4
33.1 .+ 15.5
Tyrosine
58,8 + 17.7
52.5 + 10.1
21.5 :+: 4.9
16.9 .+ 8,2
62.8 + 32.2
70.8 + 31.2
196.3 + 42,3
176.4 + 27.5
55.2 =+: 14,3
42.7 ,+ 17.3
149.7 + 74.8
177.3 + 62.1
43.1 + 13.8
47.7 + 10.4
- 8 , 8 :~ 4,0
- 7 , 9 .+ 14.7
14.1 + I t
Valine Tryptophan
2 4 . 6 + 14.2
Notes. Fasting plasma concentrations oI the individual LNAA I~IMI and their change alter 121! mm in response to a carbohydrate (CHO) and protein-rich (PROT) breakfast during placebo treatment in control IC()) and bulimic (BNI subjects, Data are mean ~ S D "p = l).050 compared to normal-weight subjects.
either in basal conditions or in response to breakfast (Table 4). CONSUMPTION AT LUNCH TIME. Placebo treatThe total weight and calorie content of food consumed at lunch time were not affected by breakfast type (Table 5). The weight and energy percentage of the separate macronutrients were not affected by breakfast type either. FOOD
ment,
Fluoxetine treatment. FXT treatment (Table 5) did not affect total weight or calorie content of lunchtime food consumed by bulimic subjects. The weight and energy percentage of consumed macronutrients were not affected by drug treatment either. There was no significant drug × breakfast type interaction with respect to their effects on food intake.
Comparison between Bulimic and Control Subjects METABOt,IC PARAMETERS. Placebo treatment. The bulimic subjects were slightly more resistant to insulin than the control subjects as calculated by the Matthews formula (R = 1.2 -+ 0.1 vs. R = 0.7 -+ 1.2, p = 0.008). The fasting serum glucose and insulin concentrations were significantly higher in bulimic subjects as well (glucose 5.0 _+ 0.4 [BN] vs. 4.7 + 0.5 [CO] raM, p = 0.03 and insulin 5.3 + 2.1 [BN] vs. 1.6 _+ 0.9 [CO] mU/L, p < 0.001). The serum glucose concentrations remained below l0 mM after the CHO load in both groups. The timeintegrated serum glucose and insulin concentrations in response to either type of breakfast were not different in both groups (AUC of glucose after the CHO-breakfast 939 :t 174 [BN] vs. 920 + 179 [CO] raM-150 min -1 and insulin AUC in response to the CHO breakfast 6385 +
Table 3. Plasma Tryptophan Ratio during Placebo Treatment CO Breakfast type
0 rain
1211 rain
Carbohydrate
0 . 0 9 4 + 0,031
11, I 1 7 " ± 0.043
Protein
0.093 + 0.025
1/.065 - I).017
BN
A iTrpt/ILNAA)
0 rain
120 m i n
0,024" ~ ().014
0 , 1 1 6 -+ 0.034
0 . 1 4 7 " _+ 0 . 0 3 8
0.027 + 0.016
0,112 ~ 0.021
0.079 ± 0.012
,5 ( T r p ) / ( L N A A ) 0 . 0 3 i f ' -+ 0 . 0 4 7 -0,033
-+ 0.018
Plasma ratio of tryptophan to other large neutral amino acids before and 12[) mm after a carboh~drale-rich or protein-rich breakfast during placebo treatment in control (CO) and bulimic IBN) subjects. Data are mean :- S D "p < 0.fX)l compared to protein-rich breakfast AN()VA re'~ealed signifikanl d~tlelcnces between both fasting and postabsorptive ratios in CO and BN (p < 0.05).
Table 4. Plasma Tryptophan Ratio during Fluoxetine Treatment CO
BN
0 rain
1211 m i n
A ('I'rpl/(LNAA)
0 rain
120 m i n
Carbohydrate
0.095 -+ 0.1125
0.118" -+ 0 . 0 4 0
0.023" : : 0 . 0 2 0
0.103 -+ 0.023
0.142" = 0.033
Protein
1/,089 ~. 0,025
0,061 ± I).016
0.029 -_ 0 . 0 3 0
0.112 + 0.021
0 . 0 8 0 +_ 0 . 0 1 6
B r e a k f a s t type
,5 ( T r p ) / ( L N A A ) 0 . 0 3 9 " -+ 0.028 -0.032
-+ 0.013
Plasma ratio of tryptophan to other large neutral amino acids before and 1211 nun after a carbohydrate-rich or protein-rich breakfast during fluoxetine treatment in control (COl and bulimic (BN) subjects. Data are mean - S D "p < 0.001 compared to protein-rich breakfast. Both tasting and postabsorptix c ratios were higher in BN compared to CO (p < 0.051. A N O V A did not revea] a significant difference from data obtained during placebo Ireatmenl cTable 3)
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Table 5. Food Selection
at
Lunchtime in Bulimic Subjects Placebo CHO
Total e n e r g y
Fluoxetine PROT
CHO
PROT
1468 + 621
1587 ÷ 955
1364 + 537
1459 -+ 524
(302 + 18(t) 50 ~ 16
~337 + 218) 48 + 1 I
(268 ± 1421 49 ± 15
(288 -+ 119) 47 -+ 8
!47 * 28~ :~5 * 15
146 ' 28) 36 - 9
(41 ± 22) 34 + I I
(41 ± 17) 35 -+ 8
(g) E n e r g y p e r c e n t a g e protein
112 6) 17 t 4
{15 +_ 111 17 ~ 4
(12 -+ 6) 17 -+ 6
(13 + 6) 19 _+ 5
(g)
~14 " 7)
115 ~ %
(14 + 7)
(16 + 6)
(g) Energy percentage carbohydrate (g/ E n e r g y p e r c e n t a g e fat
Notes, Food intake !kJ. gl and macronutricnt composition !energy percentage, gi al lunchtime alter carbohydrate (CHO) and protein (PROT) rich breakfasts in bulimic subjects treated with placebo or l]uoxcliiic Data are given a~ mean + SD.
3426 [BN] vs. 5821 -+ 3167 ICOI mU-L J.150 min data after PROT not shown). All fasting individual plasma amino acid concentrations (Table 2) were slightly lower in the bulimic subjects than in the controls, except for plasma tryptophan levels, which were somewhat higher. The differences did not reach statistical significance except for values for leucine (p 0.05). However, as a result the fasting plasma (Trp)/ (LNAA) ratio was significantly higher (34%) in BN (p 0.03, Table 3). The changes in plasma amino acids in response to breakfast were not significantly different between the two groups. The plasma (Trp)/(LNAA) ratios at 120 rain after breakfast were significantly higher (24%) in BN (p = 0.03). Fluoxetine treatment. FXT treatment did not affect any of the metabolic parameters in control subjects, in contrast to the significant effect of the drug on the glucose response in BN. Analysis of variance did not reveal a significant drug × subject type interaction with respect to metabolic parameters.
FOOD
CONSUMPTION
AT LUNCHTIME.
Placebo
treat-
ment. The total weight and calorie content of food con-
sumed at lunchtime were significantly lower in bulimic subjects (p < 0.001 and p = 0.007, respectively) (Tables 5,6). The weight of each individual macronutrient was lower in BN as well (p < 0.001). However, the macronutrient composition of lunchtime food was comparable in both groups, witness the absence of significant differences in energy percentage of protein, fat, and carbohydrate between bulimic and control subjects. Under PLAC treatment, the control subjects tended to consume less carbohydrate and more fat (both in g and as energy percentage) after the CHO breakfast. In bulimic subjects breakfast type did not have any detectable influence on subsequent food consumption. However, A N O V A failed to reveal a significanl breakfast type × subject type interaction with respect to their effects upon food intake. Fluoxetine treatment. Although FXT treatment (Tables 4,5) tended to decrease total food intake in both groups (total caloric intake was 10% less during FXT for
Table 6. Food Selection at Lunchmne in Control Subjects Placebo ('H()
Fluoxetine PROT
CHO
PROT
Total e n e r g y
2747 - 776
2~77 * 852
2502 _+ 1059
2315 + 1051
(g) Energy percentage carbohydrate
c473 1871 "~1 ~ 12
~510 - 2211 56" ! 13
(443 +_ 208) 54 ± 12
(432 _4- 1831 54 + 12
(g) E n e r g y p e r c e n t a g e fat
(81 : 191 ~,5 ~ 10
/85 : 25) ~1 ~' + 9
(79 + 261 30 + 12
(71 ± 22) 31 ± 9
(g) E n e r g y p e r c e n t a g e protein
1 2 0 : 2 121 t5 +_ 4
! 2 3 " " 121 15 ~ 4
(22 + 16) 15 + 3
(21 - 15) 15 _+ 5
(g)
~25 ~ 101
i24 : I I )
(22 ± 111
(21 ± 14)
Notes. Fnod intake (k J, g) and macronutrient ct)nl[)OSlllOtl (energy percentage, g~ at lunchtime after carbohydrate iCHO) and protein (PROT) rich breakfasts in control subjects treated with placebo or fluoxetine Data are gt,.en a> nlean ~ SD. "p - 0.058 compared to CHO breakfast J'p < 0.05 compared to CHO breakfas~ [hese data haxc heeta published prevlOLl,I3 'Ptl] Lt a[, I t)9~l
Plasma Amino Acids in Bulimia Nervo,~a
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1995;38:659-668
both groups taken together), there was no statistically significant drug effect on caloric intake. FXT abolished the effect of breakfast type on lunch food in control subjects. However, there were no significant drug × subject type interactions with respect to their effects on food intake.
Discussion The results of this experiment suggest that patients with BN might be slightly more resistant to insulin with respect to its effects on glucose metabolism than healthy control subjects. Bulimic patients had significantly elevated fasting serum glucose and insulin concentrations and a higher index of insulin resistance as calculated according to the formula of Matthews (Matthews et al 1985) compared to control subjects. Moreover, both serum glucose and insulin concentrations in response to either type of breakfast were slightly (although not significantly) higher in bulimic than in control subjects. Although all differences were small and the fasting values were within the normal range for our laboratory, the resultant chronic "hyperinsulinemia" might have metabolic consequences with respect to plasma amino acid concentrations. It has recently been shown that insulin resistance has differential effects on glucose and amino acid metabolism: glucose disposal is reduced in insulin-resistant subjects, while the influence of insulin on amino acid uptake in skeletal muscle remains unchanged (Pijl et al 1994; Luzi et al 1990; Caballero and Wurtman 1991). Since insulin induces a decline in plasma amino acid concentrations by promoting their uptake into skeletal muscle tissue, hyperinsulinemia (induced by insulin resistance) theoretically leads to (chronic) hypoaminoacidemia. Indeed, all basal plasma amino acid levels. except for the concentration of tryptophan, were lower (although not significantly) in "'hyperinsulinemic" BN patients. As a consequence, the plasma (Trp)/(LNAAI ratio in fasting conditions was (significantly) higher in the bulimic group. However, the difference was rather small (24%). On the basis of results obtained in animal experiments, it was estimated that an acute rise in the (Trp)/ (LNAA) ratio of at least 50~ or an acute fall of at least 30% is necessary to affect brain 5-HT synthesis (Ashley et al 1985). It is not known to what extent smaller but chronically present differences are able to alter brain 5-HT-mediated neurotransmission. Therefore, it remains unclear whether the 24% higher (Trp)/(LNAA) ratio in bulimic patients is likely to bring about an enhancement or: brain 5-HT synthesis in these patients. Other studies have produced contradictory data on insulin-mediated glucose and amino acid metabolism in BN. In an experiment conducted by Turner et at, the
fasting serum glucose concentration was reported to be comparable in bulimic subjects and normal controls, but the serum glucose concentrations in response to an oral carbohydrate challenge were slightly higher in bulimic patients (Turner et al 1991). Serum insulin concentrations were not mentioned in their report. In a study conducted by Schweiger et al (1987) the serum glucose and insulin concentrations in response to a balanced test meal were higher in bulimic patients. Another study using the glucose clamp technique indicates that bulimic subjects might be more resistant to insulin with respect to its effects on glucose metabolism than patients with anorexia nervosa (Kiriike et al 1990). Normal control subjects were not included in this study. In contrast, Blouin reported that fasting serum glucose and insulin levels as well as their response to an i.v. glucose challenge are comparable in BN and healthy control subjects (Blouin et al 1993). Several other experiments revealed normal glucose metabolism in bulimic patients as well (Hohlstein et al 1986; Mitchell and Bantle, 1983). On the basis of these data, it can be concluded that if glucose metabolism is altered in BN at all, the difference with control subjects is very small and of disputable clinical significance, although a longstanding slight hyperinsulinemia associated with insulin resistance might explain the lower plasma amino acid concentrations in bulimic patients. Relatively few studies have examined amino acid metabolism in BN. Lydiard et al (1988) did not find differences between the fasting (Trp)/(LNAA) ratios in bulimic subjects and normal-weight controls. Blouin et al (1993) did not observe any difference between either the fasting (Trp)/(LNAA) ratio or its response to an i.v. glucose challenge in bulimic subjects and normal-weight controls. Turner et al (1991) did not find differences between the increase of the (Trp)/(LNAA) ratio induced by an oral carbohydrate load in bulimics and normal controls either. In contrast, Kaye et al (1988) suggested that the change in the (Trp)/(LNAA) ratio in response to bingeing might contribute to the generation of satiety in bulimic subjects, since bulimics who developed an increase in the ratio during bingeing had fewer cycles of bingeing than bulimics who did not develop such an increase. It is interesting that three of nine (bulimic) subjects participating in their experiment did not develop an increase of the ratio in response to bingeing. However, since the authors did not register the type of food consumed during each binge, the failure of the (Trp)/(LNAA) ratio to rise in these subjects could have been due to the composition of the consumed binge food rather than to specific metabolic alterations. The results of the present experiment indicate that bulimics and normal controls do not exhibit different metabolic
666
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responses to either an oral cacbohydrate or an oral protein challenge. As far as subsequent food selection at lunch time was concerned, bulimic subjects consumed significantly fewer calories and grams of food than control subjects. This result has to be interpreted with much caution, since other than metabolic factors, especially the fact that the subjects knew that their food intake would be recorded, might have contributed to the difference. The macronutrient composition of the food selected for lunch was comparable in bulimic and control subjects. The calorie content, total weight, and macronutrient composition of lunch food were not significantly affected by the type of breakfast preload or FXT treatment in bulimic patients. Although fewer calories and grams of food were consumed after either breakfast type during FXT treatment in comparison with PLAC, the differences did not reach statistical significance. This is somewhat in contrast to the results obtained in control subjects, who tended to select less carbohydrate and more fat after the CHO breakfast. We found these effects of a nutritional challenge on subsequent macronutrient selection to be statistically significant in a larger study population comprising nonbulimic subjects (Pijl et al 1993). The effect was abolished by FXT treatment, indicating that 5-HT-mediated neurophysiological mechanisms in the brain could be involved in this phenomenon. The observation that bulimics do not seem to respond to the breakfast preload in a similar way, in spite of the fact that their metabolic response was comparable to that observed in control subjects, might indicate that their brain nuclei are less sensitive to serotoninergic stimuli than the central nuclei of control subjects. Although it is not known whether longstanding differences in plasma amino acid concentrations affect brain 5-HT physiology, one could hypothesize that the elevation of the plasma (Trp)/ (LNAA) ratio in bulimic patients leads to chronically enhanced 5-HT synthesis and postsynaptic 5-HT receptor downregulation in these people. This would disturb 5-HTmediated satiety mechanisms in BN. On the other hand. since feedback mechanisms regulate the synaptic release of 5-HT (Jacobs and Azmitia 1992), it seems unlikely that
small differences in precursor concentration substantially affect synaptic 5-HT concentrations in the brain. Differences between the results of experiments investigating parameters of amino acid (and perhaps glucose) metabolism in women might be partly explained by the fact that tests were performed during different phases of the menstrual cycle. It has been shown that plasma amino acid concentrations vary in different phases of the menstrual cycle, at least during high mental stress (Tuiten 1993). Since it is not known whether small but chronically present alterations in insulin sensitivity and amino acid metabolism have effects on brain physiology, it seems important to take all possible variables influencing these parameters into consideration. In summary, the results of the present experiment combined with data of previous studies indicate that no major disturbance of either glucose or amino acid metabolism as measured in response to various nutritional challenges is apparent in patients with BN. Although bulimics might have a slightly impaired glucose tolerance, the clinical importance of this finding remains questionable. The increased plasma (Trp)/(LNAA) ratios of bulimic subjects might be a metabolic consequence of their (relative) glucose intolerance. There is no scientific basis to suggest a role for increased (Trp)/(LNAA) ratios in the pathogenesis of bingeing behavior, although downregulation of postsynaptic 5-HT receptors might be an ultimate consequence. Thus, it is unlikely that bingeing is brought about by "peripheral" metabolic disturbances hampering brain 5-HT-mediated neurotransmission. However, a large body of data indicates that brain 5-HT is involved in the pathogenesis of BN. Numerous defects other than peripheral metabolic of origin, such as disturbed tryptophan transport across the blood-brain barrier, centrally located defects of 5-HT synthesis, or 5-HT receptor defects, might explain these data.
Yhis studywas supportedby a grant from the NWO Council for Medical Research. H. Pijl is an NWO research fellow.
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