- Email: [email protected]
Integrated concentrations of growth hormone, insulin, C-peptide and prolactin in human obesity
Integrated
Concentrations of Growth Hormone, Insulin, C-Peptide and Prolactin in Human Obesity Mary T. Me&as,
Giraud V. Foster, Simeon
Margolis,
and A. Avinoam
Kowarski
Twenty-four hour integrated concentrations of growth hormone (IC-GH) were significantly lower in young, obese subjects than in young subjects who were lean. Significant inverse correlations were found between IC-GH and body mass index (BMI) as well as the IC-GH and the 24 hr integrated concentrations of insulin (IC-I) and C-peptide (IC-Cl in obese subjects below 30 yr of age. Since IC-GH decreases with age, the effect of obesity on IC-GH could not be demonstrated in the older subjects; a weak inverse correlation (p < 0.05) between IC-GH and IC-C was found. Prolactin was significantly lower in the older subjects but did not correlate with IC-GH and was similar in lean and obese. Lipid deposition in adipose cells is promoted by high concentrations of insulin as well as low concentrations of growth hormone. We found a significant correlation between the IC-l/IC-GH ratio and BMI of both the young and older subjects. Correlations between these two factors do not necessarily imply a cause and effect relationship. It is plausible, however, that the elevated IC-l/IC-GH of the obese may facilitate their lipid storage and counter their efforts at weight reduction.
0
BESITY is characterized by increased basal levels of insulin and enhanced insulin release in response to various stimuli.‘*2 The elevated basal concentrations of insulin in obese subjects correlate with their degree of overweight.’ Glucose, protein or leucine ingestion and intravenous glucose, glucagon or tolbutamide all provoke greater insulin secretion in obese subjects than lean individuals.* In contrast, growth hormone (GH) response is relatively depressed in obesity. Obese subjects exhibit subnormal GH release in response to a variety of stimuli, including exercise,’ sleep, and the administration of arginine, L-dopa, methoxamine, protein and estrogens.2 Moreover, in obese subjects the magnitude of their insulin response to glucose tolerance testing correlates inversely with their GH response to insulin-induced hypoglycemia.4 Because insulin promotes fat and carbohydrate storage5,6while GH stimulates lipolysis,‘,* the combination of high insulin and low GH may aggravate the obese condition and counter efforts at weight reduction. Ratios of insulin/GH are significantly higher in ob/ob rats than in their lean littermates.’ Insulin/GH ratios have not been similarly investigated in human obesity. From the Departments of Pediatrics, Medicine. Obstetrics and Gynecoiogy. The Johns Hopkins University School of Medicine, Baltimore MD, and the Department of Pediatrics, University of Maryland School of Medicine, Baltimore, MD. Received for publication June I7. 1982. Supported in part by Research Grants HD-36077 and Training Grant AM-07109 of the National Institutes of Health, United States Public Health Service. The patients were studied at the Johns Hopkins CIinicaf Research Center of Pediatrics which is supported by Gram RR-0052 from the General Chnical Research Centers Program of the Division of Research Resources. National Institutes of Health. Address reprint requesis to A. Avinoam Kowarski. M.D.. University of Maryland School of Medicine, Howard Hall Tower I O-047, Baltimore, MD 21201. 01982 by Grune & Stratton, Inc. 0026-0495/82/3112-0008$01.00/0 1224
A continuous blood withdrawal technique is particularly suited for studies on hormonal relationships.” The method produces hormone concentrations that have been integrated over intervals of time, thus eliminating sampling error resulting from high-frequency variations. Furthermore, continuous blood withdrawal does not interfere with the normal daily activity of the subject. The hormonal values are therefore more physiologically meaningful than either basal concentrations or responses to pharmacologic stimulation tests, which only measure the capacity to secrete a hormone in response to stimuli. Using continuous blood withdrawal, we have pooled plasma over brief time intervals to identify patterns of hormone secretion as well as integrated over a full 24 hr period in order to compare average hormone concentrations. In this manner, we examined the relationships of GH, insulin, C-peptide, prolactin and insulin/GH ratios in age-matched obese and lean individuals. MATERIALS
Experimental
AND
METHODS
Subjects
Fourteen males and 54 female volunteers, ages 15-58 yr, were studied. All were in good health, on no medications and experienced no recent changes in body weight. The height and weight of each subject was used to calculate body mass index (BMI), wt/h?. Twenty-three subjects with a BMI above 25.0 were considered obese” (range 25.0 to 60.0, mean +_ 1 SD: 33.6 r 7.5). The remaining 45 subjects, whose BMI ranged from 16.9 + 24.4 (mean 20.6 t 1.8)were classified as lean. Eighty-eight percent of the obese and 76% of the lean subjects were female. All subjects had normal fasting levels of plasma glucose.
Experimental
Protocol
Subjects were admitted to the Pediatric Clinical Research Center of the Johns Hopkins Hospital. A weight-maintaining diet was provided on the research unit. No differences were noted in the activity level, recorded at 30 min intervals throughout the day, between the lean and obese subjects. The day was spent primarily watching television, reading and ambulating within the premises of the research unit. Blood was withdrawn through a nonthrombogenic catheter using
Metabolism,
Vol. 3 1, No. 12 (December), 1982
.OW
GH AND
HIGH
INSULIN:GH
RATIO
IN OBESITY
1225
0600
Age 8hiL
No 4 252 yrs 203
I2QO
1800
2400
&JO
No 5 26 2 yr5 217
o(ioo
1200
NO3 19.6 yrs 403
1800
2400
25 5 yrs 348
401
2 3
20
20
;,
7
Fig. 1. Twenty-four hour integrated profiles of prolactin (IC-PRL) and growth hormone UC-GH) in age-matched lean INo. 1,4,5, and 7) and obese (No. 2,3.6. and 7) subjects. The blackened area denotes time of sleep. Subject No. 2 is male and the remaining subjects are female. The arrow (-_) indicates the time the 24 hr continuous blood withdrawal was initiated.
0
0 ,.-
(kQ._
‘IL_
wo0 0600 I200 1800 2400 NO 7 A9e
EMI
our technique of constant withdrawal.‘0 Samples were collected in tubes containing EDTA which were changed every 30 min. Plasma was separated and frozen at 3 hr intervals. At the end of the study, the 24 hr integrated pool was prepared by combining equal aliquots of plasma from each of the 30 min collections.
&I
0600
281 yrr 196
Laboratory
and Statistical
1200
f800
2400
NO 6 37 2 yra 349
Methods
GH, insulin, C-peptide and prolactin were measured described double antibody immunoassays.‘*-‘5 Pearson product-moment correlations are reported
by previously between
hor-
MEISTAS ET AL.
1226
mones and BMl
and the Student’s t test was used to compare lean
and obese, as well as the younger and older subjects.
Rank
sum testI was used to analyze the difference in
The Wilcoxon IC-I/IC-GH.
Table 1. Twenty-four C-peptide,
Growth
hr Integrated
Concentrations
of Insulin,
Hormone and Prolactin in Obese and Lean Subjects Statistical
RESULTS
Lean
Figure 1 illustrates the 24 hr profiles of GH and prolactin for 4 lean and 4 obese subjects, grouped according to age. GH levels were much lower in obese than in lean subjects. Both basal and pulsatile prolactin concentrations were markedly elevated in 1 of the the obese subjects (No. 6). In the remaining subjects, prolactin secretion peaks, occurring primarily between 0100 and 0800 hr, were similar in lean and obese subjects. The 24 hr integrated concentrations of insulin (ICI), C-peptide (IC-C), GH (IC-GH), prolactin (ICPRL) and IC-I/IC-GH are presented in Table 1. Because a significant decrease in GH secretion has been reported after the age of 30,” the results were calculated independently for subjects above and below 30 yr of age. In the younger group the IC-I, IC-C and IC-I/IC-GH of the obese subjects were significantly higher, and IC-GH was significantly lower, than the corresponding values of the lean. ICI, IC-C, IC-GH and IC-I/IC-GH values in the lean and obese subjects over 30 yr of age were not significantly different. IC-PRL was similar in obese and lean subjects within each age group, but IC-PRL was higher in the younger than older subjects (p < 0.0001). IC-GH was significantly lower, IC-I/IC-GH was significantly higher and IC-I and IC-C were not significantly different in the older when compared with the younger lean subjects. In the subjects below 30 yr of age, IC-GH correlated signficantly and inversely with ICI, IC-C and BMI (Table 2). In the subjects older than 30 yr, a significant correlation (p < 0.05) was found only between IC-GH and IC-C. BMI correlated with IC-I/IC-GH in both the younger (Fig. 2) and older groups of subjects (r = 0.56, p < 0.0001). DISCUSSION
Copinschi et al.” previously reported 24 hr integrated profiles of GH in lean and obese subjects. These authors found no significant differences in the mean IC-GH between lean and obese subjects. The discrepancy between their results and ours may be due to the
Table 2. Linear Regression
Analysis of 24 hr Integrated K-l
IC-GH, Under 30 yr ” = 45 IC-GH, Over 30 yr n = 23
Difference
Obese
Under 30 i 5.7’
22.9
+ 4.2
Age n
25.0
BMI (kg/m*)
20.4
35.2
+ 5.9
p < 0.0001
K-l I/.Wnl)
1.22 2 0.38
2.48
I
p < 0.0001
IC-C (ng/mlj
2.94
+ 0.63
3.69
c 0.93
IC-GH (ng/ml)
5.31
* 1.91
2.97
k 1.34
IC-I/GH
0.26
+ 0.11
1.30 * 1.26
IC-PRL Ins/ml)
20.4
+ 6.4
18.9 + 6.5
37
13
i 2.0
1.49
p < 0.001 p < 0.000 1 p < 0.0001 N.S.
Over 30 Age n
37.6
r 7.1
39.8
8
BMI (kg/m*)
21.7
IC-I tU/mlJ
1.08 -r 0.24
+ 2.1
ICC (ng/ml)
3.10
IC-GH (ng/ml)
t 7.4 10
30.8
+ 8.6
p = 0.01
1.44 t 0.64
N.S.
+ 0.40
3.4 + 1.10
N.S.
2.19
+ 1.12
2.2 * 1.0
N.S.
IC-l/GH
0.59
+ 0.25
IC-PRL lng/ml)
13.5 -+ 1.5
0.73
+ 0.45
N.S. N.S.
15.0 + 3.3
‘All reported values are mean + 1 SD.
fact that these investigators failed to take into account the effects of age. Since GH falls with advancing age, ‘7.‘9-2’in the present study we have analyzed the results of younger and older subjects separately. When 24 hr GH profiles were compared in obese and lean subjects in the younger age group, a significant difference was apparent. Because the IC-GH of the older subjects was not significantly affected by obesity, it is possible that the previously reported similarity of mean IC-GH between lean and obese subjects” was due to the greater number of older subjects included in their study. In contrast to our findings with GH, the 24 hr profile of prolactin levels were similar in obese and lean subjects. This result is consistent with previous studies which have demonstrated normal mean integrated,” basa122-26 and nocturnal” prolactin concentrations in obesity, as well as normal prolactin responses to TRH22.23,28 in obese subjects. Other workers have reported subnormal prolactin responses to insulininduced hypoglycemia24~28 arginine” and TRHz4 in obesity. However, the abnormal prolactin secretory response to these latter stimuli did not influence the secretory pattern of the hormone during our 24 hr period of observation.
Concentrations K-C
of GH with Insulin. C-peptide,
Prolactin and BMI BMI
IC-PRL
r = -0.470
r = -0.461
r = 0.020
r - -0.496
p < 0.01
p < 0.01
N.S.
p < 0.01
r = -0.330
r = -0.483
r = 0.246
r-
N.S.
p < 0.05
N.S.
N.S.
-0.039
LOW GH AND HIGH INSULIN:GH RATIO IN OBESITY
1227
. .
0
100
20.0
300
40.0
Body Mass Index (kg/m? Fig. 2.
Correlation
of body mass index with the 24 hr integrated
Our demonstration of a normal 24 hr secretory pattern of prolactin in obese subjects, along with previous reports that basal and post-stimulation levels of TSH,=.%o pulsatile gonadotropin release and LH and FSH responses to LHRH are all normal in obesiGH secretion in ty, 3’m33suggest that the diminished obesity is not due to a generalized suppression of hypothalamic-pituitary function. A loss of the normal nocturnal rise in GH concentrations’* and a diminished GH response to insulininduced hypoglycemia34 have been reported in obese subjects. Furthermore, normal weight subjects who became obese by force-feeding developed an impaired GH release with euglycemic recovery following oral glucose administration.3s Indeed, an inverse relationship between GH concentrations and degree of obesity was also evident in our young, obese subjects. Because insulin has somatomedin-like properties,36 it is possible that the high plasma insulin concentrations accompanying obesity could directly suppress GH secretion. Alternately, since C-peptide reflects insulin secretory activity,37 the inverse correlations found between JC-C and IC-GH in our subjects could be interpreted as evidence that the secretory rates of insulin and GH may be governed by the same factors. Plasma free fatty acids are elevated in obese subjects38*39 and could function in this capacity.40 Another hormone, metabolite or unidentified factor may also serve as an intermediary in this regard. In contrast with our younger subjects, the obese and lean subjects over 30 yr of age had comparable IC-GH. This may be partly ascribable to the (insignificantly)
ratio of insulin/growth
hormone in subjects less than 30 yr of age.
lower mean BMI in the older compared with the younger subjects. It may be also due, in part, to a reduced discrimination of the GH immunoassay at low plasma concentrations. Thus, differences in the ICGH of the older subjects may remain undetected because of the limitations of the present methodology. A resistance to the biologic activity of insulin has been described in hyperinsulinemic obese subjects.41 43 However, this resistance may selectively affect glucose (rather than lipid) membrane transport, metabolism and storage. Thus, high insulin and low GH concentrations could still function synergistically in promoting further fat storage in the younger obese subjects. The weak correlation found between BMI and ICI/IC-GH in the subjects over 30 may merely reflect our previously reported association of BMI with 1C-I.44 Because GH was appropriate for age in obese subjects over 30 yr of age, it is unlikely that this hormone alone is responsible for their excess lipid storage. It may, however, play a permissive role in the presence of high insulin concentrations in this regard. ACKNOWLEDGMENT The authors would assistance and Barbara tion of the manuscript.
like to thank Beverly S. Mace for secretarial
Smith for technical help in the prepara-
REFERENCES
1. Bagdade J: Basal insulin and obesity. Lancet 11:63&631, 1968 2. Glass AR, Burman KD, Dahms WT, et al: Endocrine function in human obesity. Metabolism 30:89-104, 1981 3. Bogardus C. LaGrange BM, Horton ES. et al: Comparison of carbohydrate-containing and carbohydrate restricted hypocaloric
122% diets in the treatment of obesity: endurance and metabolic fuel homeostasis during strenuous exercise. J Clin Invest 68:399-404, 1981 4. Komorowski JM, Pawlikowski M: Relationship between insulin and somatotropin in obesity. Endokrinologie 73:209-213, 1979 5. Fain JN, Rosenberg L: Antilipolytic action of insulin on fat cells. Diabetes 21(Suppl2):414425, 1972 6. Rodbell M: Metabolism of isolated fat cells. Effects of hormones on glucose metabolism and lipolysis. J Biol Chem 239:375380.1964 7. Felig P, Marliss EL, Cahill, Jr GF: Metabolic response to human growth hormone during prolonged starvation. J Clin Invest 50:411-421,1971 8. Henneman DH, Henneman PH: Effects of human growth hormone on levels of blood and urinary carbohydrate and fat metabolites in man. J Clin Invest 39: 1239-l 245, 1960 9. Mendel VE: Influence of the insulin-to-growth hormone ratio on body composition of mice. Am J Physiol238:E231-E234, 1980 10. Kowarski AA, Thompson RG, Migeon CJ, et al: Determinations of integrated plasma concentrations and true secretion rates of growth hormone. J Clin Endocrinol Metab 32:356-360, 1971 11. Bray GA: Definition, measurement and classification of the syndromes of obesity. Int J Obesity 2:99, 1978 12. Schalch DS, Parker ML: A sensitive double antibody immunoassay for human growth hormone in plasma. Nature 203:11411142,1964 13. Morgan CR, Lazarow A: Immunoassay of insulin: two antibody system. Diabetes 12:l I S-126, 1963 14. Kaneko T, Oka H, Munemura M, et al: Radioimmunoassay of human proinsulin and C-peptide using synthetic human connecting peptide. Endocrinol Jpn 21:141-145, 1974 15. Hwang P, Guyda H, Freisen H: Radioimmunoassay for human prolactin. Proc Nat1 Acad Sci 68:1902-1906, 1971 16. Colton T: Nonparametric methods, in Colton T (ed): Statistics in Medicine, Boston, Little, Brown & Co, 1974, pp 221-222 17. Rudman D, Kutner MH, Rogers CM, et al: Impaired growth hormone secretion in the adult population. Relation to age and adiposity. J Clin Invest 67:1361-1369, 1981 18. Copinschi G, De Laet MH, Brion JP: Simultaneous study of cortisol, nyctohemeral variations in normal and obese subjects: Influence of prolonged fasting in obesity. Clin Endocrinol (Oxf) 9: 15-26, 1978 19. Finkelstein JW, Roffwarf HP, Boyar RM, et al: Age-related change in the twenty-four hour spontaneous secretion of growth hormone. J Clin Endocrinol Metab 35:665-670, 1972 20. Carbon HE, Gillin JC, Gorden P, et al: Absence of sleeprelated growth hormone peaks in aged normal subjects and in acromegaly. J Clin Endocrinol Metab 34:1102-l 105, 1972 21. Bazaarre TL, Johanson AJ, Huseman CA, et al: Human GH changes with age, in Growth Hormone and Related Peptides. Proceedings of the Third International Symposium. Excerpta Med Int Cong Ser 381:261-270, 1976 22. Carbon HE, Drenick EJ, Chopra IJ: Alterations in basal and TRH stimulated serum levels of thyrotropin. prolactin, and thyroid hormones in starved obese men. J Clin Endocrinol Metab 45:707713,1977 23. Wilcox RG: Triiodothyronine. TSH and prolactin in obese women. Lancet 1:1027-1029.1977 24. Kopelman PG, White N, Pilkington TRE, et al: Impaired
MEISTAS ET AL.
hypothalamic control of prolactin secretion in massive obesity. Lancet 1:747-750.1979 25. Drenick EJ, Carlson HE, Robertson GL, et al: The role of vasopressin and prolactin in abnormal salt and water metabolism of obese patients before and after fasting and during refeeding. Metabolism 26:309-317, 1977 26. Harrower ADB, Yap PL, Nairm IM: Growth hormone, insulin and prolactin secretion in anorexia nervosa and obesity during bromocriptine treatment. Br Med J 2:156-159, 1977 27. Kalucy RS, Crisp AH, Chard T: Nocturnal hormonal profiles in massive obesity, anorexia nervosa, and normal females. J Psychosom Res 20595-604, 1976. 28. Cavagnini F, Marachini C, Pinto M, et al: Prolactin secretion in obese patients. Lancet 2: 1020, 1979 29. Azizi F: The effect of dietary composition on fasting induced changes in serum thyroid hormones and thyrotropin. Metabolism 27:935-942,1978 30. Burman KD, Harvey GS, O’Brien JT: Glucose modulation of alterations in peripheral thyroid hormone metabolism induced by fasting. Metabolism 28:291-299, 1979 31. Kopelman PG, Pilkington TRE, White N, et al: Abnormal sex steroid secretion and binding in massively obese women. Clin Endocrinol (Oxf) 12:363-369, 1980 32. Glass AR, Dahms WT, Abraham G, et al: Secondary amenorrhea in obesity: Etiologic role of weight-related androgen excess. Fertil Steril 30:243-244, 1978 33. Newmark SR, Rossini A, Naftolin FI, et al: Gonadotropin profiles in fed and fasted obese women. Am J Obstet Gynecol 133:75-80,1979 34. Beck P, Koumans JHT, Winterling CA: Studies of insulin and growth hormone secretion in human obesity. J Lab Clin Med 64:654-667,1964 35. Sims EA, Goldman RF, Gluck GM, et al: Experimental obesity in man. Trans Assoc Am Physicians 81:153-170, 1968 36. Phillips LS, Vassilopoulou-Sellin R: Somatomedins. N Engl J Med 302:371-380,198O 37. Meistas MT, Rendell M, Margolis S, et al: Estimation of the secretion rate of insulin from the urinary excretion rate of c-peptide: Study in obese and diabetic subjects. Diabetes 31:449-453, 1982 38. Kreisberg RA, Boshell BR, DiPlacido J, et al: Insulin secretion in obesity. N Engl J Med 276:3 14-3 19, 1967 39. Rabinowitz D, Zierler KL: Forearm metabolism in obesity and its reponse to intra-arterial insulin. Characterization of insulin resistance and evidence for adaptive hyperinsulinism. J Clin Invest 41:2173-2181.1962 40. Gomez F, Jequier E, Chaliot V, et al: Carbohydrate and lipid oxidation in normal human subjects: its influence on glucose tolerance and insulin response to glucose. Metabolism 21:381-391. 1972 41. Rabinowitz, D: Some endocrine and metabolic aspects of obesity. Ann Rev Med 21:241-258, 1970 42. Olefsky JM, Kolterman OG: Mechanisms of insulin resistance in obesity and non-insulin dependent (type II) diabetes. Am J Med 70:151-168, 1981 43. Kolterman OG, Insel J, Saekow M, et al: Mechanisms of insulin resistance in human obesity. Evidence for receptor and postreceptor defects. J Clin Invest 65:1272-1284, 1980 44. Meistas M, Margolis M, Kowarski AA: Hyperinsulinemia of obesity is due to decreased clearance of insulin. Am J Phys (in press)
Concentrations of Growth Hormone, Insulin, C-Peptide and Prolactin in Human Obesity Mary T. Me&as,
Giraud V. Foster, Simeon
Margolis,
and A. Avinoam
Kowarski
Twenty-four hour integrated concentrations of growth hormone (IC-GH) were significantly lower in young, obese subjects than in young subjects who were lean. Significant inverse correlations were found between IC-GH and body mass index (BMI) as well as the IC-GH and the 24 hr integrated concentrations of insulin (IC-I) and C-peptide (IC-Cl in obese subjects below 30 yr of age. Since IC-GH decreases with age, the effect of obesity on IC-GH could not be demonstrated in the older subjects; a weak inverse correlation (p < 0.05) between IC-GH and IC-C was found. Prolactin was significantly lower in the older subjects but did not correlate with IC-GH and was similar in lean and obese. Lipid deposition in adipose cells is promoted by high concentrations of insulin as well as low concentrations of growth hormone. We found a significant correlation between the IC-l/IC-GH ratio and BMI of both the young and older subjects. Correlations between these two factors do not necessarily imply a cause and effect relationship. It is plausible, however, that the elevated IC-l/IC-GH of the obese may facilitate their lipid storage and counter their efforts at weight reduction.
0
BESITY is characterized by increased basal levels of insulin and enhanced insulin release in response to various stimuli.‘*2 The elevated basal concentrations of insulin in obese subjects correlate with their degree of overweight.’ Glucose, protein or leucine ingestion and intravenous glucose, glucagon or tolbutamide all provoke greater insulin secretion in obese subjects than lean individuals.* In contrast, growth hormone (GH) response is relatively depressed in obesity. Obese subjects exhibit subnormal GH release in response to a variety of stimuli, including exercise,’ sleep, and the administration of arginine, L-dopa, methoxamine, protein and estrogens.2 Moreover, in obese subjects the magnitude of their insulin response to glucose tolerance testing correlates inversely with their GH response to insulin-induced hypoglycemia.4 Because insulin promotes fat and carbohydrate storage5,6while GH stimulates lipolysis,‘,* the combination of high insulin and low GH may aggravate the obese condition and counter efforts at weight reduction. Ratios of insulin/GH are significantly higher in ob/ob rats than in their lean littermates.’ Insulin/GH ratios have not been similarly investigated in human obesity. From the Departments of Pediatrics, Medicine. Obstetrics and Gynecoiogy. The Johns Hopkins University School of Medicine, Baltimore MD, and the Department of Pediatrics, University of Maryland School of Medicine, Baltimore, MD. Received for publication June I7. 1982. Supported in part by Research Grants HD-36077 and Training Grant AM-07109 of the National Institutes of Health, United States Public Health Service. The patients were studied at the Johns Hopkins CIinicaf Research Center of Pediatrics which is supported by Gram RR-0052 from the General Chnical Research Centers Program of the Division of Research Resources. National Institutes of Health. Address reprint requesis to A. Avinoam Kowarski. M.D.. University of Maryland School of Medicine, Howard Hall Tower I O-047, Baltimore, MD 21201. 01982 by Grune & Stratton, Inc. 0026-0495/82/3112-0008$01.00/0 1224
A continuous blood withdrawal technique is particularly suited for studies on hormonal relationships.” The method produces hormone concentrations that have been integrated over intervals of time, thus eliminating sampling error resulting from high-frequency variations. Furthermore, continuous blood withdrawal does not interfere with the normal daily activity of the subject. The hormonal values are therefore more physiologically meaningful than either basal concentrations or responses to pharmacologic stimulation tests, which only measure the capacity to secrete a hormone in response to stimuli. Using continuous blood withdrawal, we have pooled plasma over brief time intervals to identify patterns of hormone secretion as well as integrated over a full 24 hr period in order to compare average hormone concentrations. In this manner, we examined the relationships of GH, insulin, C-peptide, prolactin and insulin/GH ratios in age-matched obese and lean individuals. MATERIALS
Experimental
AND
METHODS
Subjects
Fourteen males and 54 female volunteers, ages 15-58 yr, were studied. All were in good health, on no medications and experienced no recent changes in body weight. The height and weight of each subject was used to calculate body mass index (BMI), wt/h?. Twenty-three subjects with a BMI above 25.0 were considered obese” (range 25.0 to 60.0, mean +_ 1 SD: 33.6 r 7.5). The remaining 45 subjects, whose BMI ranged from 16.9 + 24.4 (mean 20.6 t 1.8)were classified as lean. Eighty-eight percent of the obese and 76% of the lean subjects were female. All subjects had normal fasting levels of plasma glucose.
Experimental
Protocol
Subjects were admitted to the Pediatric Clinical Research Center of the Johns Hopkins Hospital. A weight-maintaining diet was provided on the research unit. No differences were noted in the activity level, recorded at 30 min intervals throughout the day, between the lean and obese subjects. The day was spent primarily watching television, reading and ambulating within the premises of the research unit. Blood was withdrawn through a nonthrombogenic catheter using
Metabolism,
Vol. 3 1, No. 12 (December), 1982
.OW
GH AND
HIGH
INSULIN:GH
RATIO
IN OBESITY
1225
0600
Age 8hiL
No 4 252 yrs 203
I2QO
1800
2400
&JO
No 5 26 2 yr5 217
o(ioo
1200
NO3 19.6 yrs 403
1800
2400
25 5 yrs 348
401
2 3
20
20
;,
7
Fig. 1. Twenty-four hour integrated profiles of prolactin (IC-PRL) and growth hormone UC-GH) in age-matched lean INo. 1,4,5, and 7) and obese (No. 2,3.6. and 7) subjects. The blackened area denotes time of sleep. Subject No. 2 is male and the remaining subjects are female. The arrow (-_) indicates the time the 24 hr continuous blood withdrawal was initiated.
0
0 ,.-
(kQ._
‘IL_
wo0 0600 I200 1800 2400 NO 7 A9e
EMI
our technique of constant withdrawal.‘0 Samples were collected in tubes containing EDTA which were changed every 30 min. Plasma was separated and frozen at 3 hr intervals. At the end of the study, the 24 hr integrated pool was prepared by combining equal aliquots of plasma from each of the 30 min collections.
&I
0600
281 yrr 196
Laboratory
and Statistical
1200
f800
2400
NO 6 37 2 yra 349
Methods
GH, insulin, C-peptide and prolactin were measured described double antibody immunoassays.‘*-‘5 Pearson product-moment correlations are reported
by previously between
hor-
MEISTAS ET AL.
1226
mones and BMl
and the Student’s t test was used to compare lean
and obese, as well as the younger and older subjects.
Rank
sum testI was used to analyze the difference in
The Wilcoxon IC-I/IC-GH.
Table 1. Twenty-four C-peptide,
Growth
hr Integrated
Concentrations
of Insulin,
Hormone and Prolactin in Obese and Lean Subjects Statistical
RESULTS
Lean
Figure 1 illustrates the 24 hr profiles of GH and prolactin for 4 lean and 4 obese subjects, grouped according to age. GH levels were much lower in obese than in lean subjects. Both basal and pulsatile prolactin concentrations were markedly elevated in 1 of the the obese subjects (No. 6). In the remaining subjects, prolactin secretion peaks, occurring primarily between 0100 and 0800 hr, were similar in lean and obese subjects. The 24 hr integrated concentrations of insulin (ICI), C-peptide (IC-C), GH (IC-GH), prolactin (ICPRL) and IC-I/IC-GH are presented in Table 1. Because a significant decrease in GH secretion has been reported after the age of 30,” the results were calculated independently for subjects above and below 30 yr of age. In the younger group the IC-I, IC-C and IC-I/IC-GH of the obese subjects were significantly higher, and IC-GH was significantly lower, than the corresponding values of the lean. ICI, IC-C, IC-GH and IC-I/IC-GH values in the lean and obese subjects over 30 yr of age were not significantly different. IC-PRL was similar in obese and lean subjects within each age group, but IC-PRL was higher in the younger than older subjects (p < 0.0001). IC-GH was significantly lower, IC-I/IC-GH was significantly higher and IC-I and IC-C were not significantly different in the older when compared with the younger lean subjects. In the subjects below 30 yr of age, IC-GH correlated signficantly and inversely with ICI, IC-C and BMI (Table 2). In the subjects older than 30 yr, a significant correlation (p < 0.05) was found only between IC-GH and IC-C. BMI correlated with IC-I/IC-GH in both the younger (Fig. 2) and older groups of subjects (r = 0.56, p < 0.0001). DISCUSSION
Copinschi et al.” previously reported 24 hr integrated profiles of GH in lean and obese subjects. These authors found no significant differences in the mean IC-GH between lean and obese subjects. The discrepancy between their results and ours may be due to the
Table 2. Linear Regression
Analysis of 24 hr Integrated K-l
IC-GH, Under 30 yr ” = 45 IC-GH, Over 30 yr n = 23
Difference
Obese
Under 30 i 5.7’
22.9
+ 4.2
Age n
25.0
BMI (kg/m*)
20.4
35.2
+ 5.9
p < 0.0001
K-l I/.Wnl)
1.22 2 0.38
2.48
I
p < 0.0001
IC-C (ng/mlj
2.94
+ 0.63
3.69
c 0.93
IC-GH (ng/ml)
5.31
* 1.91
2.97
k 1.34
IC-I/GH
0.26
+ 0.11
1.30 * 1.26
IC-PRL Ins/ml)
20.4
+ 6.4
18.9 + 6.5
37
13
i 2.0
1.49
p < 0.001 p < 0.000 1 p < 0.0001 N.S.
Over 30 Age n
37.6
r 7.1
39.8
8
BMI (kg/m*)
21.7
IC-I tU/mlJ
1.08 -r 0.24
+ 2.1
ICC (ng/ml)
3.10
IC-GH (ng/ml)
t 7.4 10
30.8
+ 8.6
p = 0.01
1.44 t 0.64
N.S.
+ 0.40
3.4 + 1.10
N.S.
2.19
+ 1.12
2.2 * 1.0
N.S.
IC-l/GH
0.59
+ 0.25
IC-PRL lng/ml)
13.5 -+ 1.5
0.73
+ 0.45
N.S. N.S.
15.0 + 3.3
‘All reported values are mean + 1 SD.
fact that these investigators failed to take into account the effects of age. Since GH falls with advancing age, ‘7.‘9-2’in the present study we have analyzed the results of younger and older subjects separately. When 24 hr GH profiles were compared in obese and lean subjects in the younger age group, a significant difference was apparent. Because the IC-GH of the older subjects was not significantly affected by obesity, it is possible that the previously reported similarity of mean IC-GH between lean and obese subjects” was due to the greater number of older subjects included in their study. In contrast to our findings with GH, the 24 hr profile of prolactin levels were similar in obese and lean subjects. This result is consistent with previous studies which have demonstrated normal mean integrated,” basa122-26 and nocturnal” prolactin concentrations in obesity, as well as normal prolactin responses to TRH22.23,28 in obese subjects. Other workers have reported subnormal prolactin responses to insulininduced hypoglycemia24~28 arginine” and TRHz4 in obesity. However, the abnormal prolactin secretory response to these latter stimuli did not influence the secretory pattern of the hormone during our 24 hr period of observation.
Concentrations K-C
of GH with Insulin. C-peptide,
Prolactin and BMI BMI
IC-PRL
r = -0.470
r = -0.461
r = 0.020
r - -0.496
p < 0.01
p < 0.01
N.S.
p < 0.01
r = -0.330
r = -0.483
r = 0.246
r-
N.S.
p < 0.05
N.S.
N.S.
-0.039
LOW GH AND HIGH INSULIN:GH RATIO IN OBESITY
1227
. .
0
100
20.0
300
40.0
Body Mass Index (kg/m? Fig. 2.
Correlation
of body mass index with the 24 hr integrated
Our demonstration of a normal 24 hr secretory pattern of prolactin in obese subjects, along with previous reports that basal and post-stimulation levels of TSH,=.%o pulsatile gonadotropin release and LH and FSH responses to LHRH are all normal in obesiGH secretion in ty, 3’m33suggest that the diminished obesity is not due to a generalized suppression of hypothalamic-pituitary function. A loss of the normal nocturnal rise in GH concentrations’* and a diminished GH response to insulininduced hypoglycemia34 have been reported in obese subjects. Furthermore, normal weight subjects who became obese by force-feeding developed an impaired GH release with euglycemic recovery following oral glucose administration.3s Indeed, an inverse relationship between GH concentrations and degree of obesity was also evident in our young, obese subjects. Because insulin has somatomedin-like properties,36 it is possible that the high plasma insulin concentrations accompanying obesity could directly suppress GH secretion. Alternately, since C-peptide reflects insulin secretory activity,37 the inverse correlations found between JC-C and IC-GH in our subjects could be interpreted as evidence that the secretory rates of insulin and GH may be governed by the same factors. Plasma free fatty acids are elevated in obese subjects38*39 and could function in this capacity.40 Another hormone, metabolite or unidentified factor may also serve as an intermediary in this regard. In contrast with our younger subjects, the obese and lean subjects over 30 yr of age had comparable IC-GH. This may be partly ascribable to the (insignificantly)
ratio of insulin/growth
hormone in subjects less than 30 yr of age.
lower mean BMI in the older compared with the younger subjects. It may be also due, in part, to a reduced discrimination of the GH immunoassay at low plasma concentrations. Thus, differences in the ICGH of the older subjects may remain undetected because of the limitations of the present methodology. A resistance to the biologic activity of insulin has been described in hyperinsulinemic obese subjects.41 43 However, this resistance may selectively affect glucose (rather than lipid) membrane transport, metabolism and storage. Thus, high insulin and low GH concentrations could still function synergistically in promoting further fat storage in the younger obese subjects. The weak correlation found between BMI and ICI/IC-GH in the subjects over 30 may merely reflect our previously reported association of BMI with 1C-I.44 Because GH was appropriate for age in obese subjects over 30 yr of age, it is unlikely that this hormone alone is responsible for their excess lipid storage. It may, however, play a permissive role in the presence of high insulin concentrations in this regard. ACKNOWLEDGMENT The authors would assistance and Barbara tion of the manuscript.
like to thank Beverly S. Mace for secretarial
Smith for technical help in the prepara-
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