Food intake: Regulation by plasma amino acid pattern

Life Sciences Vol. 8, Part II, pp . 1-9, 1969 . Printed in Great Britain.

FOOD INTAKE :

Pergamon Press

REGULATION BY PLAS14A AMINO ACID PATTERN*

Philip M-B . Leung and Quinton R . Rogers Department of Physiological Sciences, School of Veterinary Medicine University of California, Davis, California 95616

(Received 27 May 1968 ; in final form 21 October 1968) Apart from the glucostatic (1) and lipostatic (2) theories of food intake regulation it has been suggested that the central mechanism regulating food intake may in part be based on a metering of plasma amino acids (3, 4) .

Amino

acid imbalances have been very useful in studying this effect and can be most readily created by adding a surplus of all but one of the indispensable amino acids to a low protein diet .

One of the earliest effects of an amino acid

imbalance is the effect on food intake depression (5) .

The alteration in the

feeding behavior of animals ingesting amino acid imbalanced diets has been attributed directly or indirectly to the changes in the plasma amino acid pattern which has been observed consistently in animals fed such diets (4) .

The

food intake depression of rats fed amino acid imbalanced diets is rapid, occurring within 5-12 hours .

This fall in food intake has been shown to

be associated with a fall in the most limiting amino acid in the plasma of animals fed the unbalanced diets (4, 6, 7) . If some basic mechanism regulating food intake is affected by an amino acid imbalance, then the fall of the most limiting amino acid in the blood plasma could provide an adequate signal triggering the central food intake regulatory mechanism which might ultimately curtail the voluntary intake of the diet .

The fact that the depression in food intake in an amino acid

*This investigation was supported in part by Grant Number 3T4 from the Nutrition Foundations Inc . and by Grant number A?i-11066 from the National Institutes of Health .

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imbalance resembles that observed in animals fed an amino acid deficient (8, 9, 10) diet or one devoid (11, 12) completely of an indispensable amino acid which, when force-fed, causes the development of pathologic lesions and other abnormalities, leads to the belief that the inhibition of food intake of animals fed an amino acid imbalanced, deficient or devoid diet may all be the result o£ the same type of protective response involving homeostatic control within the animal body . The experiments presented here provide evidence that the food intake regulatory mechanism sensitive to the change in the concentration of the growth limiting amino acid in the blood plasma acts centrally and not peri pherally to influence the feeding behavior of the animals ingesting amino acid imbalanced diets .

The results show that the infusion of the growth

limiting amino acid into the carotid artery (CA) which leads directly to the brain alleviates the deleterious effect of an amino acid imbalance ; whereas, the same amount of the most limiting amino acid infused into the jugular vein (JV), which leads equally to all parts of the body, has no beneficial effects in food intake of animals ingesting the imbalanced diets . Materials and Methods Sprague-Dawley, albino, male rats (230-240 grams) were used in all experiments .

A polyethylene cannula was permanently implanted into the right carotid

artery or the jugular vein of each animal prior to the infusion experiment . The cannulas extended subcutaneously from the point of entry into the carotid artery or the jugular vein along the ventral midline up the side and emerged through an incision between the ears of the animals .

An elastic rubber band

was attached to the cannula six inches from the anchoring point of the incision end was fastened with slight tension to the top of a specially constructed cage 12" x 8" x 24" .

This technique was important to insure the free movement

of the animal inside the cage and at the same time to safe-guard the tampering of the cannula by the animal since the cannula stretched up or down as the animal stood up or lay down inside the cage .

The animals were fed ad libi tum

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and had free access to their water at all times . the following (in percent) :

3

The control diets contained

Corn oil (Mazola, Corn Products Co ., New York),

5 ; vitamin diet fortification mixture (Nutritional Bioche icals, Cleveland, Ohio), 1 ; salt mixture (13), 5 ; choline chloride, 0 .1 ; free amino acids as the protein source, both dispensable and indispensable,

(see Table 1),

and a sucrose-starch mixture (1 :2) as the cârbohydrate to make up 100% . The unbalanced diets were prepared by adding a mixture of indispensable amino acids lacking in the most limiting amino acid, threonine or isoleucine, to the respective appropriate control diets (see Table 1) .

The corrected diets

TABLE 1 Composition of Indispensable Amino Acids in Basal and Imbalanced Diets} Amino

Basal

Isoleu

Basal

Imbal

Acids

Thr Imbal

% of diet

% of diet

% of diet

% of diet

L- His " HC1 " H20

0 .22

0.62

0 .30

1.10

L-Ileu

0 .30

0.30

0.40

1 .40

L-Leu

0 .50

1 .20

0 .60

2.00

L- Lys " HC1

0 .60

1 .40

0 .80

2 .40

L-Met

0.25

0 .55

0 .30

0 .90

L-Cys

0 .20

0 .40

0 .20

0 .60

L-Phe

0.25

0 .75

o .45

1 .45

UTyr

0 .25

0 .55

0 .25

0 .85

L-Thr

0.20

0 .70

0 .20

0 .20

L-Trp

0 .08

0 .18

0 .10

0 .30

L-Val

o.45

0 .95

0 .50

1.50

Sodium Acetate

0 .35

0 .98

0 .47

1.73

Total

3.65

8.58

4 .57

14 .43

*The dispensable amino acid mixture used in all .the amino acid diets consists of (in % of diet) : L-arginine " HC1, 1 .0 ; L-asparagine, 1 .0 ; L-serine, 0 .35 ; Lproline, 1.0 ; glycine, 1.0 ; L-glutamic acid, 3 .0 ; L-alanine, 0 .35 ; and sodium acetate, 0.38.

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had the same composition as the imbalanced diets except that additional quantities of the limiting amino acid, threonine or isoleucine, was added in the amount of 0 .4% or 0 .5% of the diet, respectively . Results and Discussion The food intake of normal intact animals (5 rats per group) ingesting the threonine-imbalanced or the isoleucine-imbalanced diets ad libiti n n was reduced by 40-50% initially when compared to their respective controls (FIG . 1) .

The

food intake of rats fed the corrected diets in which the limiting amino acids were added back was not depressed .

The growth of the animals.i n each group

reflected the quantity of food ingested .

Rate adapted to the isoleucine-

imbalanced diet faster than they did to the threonine-imbalanced diet showing that the latter was somewhat more severe .

Q O W Y

Q bz O O Li-

. FIG Food intake of rats fed ad libitum the basal, threonine-imbalanced (- thr), corrected diet (+ thr , isoleucine-imbalaneed (- ileu) or corrected diet (+ ileu) .

FOOD INTAKE

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In the infusion experiment, before saline or the growth limiting amino acids were infused into either the carotid artery (CA) or the jugular vein (JV) (4 rats per group) during the experimental period, the rats were prefed the threonine-control diet for at least four days postoperatively and saline was infused (1 .3 - 1 .4 ml/day) .

The food intake of each animal was measured

and the average value of the last two days before switching to the imbalanced diet served as the baseline (expressed as 100%) for each rat .

The daily food

intake of each individual rat in each group after the second day of operation was relatively constant .

The average baseline control values for the CA and

the JV-cannulated groups (preoperative average weight, 236 g) infused with threonine were 11 .7 and 14 .3 g respectively, while those for the similarly cannulated animals (preoperative average weight, 242 g) infused with saline throughout the experimental period were 13 .7 and 15 .6 g respectively .

It

was noted that the CA-cannulated rats ate consistently about 10-20w less of the control diet than the JV-canrulated rats during the saline infused control period, probably due to the restriction of the blood flow to the brain . On the day of the feeding of the threonine-imbalanced diet after establishing the baseline, two groups of (CA- and JV-cannulated) animals continued to receive saline throughout the experimental period, while the other two groups of (CA- and JV-cannulated) rats were continuously infused with Lthreonine, (pH 7 .4 in saline, 1 .3 ml (4 mF)/24 hours) beginning 4 hours prior to the feeding of the iebalanced diet . The food intake of the CA- or JV-cannulated rats ingesting the threonine imbalanced diet and infused continuously with saline was initially reduced 40`n as compared to their own respective baseline control values .

Their food

intake remained low throughout the 4 day experimental period (FIG . 2) .

How-

ever, the food intake of the CA-cannulated rats ingesting the imbalanced diet and infused with threonine was not depressed ; while the JV-cannulated animals,

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receiving the same amount of threonine, exhibited the same initial reduction in food intake of 30-40% (FIG . 2) .

Adaptation to the threonine-imbalanced

diet which occurred in ad libitum fed intact animals after the initial depression in food intake (FIG . 1) occurred somewhat earlier in the JV-cannulated animals infused with threonine .

This can also be seen when the group is com-

pared to either the CA- or JV-cannulated animals, infused with saline and fed the imbalanced diet .

The more rapid adaptation in the JV-threonine infused

group is probably a result of a partial correction of the imbalance due to the threonine that was infused peripherally .

After feeding the threonine imbal-

anted diet for 4 days, both the CA- and JV-cannulated rats that had been infused with threonine or saline were switched to the isoleucine control diet for two days to obtain a new control food intake value while infusing saline . The average control values for the isoleucine infused CA- and JV-cannulated rate were 14 .0 and 15 .5 g respectively, while those for the saline infused CA- and JV-cannulated animals were 17 .5 and 19 .3 g respectively .

Again

the CA-cannulated rats ate 10-205 less of the isoleucine-control diet than the JV-cannulated animals in the saline infused control period .

All animals

were then fed the isoleucine-imbalanced diet for another 4 days and were continuously infused with either saline or isoleucine (pH 7 .4 in saline, 1 .3 ml (4 mg)/24 hours) .

The food intake of the CA- or JV-cannulated rats

fed the isoleucine imbalanced diet and infused with saline was again reduced initially by about 30-40% .

The food consumption of the saline infused animals

receiving the isoleucine imbalanced diet continued to be depressed for 3 days before gradual adaptation occurred .

The infusion of the limiting amino

acid (isoleucine) again prevented the food intake depression of the CAcannulated but not that of the JA-cannulated animals ingesting the isoleucineimbalanced diet (FIG . 2) .

Adaptation to the isoleucine imbalanced diet again

occurred earlier and somewhat more rapid in the JV-cannulated rats infused with the limiting amino acid as compared to the saline infused CA- or JVcannulated animals .

0n the fifth day of feeding of the imbalanced diet, the

Vol . 8, No. 2 180

FOOD INTAKE 180 ,

,

160 12. O 12

o

0 W

X

W Y Q FZ

o

o w

7

160 o

140

K Fz O O

120

140 120

4

o

100

1

80

w

Y Q é-z

60

O

40

o o

100 80 60 40

U-

20

20

0

1

2

3

4

0'

DAYS AFTER FEEDING THREONINE - IMBALANCED DIET

1

2

3

4

5

DAYS AFTER FEEDING ISOLEUCINE - IMBALANCED DIET FIG .

6

2

Food intake of rate fed the threonine-inbalanced diet or isoleucineimbalanced diet and infused with saline, threonine or isoleucine through the carotid artery (CA) or the jugular vein (JV) . o

o CA, Thr (4 mg/day)

e

e CA, Ileu

(4

o ------ o CA, Saline

9 ----- 9

e

A JV, Thr (4 mg/day)

A

A

A JV, saline

A ------ A JV, saline

mg/day)

CA, saline

A JV, Ileu (4 mg/day)

CA-cannulated animals were infused with saline instead of isoleucine for two days while the JV-cannulated animals received 10 times their usual amount of infused isoleucine (4o mg/day) . animals

The food intake of the CA-cannulated

was reduced to about that of the saline infused CA-cannulated group

while the food intake of the JV-cannulated animals was greatly stimulated as compared to the saline infused CA- or JV-cannulsted animals (FIG . 2) .

The

usual adaptation to the imbalanced diet was probably the cause for the less severe food intake depression of the CA-cannulated animals when saline was substituted for isoleucine after the 4th day feeding of the isoleucineimbalanced diet . Our studies clearly indicated that a small amount of the most limiting

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amino acid infused into blood that is directly nourishing the brain facilitated the voluntary food consumption of amino acid imbalanced diets . The same amount of the most limiting amino acid infused equally into all parts of the body did not alleviate the food intake depression caused by the amino acid imbalanced diets .

Higher amounts of the limiting amino

acid was needed to stimulate food intake of rats infused through the jugular vein .

This provides evidence that the level of the most limiting

amino acid in the blood plasma of rats ingesting the imbalanced diets influences a central, food intake regulatory mechanism and points to the possible existence of receptors in the central nervous system which are sensitive to the concentration of the most limiting amino acid in the blood plasma .

An elucidation of the mechanism whereby the signals are executed

and how they ultimaltely lead to a reduction in the intake of a poorly nutritious diet should lead to a better understanding of the influence of the nutrition of an animal on its behavior . Summary Food intake of rats fed diets containing a large amount of indispensable amino acids low in the growth limiting one was reduced 40-50% below that of the control in normal or cannulated rats infused with saline .

When the

growth limiting amino acid was infused, the food intake of the carotid artery-cannulated rats was not depressed, while the limiting amino acid infused into the jugular vein did not prevent the marked reduction in food intake of rats fed the imbalanced diets .

These results provide evidence

for a food intake regulatory function of some portion of the brain which is sensitive to the concentration of the growth limiting amino acid in blood. References 1.

J. Meyer, Physiol. Rev.

, 472 (1953) .

2.

G. C. Kennedy, Proc . Ray . Soc .

London Ser . B. 140, 578 (1953) .

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9

S . M . Mellinkoff, M. Frankland, D . Boyle, M . Greipel, Stanford Med . Bull . 13, 117 (1955) . 4.

A . E . Harper, P . Leung, A . Yoshida, and Q . R . Rogers .

Federation Proc .

23, 1087 (1964) . 5.

A . E . Harper, In , Mammalian Protein Metabolism . Allison Ed .

6.

H . ti . Munro and J . B .

(Academic Press, Inc ., New York, 1964) Vol . II, p . 87 .

U . S . Kumta and A . E . Harper .

Proc . Soc . Exptl . Biol . Med . 110, 512

(1962) . 7.

J . C . Sanahuja and A . E . Harper .

8.

E . G . Willcock and F . G . Hopkins .

9.

W . C . Rose .

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L . E . Frazier, R . W . Wissler, C . H . Steffee, R . L . Woolridge, and P . R . Cannon .

Amer . J . Physiol . 204, 686 (1963) . J . Physiol ., London

35,

88 (1906) .

Physiol . Rev . 18, 109, (1938) .

J . Nutr . ,333, 65, (1947) "

11 .

H . Sidransky and M . Rechcigl, Jr .

12 .

H . Sidransky and E . Verney .

13 .

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J . Nutr . 78, 269 (1964) .

Arch . Pathol . 76, 134 (1964) . J . Nutr . 87, 267 (1965) .