Leptin levels in human and rodent: measurement of plasma leptin and ob RNA in obese and weight-reduced subjects

Plasma insulin rise precedes rise in ob mRNA expression and plasma leptin in gold thioglucose-obese mice

Circulating leptin levels are strongly related to the degree of adiposity, with hyperleptinemia being associated with hyperinsulinemia. In the gold thioglucose-injected mouse (GTG), hyperinsulinemia is an early abnormality in the development of insulin resistance and obesity. In this study, hyperinsulinemia occurred 1 wk post-GTG [GTG, 199 +/- 43; age-matched controls (CON), 53 +/- 5 microU/ml; P < 0.001], with leptin levels not rising until 2 wk post-GTG (CON, 3.2 +/- 0.3; GTG, 9.9 +/- 1.7 ng/ml; P < 0.001) in parallel with increases in the size of different fat pads and increased expression of ob mRNA. The ratio of serum leptin to fat pad weight was significantly higher in GTG mice 12 wk postinjection. Starvation-induced reductions in serum leptin (50%), glucose (50%), and insulin (74%) were greater than decreases in fat pad weight (18%). Adrenalectomy decreased both adiposity and serum leptin within 1 wk in both CON and GTG and altered the serum leptin level-to-fat pad weight ratio in CON. Thus hyperinsulinemia preceded increased ob expression and hyperleptinemia, which occurred in parallel with increasing adiposity, consistent with the role of leptin as an indicator of energy supplies. Changes in hormonal and nutritional status may modify this relationship.

Gene-environment interactions in hypertension

Environmental factors such as stress, diet, and physical activity have long been recognized as playing an important role in the pathogenesis of essential hypertension. Individuals may vary in their response to these factors depending on differences in genes determining physiologic systems that mediate the response. In this article we discuss gene-environment interactions that contribute to the development of essential hypertension (environmental susceptibility to hypertension) and those that are involved in control of the disease (pharmacogenetics).

Neuroendocrine regulation of food intake

Maintenance of appropriate stores of metabolic fuels depends on carefully matching caloric intake to caloric expenditure. Achieving such 'energy balance' is a product of complex interactions of peripheral hormones with effector systems in the central nervous system (CNS) that regulate food intake and energy expenditure. Leptin is a hormone that is made in the adipocytes, circulates in the blood and interacts with receptors in the CNS. These receptors can be found in two different types of systems. One effector system is termed 'anabolic' and is activated by low levels of leptin during negative energy balance. This system (exemplified by the hypothalamic neuropeptide Y system) increases food intake and decreases energy expenditure to facilitate the regaining of lost energy stores. The other effector system is termed 'catabolic' and is activated by high levels of leptin during positive energy balance. This system (exemplified by the hypothalamic melanocortin and corticotrophin-releasing hormone systems) decreases food intake and increases energy expenditure to facilitate the loss of excess energy stores. Further understanding of these systems is necessary to develop adequate treatments for disorders of energy balance, such as obesity and wasting.

Pathogenesis of non-insulin-dependent (type II) diabetes mellitus (NIDDM) - genetic predisposition and metabolic abnormalities

Non-insulin-dependent diabetes mellitus (NIDDM), also known as type II diabetes, is characterized by abnormal glucose homeostasis, resulting in hyperglycemia, and is associated with microvascular, macrovascular, and neuropathic complications. NIDDM is a complex disease with many causes. Both genetic and environmental factors play important roles in the pathogenesis of NIDDM. Cumulative evidence on the high prevalence of NIDDM in certain ethnic groups, the high concordance rate for the disease in monozygotic twins, familial aggregation, and familial transmission patterns suggests that the genetic component plays an important etiological role in the development of NIDDM. In genetically predisposed individuals, there is a slow progression from a normal state to hyperglycemia, largely due to a combination of insulin resistance and defects in insulin secretion. Although numerous candidate genes responsible for insulin resistance and for the defects in insulin secretion have been reported, no specific gene(s) accounting for the majority of cases of the common type of NIDDM has been identified. Considerable evidence indicates that environmental and other factors, including diet, stress, physical activity, obesity and aging, also play an important role in the development of the disease. In conclusion, the pathogenic process of NIDDM depends on a complex interaction between genetic and environmental factors.

Interacting appetite-regulating pathways in the hypothalamic regulation of body weight

Various aspects of the complex spatio-temporal patterning of hypothalamic signaling that leads to the development of synchronized nocturnal feeding in the rat are critically examined. Undoubtedly, as depicted in Fig. 7, a distinct ARN in the hypothalamus is involved in the control of nocturnal appetite. At least four basic elements operate within this ARN. These are: 1) A discrete appetite-driving or orexigenic network of NPY, NE, GABA, GAL, EOP, and orexin transduces and releases appetite-stimulating signals. 2) Similarly, anorexigenic signal-producing pathways (e.g., CRH, GLP-1, alpha MSH, and CART) orchestrate neural events for dissipation of appetite and to terminate feeding, possibly by interrupting NPY efflux and action at a postsynaptic level within the hypothalamus. It is possible that some of these may represent the physiologically relevant "off" switches under the influence of GABA alone, or AgrP alone, or in combination with NPY released from the NPY-, GABA-, and AgrP-coproducing neurons. 3) Recent evidence shows that neural elements in the VMN-DMN complex tonically restrain the orexigenic signals during the intermeal interval; the restraint is greatly aided by leptin's action via diminution of orexigenic (NPY) and augmentation of anorexigenic (GLP-1, alpha MSH, and CART) signals. Since interruption of neurotransmission in the VMN resulted in hyperphagia and development of leptin resistance, it seems likely that the VMN is an effector site for the restraint exercised by leptin. The daily rhythms in leptin synthesis and release are temporally dissociable because the onset of daily rise in leptin gene expression in adipocytes precedes that in leptin secretion. Nevertheless, these rhythms are in phase with daily ingestive behavior because the peak in circulating leptin levels occurs during the middle of the feeding period. These observations, coupled with the fact that circulating levels of leptin are directly related to adiposity, pose a new challenge for elucidating the precise role of leptin in daily patterning of feeding in the rat. 4) A neural timing mechanism also operates upstream from the ARN in the daily management of energy homeostasis. Although the precise anatomical boundaries are not clearly defined, this device is likely to be composed of a group of neurons that integrate incoming internal and external information for the timely onset of the drive to eat. Evidently, this network operates independently in primates, but it is entrained to the circadian time keeper in the SCN of rodents. Apart from its role in the onset of drive to eat, the circadian patterns of gene expression of NPY, GAL, and POMC denote independent control of the timing device on the synthesis and availability for release of orexigenic signals. The VMN-DMN-PVN complex is apparently an integrated constituent of the timing mechanism in this context, because lesions in each of these sites result in loss of regulated feeding. The accumulated evidence points to the PVN and surrounding neural sites within this framework as the primary sites of release and action of various orexigenic and anorexigenic signals. A novel finding is the identification of the interconnected wiring of the DMN-mPVN axis that may mediate leptin restraint on NPY-induced feeding. The chemical phenotypes of leptin and NPY target neurons in this axis remain to be identified. These multiple orexigenic and anorexigenic pathways in the hypothalamic ARN appear to represent redundancy, a characteristic of regulated biological systems to provide a "fail-safe" neural mechanism to meet an organism's constant energy needs for growth and maintenance. Within this formulation, the coexisting orexigenic signals (NPY, NE, GAL, GABA, and AgrP) represent either another level of redundancy or it is possible that these signals operate within the ARN as reinforcing agents to varying degrees under different circumstances. (ABSTRACT TRUNCATED)

Association of the serum leptin concentration with weight loss in chronic hemodialysis patients

Circulating leptin, which is partly cleared by the kidney, has been reported to increase with chronic renal failure and thus may play a role in the weight loss of patients with chronic renal failure. We investigated the association of body weight loss with the serum leptin concentration in Japanese hemodialysis patients. The relationship between serum leptin and the body mass index (BMI) or body fat mass was compared among 181 patients undergoing hemodialysis and 185 control subjects. There was no difference in the serum leptin concentration between the hemodialysis patients (HD) and controls (C) for either the men (3.9 +/- 0.2 ng/mL for HD, n=117; 3.9 +/- 0.3 ng/mL for C, n=89; NS) or women (8.9 +/- 1.2 ng/mL for HD, n=64; 7.4 +/- 0.5 ng/mL for C, n=96; NS), whereas BMI of the hemodialysis patients was significantly lower than that of the controls for both the men (20.1 +/- 0.2 kg/m2 for HD, 22.4 +/- 0.3 kg/m2 for C, P < 0.001) and women (19.2 +/- 0.3 kg/m2 for HD, 22.0 +/- 0.4 kg/m2 for C, P < 0.001). The serum leptin/body fat mass ratio was significantly correlated with the weight change of the patients during a follow-up evaluation period of 17 months (r = -0.37, P < 0.05 for men, n=27 and r = -0.53, P < 0.005 for women, n=28), indicating the possibility that a relatively high level of serum leptin had induced weight loss in the hemodialysis patients. The serum leptin/body fat mass ratio also showed a significant inverse correlation with the duration of hemodialysis (r = -0.31, P < 0.05 for men and r = -0.49, P < 0.05 for women). A multiple regression analysis indicated that the body fat mass was significantly correlated with serum leptin concentration, whereas the fat distribution did not have any relationship with leptin. These data indicate that a high level of serum leptin relative to the body fat mass might be associated with weight loss in long-term hemodialysis patients. The serum leptin level relative to the body fat mass also seems to have been affected by the duration of hemodialysis.

Serum leptin levels in women throughout pregnancy and the postpartum period and in women suffering spontaneous abortion

OBJECTIVE

In pregnancy, important changes occur in the body weight of the mother, caused by sodium and water retention and by an increase in body fat tissue, but the mechanisms that regulate maternal and foetal changes in fat mass are poorly understood. Leptin is a hormone produced by adipocytes in order to regulate food intake and energy expenditure at the hypothalamic level in man. In order to verify whether leptin participates in the changes in body composition during pregnancy and postpartum, 630 healthy women were studied at specific time periods and leptin and auxological parameters were determined.

DESIGN

A cross-sectional study in which leptin levels were measured in women at specific time periods related to pregnancy. Each woman was assessed only once.

PATIENTS

630 women participated in the study, and were divided into categories as follows: Group A, 29 internal controls, with no previous or current pregnancy; Group B, 73 women in the first trimester of pregnancy; Group C, 60 women in the 24 h before delivery; Group D, 212 women in the 24 h postpartum; Group E, 93 women in the eightH postpartum week (2 months group); Group F, 71 women in the sixteenth postpartum week (4 months group); Group G, 20 women in the sixth month postpartum; Group H, 23 women one year postpartum; Group I, 20 women two years postpartum; finally Group J, of 29 women who had suffered spontaneous abortion in the first trimester of pregnancy and were studied in the 24 h after the stillborn delivery.

MEASUREMENTS

Serum leptin levels were measured in duplicate by radioimmunoassay using commercial kits. Height and weight was measured and BMI (kg/m2) calculated.

RESULTS

Compared with serum leptin in the control group (11.7 +/- 1.0 micrograms/l), a non significant (NS) increase was observed in the first trimester of pregnancy (14.3 +/- 1.4 micrograms/l), with no parallel changes in body weight. A reduction in leptin occurred in the 24 h after delivery (9.4 +/- 1.4 micrograms/l, P = 0.02). After delivery a progressive increase in leptin concentrations was observed, 13.3 +/- 1.5 micrograms/l at two months (NS) and 17.4 +/- 2.6 micrograms/l at four months (P = 0.035 vs controls). Afterwards leptin values decreased towards normal values at 6, 12 and 24 months after delivery 14.4 +/- 1.8 micrograms/l; 12.9 +/- 1.6 micrograms/l; and 10.1 +/- 1.1 micrograms/l respectively (all NS). With the exception of the postpartum group, a significant correlation was observed between leptin concentrations and body weight or BMI in each group of women studied. In the women who suffered spontaneous abortion in the first trimester of pregnancy a reduction in leptin levels occurred (8.8 +/- 1.0 micrograms/l, P = 0.001 vs first trimester group).

CONCLUSION

Serum leptin concentrations rose slightly during pregnancy, fell following delivery and subsequently increased during the first six months postpartum. These variations were unrelated to changes in body composition, and may be responsible for the postpartum weight gain observed in some women. Abnormally low serum leptin levels were observed in women suffering spontaneous abortion in the first trimester of pregnancy.

Plasma leptin and insulin in C57BI/6J mice on a high-fat diet: relation to subsequent changes in body weight

It has been proposed that leptin and insulin through central effects are involved in the regulation of energy balance and body weight. Whether circulating leptin or insulin levels predict subsequent changes in body weight is, however, not known. We examined plasma leptin and insulin at 2, 3, 6, 9 and 12 months of age in C57BI/6J mice given a normal diet (n = 12) or a high-fat diet (58% fat on a caloric base; n = 15). Plasma leptin levels increased by age and correlated with body weight in the entire material (r = 0.81, P < 0.001). Also plasma insulin increased by high-fat diet and correlated across all age periods with body weight (r = 0.56, P < 0.001). In mice, given normal diet, plasma leptin or insulin did not correlate to subsequent changes in body weight at any of the time points studied. However, in mice given the high-fat diet, plasma leptin at 6 (r = -0.57, P = 0.027) and 9 months of age (r = -0.56, P = 0.042) as well as plasma insulin at 6 (r = - 0.51, P = 0.049) and 9 months (r = -0.58, P = 0.037) correlated inversely to the change in body weight during the subsequent 3-month period. Hence, both leptin and insulin are negative predictors for future weight gain in high-fat fed mice. This suggests that when the regulation of body weight is challenged by a high-fat diet, leptin and insulin act to restrain or prevent future weight gain. This in turn may suggest that impairment of these (probably central) actions of leptin and insulin might underlie excessive increase in body weight under such conditions.

Effect of diet on the response to leptin in the marsupial Sminthopsis crassicaudata

The aim of this study was to determine in the marsupial Sminthopsis crassicaudata 1) the effect of leptin on food intake, body fat stores, and metabolism and 2) whether leptin can prevent a diet-induced increase in adiposity. In response to 21 days of feeding with mealworms (2.99 kcal/g, 30% fat), body weight (P < 0. 0001) and tail width (P < 0.0001) increased, compared with control animals fed with laboratory diet (1.01 kcal/g, 20% fat). Subsequently, S. crassicaudata were randomly allocated to receive either laboratory diet or a choice between laboratory diet and mealworms. For 13 days, one-half of the animals in each dietary group received intraperitoneal human leptin (2.5 mg/kg twice daily), while the other one-half received phosphate-buffered saline. In animals receiving laboratory diet alone, leptin induced a decrease in body weight (P < 0.0001), tail width (P < 0.0001), and energy intake (P < 0.01). In animals receiving both laboratory diet and mealworms, leptin had no effect on body weight or tail width, although the proportion of laboratory diet eaten was reduced (P = 0. 0001), and there was a nonsignificant fall in overall energy intake (P = 0.07). We conclude that in S. crassicaudata, 1) a high-calorie, higher-fat diet induces an increase in adiposity and 2) leptin induces weight loss, but 3) an increase in dietary calories and fat content is associated with resistance to the actions of leptin.

Physiological perspectives on leptin as a regulator of reproduction: role in timing puberty

How nutrition regulates reproductive activity remains a major unsolved question of reproductive biology. Reducing the level of nutrition during adulthood can lead to infertility, primarily through reduction of GnRH secretion. Inquiry about such a mechanism has its roots in the search for cues timing the onset of fertility, because the tempo of sexual maturation is much more closely associated with body growth than with chronological age. Growth depends on the quantity and quality of food intake. When food availability is low, small, short-lived species with high metabolism and reduced growth may not even attain puberty before they die. In longer-lived species, puberty is delayed for months or even years until more food becomes available. To appreciate fully how the pubertal progression is timed will require understanding how peripheral signals relating information about energy metabolism are sensed by the brain and how such information is routed through pathways controlling GnRH secretion. Here, we provide some background and physiologic perspective on the question of whether the fat-derived hormone leptin is the unique peripheral signal, is an important signal, is but one of a constellation of signals, or is not a signal timing puberty.

Leptin's actions on the reproductive axis: perspectives and mechanisms

Energy availability influences reproductive fitness. The activity of the reproductive axis is sensitive to the adequacy of nutrition and the stores of metabolic reserves. The adipocyte-derived hormone leptin is postulated to reflect the state of nutrition and energy reserves and serve as a metabolic gate to the reproductive system. Genetically obese ob/ob mice (lacking endogenous leptin) are infertile, and treatment of these animals with exogenous leptin stimulates the activity of the reproductive endocrine system and induces fertility in both sexes. Severely food-restricted animals have reduced circulating levels of leptin, which are associated with markedly reduced secretion of the gonadotropins, LH, and FSH. Treatment of food-restricted mice, rats, sheep, and monkeys with exogenous leptin reverses the diet-induced inhibition of gonadotropin secretion. Leptin has also been suggested to have a role in timing the onset of puberty in several species, although evidence that leptin is the primary metabolic signal for initiating the onset of puberty in any species is controversial. Notwithstanding this debate, it is undisputed for all species studied to date that adequate levels of leptin in the circulation are essential (but not sufficient) for pubertal progression and that leptin treatment can reverse the delay in sexual maturation caused by food restriction. Double-label in situ hybridization studies in the brain of the mouse, rat, and monkey have revealed that hypothalamic neurons expressing proopiomelanocortin and neuropeptide Y coexpress the leptin receptor, whereas no evidence has been adduced that GnRH neurons express this receptor. Together, these observations suggest that leptin is a metabolic signal to the neuroendocrine reproductive system and that under conditions of inadequate energy reserves, low leptin levels act as a metabolic "gate" to inhibit the activity of the neuroendocrine reproductive axis in both sexes.

Changes in leptin concentration during the early postnatal period: adjustment to extrauterine life?

There are substantial alterations in fuel homeostasis immediately after birth. Leptin is a putative regulator of energy metabolism. Consequently, the aim of this study was to examine whether there are changes in circulating leptin concentrations during the early postnatal period. Umbilical cord mixed blood samples were taken at delivery, and a venous blood sample was obtained at 3 d of age from 38 healthy newborn infants (20 male, 18 female; gestational age 36.3 to 41.9 wk) for analysis of leptin concentration with radioimmunoassay. Cord plasma leptin concentration was 9.7+/-5.2 microg/L (mean+/-SD), with no gender difference between male (8.6+/-4.6 microg/L) and female (10.9+/-5.6 microg/L) infants. In male newborns, cord plasma leptin concentration correlated with arm circumference (r = 0.48, p < 0.05), and in female newborns with body mass index (r = 0.62, p < 0.01), thickness of the s.c. fat (r = 0.54, p < 0.05), and arm circumference (r = 0.72, p < 0.01). By the third postnatal day, plasma leptin decreased similarly in male (to 1.8+/-0.4 microg/L, p < 0.001) and female (to 2.3+/-0.8 microg/L, p < 0.001) infants, when there was a significant gender difference in leptin levels (p = 0.01). At 3 d of age, plasma leptin correlated with weight (r = 0.49, p < 0.05) and arm circumference (r = 0.49, p < 0.05) in female but not in male newborns. In conclusion, 1) circulating leptin already correlates with adiposity at birth in female but not in male newborn infants and 2) leptin decreases markedly in both genders by the third postnatal day, and the gender difference with higher leptin levels in females develops by that time. Thus, the postnatal decrease in plasma leptin concentration may be a physiologically feasible adaptation to profound alterations in fuel homeostasis during the first days of extrauterine life.

Cord blood leptin concentrations in relation to intrauterine growth

OBJECTIVE

Leptin, a hormone that signals the amount of energy stores to the brain, has recently been shown to play a role in the regulation of several hypothalamic pituitary axes, including the growth hormone axis. To investigate a potential association between cord blood leptin concentrations and intrauterine growth we measured leptin concentrations in the cord blood of small for gestational age (SGA), appropriate for gestational age (AGA) and large for gestational age (LGA) healthy newborns.

PATIENTS AND MEASUREMENTS

Cord blood leptin concentrations were evaluated in 25 SGA, 100 AGA, and 45 LGA, neonates.

RESULTS

Leptin was detectable in all newborns in concentrations comparable with those found in adults. Moreover, SGA newborns had lower leptin concentrations (3.70 +/- 1.81 micrograms/l) than AGA (5.65 +/- 4.98 micrograms/l) and LGA newborns (11.99 +/- 7.06 micrograms/ l)(P < 0.01). Cord blood leptin concentrations were significantly associated with ponderal index, cord blood insulin concentrations, placental weight and maternal serum leptin concentrations. Importantly, the association between cord blood leptin concentrations and intrauterine growth status persisted after adjusting for adiposity, placental weight, maternal serum leptin concentrations and cord blood insulin concentrations.

CONCLUSIONS

Cord blood leptin concentrations are independently associated with intrauterine growth. Future studies are needed to elucidate the underlying mechanism and clarify the role of leptin in regulating growth and controlling appetite in newborns.

Serum leptin and weight gain over 8 years in African American and Caucasian young adults

OBJECTIVE

There is considerable interest in how to prevent weight gain in adulthood. Leptin, a peptide hormone expressed in adipose tissue, is believed to signal the central nervous system about the level of body fat stores, and thereby may control appetite. Little information exists on whether the serum leptin concentration influences long-term weight changes in the free-living population.

RESEARCH METHODS AND PROCEDURES

From an ongoing cohort study of young African American and white adults, we selected a sample of participants (n=492), stratified on sex, race, and weight changes over 8 years. Serum leptin was measured on stored specimens using a radioimmunoassay. Weight change was modeled in relation to baseline leptin concentrations.

RESULTS

Cross-sectionally, leptin concentration was associated positively with body mass index, negatively with physical activity level, and was higher in women than men. These variables explained 72% of the variance in serum leptin. Over the 8 years, the sample gained an average of 7.8 kg (standard deviation = 10.8). There was no evidence that 8-year weight change was associated with initial leptin concentration: 8-year weight change was only 0.5 kg less (95% confidence interval =-1.8 to 0.8, p = 0.47) per each 10 ng/ mL increment (approximately one standard deviation) of baseline leptin. In contrast, leptin change correlated highly (r=0.62) with weight change.

DISCUSSION

Our data corroborate evidence that adiposity determines leptin levels but do not support the hypothesis that leptin deficiency plays an important role in obesity in the general population.

Lowering of circulating insulin and leptin is closely associated following weight reduction after vertical banded gastroplasty in obese women

AIM

To explore whether the lowering of insulin relates to changes in leptin following weight reduction after vertical banded gastroplasty (VBG) in seven grossly obese women (BMI 45.9 +/- 1.5 kg/m2).

RESULTS

At 6-24 months after VBG, BMI was reduced to 31.3 +/- 0.9 kg/m2 (p < 0.018), leptin from 43 +/- 9 to 12 +/- 2 ng/ml (p < 0.018) and insulin from 172 +/- 19 to 71 +/- 2 pmol/l (p=0.018). The reduction in leptin correlated to the reduction in insulin (r = 0.94, p = 0.002) but not to that of BMI (r = 0.05, NS).

CONCLUSION

Reduction in insulin seems more important for reduction in leptin than lowering of BMI, suggesting a close association between leptin and insulin in obese subjects.

Serum leptin levels and their association with several factors related to arteriosclerosis among medical students in Japan

The association between serum leptin levels and several factors related to arteriosclerosis were studied in subjects who were Japanese medical students taking no medications. The group was comprised of 75 males and 35 females. The age distribution in males was 21.4 to 29.8 years (median age, 24.0 years) and in females was 21.3 to 29.9 years (median age, 22.9 years). Statistical analyses were performed using the Wilcoxon rank sum test and Spearman correlation. Median levels of serum leptin were 3.3 ng/ml in males and 7.6 ng/ml in females. The largest correlation observed was widi percent body fat irrespective of sex [males; r=0.775, p<0.001, females; r=0.553, p<0.001]. However, body mass index (BMI) was similarly well correlated with serum leptin [r=0.631, p<0.001] in males but not in females [r=0.305, p=0.075]. A negative correlation was observed between high density lipoprotein cholesterol (HDL-C) and leptin in both sexes [males; r=-0.298, p<0.01, females; r=-0.363, p<0.05] .respectively. Percent body fat, BMI, diastolic blood pressure, apolipoprotein B, and HDL-C were the most significant factors in males. Percentage of body fat and HDL-C were the most important factors in females.

Metabolic responses to intracerebroventricular leptin and restricted feeding

Leptin is a hormone secreted by adipocytes, which plays an important role in the control of food intake and metabolic processes. In the current study, a dose-dependent relationship was shown between a bolus intracerebroventricular rat recombinant leptin administration and reductions in food intake and body weight in Sprague-Dawley rats. During the 24 h postinjection period, food intake was decreased by 24, 26, and 52% with 0.625, 2.5, and 10 microg of leptin, respectively. Body weight was reduced by 2, 3, and 5% at 24 h after leptin administration at the doses of 0.156, 2.5, and 10 microg, respectively. Furthermore, indirect calorimetry demonstrated that five daily i.c.v. injections of leptin resulted in an increase in heat production per unit of metabolic body size and fat oxidation by approximately 10 and 48%, respectively. In contrast, food-restricted rats that consumed the equivalent amount of food as leptin-treated rats for 5 days decreased their energy expenditure by 10%. Food restriction was found to decrease respiratory quotient in a similar pattern as the leptin administration. When ad lib feeding was resumed, food-restricted rats quickly recovered their normal food intakes, body weights, and metabolism. Conversely, leptin treatment has prolonged effects on body weight resulting from different metabolic responses than food restriction. Leptin not only suppresses food intake, but also enhances energy expenditure to reduce fat depots.

Plasma leptin concentrations in lean and obese human subjects and Prader-Willi syndrome: comparison of RIA and ELISA methods

Immunoassays for circulating leptin are important research tools for examining the role and regulation of leptin expression in human obesity. However, uncertainty exists regarding the comparability between studies of reported plasma or serum leptin concentrations. The purpose of the present study was to directly compare plasma leptin concentrations by using two of the most widely reported immunoassay methods-namely, a commercially available radioimmunoassay (RIA) and a proprietary enzyme-linked immunosorbent assay (ELISA). Plasma leptin concentrations were measured in healthy lean and obese volunteers and in patients with Prader-Willi syndrome (PWS). Over a wide range of plasma concentrations (2 to 70 ng/mL), leptin measurements obtained with the RIA and ELISA methods were highly correlated (r = 0.957, P<.0001) and were essentially indistinguishable. Leptin levels measured by RIA and ELISA were highly correlated with body mass index (BMI) overall (r = 0.784, P<.0001 and r = 0.732, P<.0001, respectively) and in the lean and obese subgroups. When compared with the results in the lean individuals (mean +/- SEM, 11.6+/-3.2 ng/mL), plasma leptin was significantly higher in both the obese (35.5+/-4.0 ng/mL, P<.0001) and the PWS subjects (30.7+/-6.9 ng/mL, P<.05). However, after we controlled for differences in BMI, the leptin levels were similar in all three groups. In conclusion, we found that the RIA and ELISA used in the present study yield plasma leptin concentrations that are essentially indistinguishable. Our findings should facilitate comparisons of leptin levels measured by these two widely used immunoassays in previous and future studies that examine the role of leptin in body weight regulation.

Acute effect of leptin on hepatic glycogenolysis and gluconeogenesis in perfused rat liver

Leptin circulates in blood and is involved in body weight control primarily via hypothalamic receptors. To examine its direct metabolic action, effects of short-term portal leptin infusion: 1) on postprandial basal and epinephrine-stimulated glycogenolysis; and 2) on postabsorptive lactate-stimulated gluconeogenesis were studied in isolated perfused rat livers. Incremental epinephrine (150 pmol x min-1 x g-1 liver)-stimulated glucose release (in micromol/g liver within 30 minutes; control: 28.3 +/- 2.8) was suppressed (P <.05) by 44% (15.8 +/- 1.6), by 48% (14.6 +/- 4.1), and by 53% (13.3 +/- 2.1) during insulin (3 pmol x min-1 x g-1 liver), leptin (30 pmol x min-1 x g-1 liver), and simultaneous leptin + insulin infusion. Perfusate cyclic adenosine monophosphate increased approximately twofold during epinephrine stimulation in all groups. Neither leptin nor insulin affected hepatic lactate production, bile flow, or portal pressure in the fed state. In the postabsorptive state (20-hour fasting), rates of lactate (10 mmol/L)-dependent hepatic glucose release (in micromol. min-1 x g-1 liver; control: 0.12 +/- 0.01) were increased (P <.01) to 0.35 +/- 0.02 and to 0.24 +/- 0.01 by glucagon (3 pmol x min-1 x g-1 liver) and by leptin (15 pmol x min-1 x g-1 liver), respectively. In parallel, lactate uptake rates (in micromol x min-1 x g-1 liver) were higher in the presence of both glucagon (0.90 +/- 0. 03) and leptin (0.84 +/- 0.02) compared with control (0.68 +/- 0.04). In conclusion, leptin modulates hepatic glucose fluxes and may contribute to direct humoral regulation of liver glycogen stores in the fasted as well as in the fed state.

Heritability of plasma leptin levels: a twin study

OBJECTIVE

To examine the influence of genetic factors on plasma leptin levels.

SUBJECTS AND METHODS

We measured plasma leptin levels, body mass index and body fat distribution in healthy young female monozygotic (n = 19) and dizygotic (n = 14) twins. The twin zygosity was verified by determination of short tandem repeat and amplified fragment length polymorphism systems. The genetic analysis included analysis of variance-based and maximum likelihood-based methods.

RESULTS

Plasma leptin levels were correlated significantly with body mass index (r = 0.59, P < 0.001), waist circumference (r = 0.54, P < 0.001) and hip circumference (r = 0.63, P < 0.001), but not with age (r = -0.17) or the waist:hip ratio (r = 0.02). The heritability estimates derived from intraclass correlations were significant for body mass index (P = 0.001), waist circumference (P = 0.004), hip circumference (P = 0.01) and plasma leptin levels (P = 0.005), but not for the waist:hip ratio (P = 0.22). In the maximum likelihood-based path analysis, heritability was estimated at 79% for body mass index and at 73% for plasma leptin levels. After adjustment for body mass index, the heritability estimate for leptin levels from the model-fitting approach was 55%.

CONCLUSIONS

Genetic factors are major determinants of plasma leptin levels in humans and may account for as much as half of the variance in leptin levels.

Effects of food restriction on glucose tolerance, insulin secretion, and islet-cell proliferation in pregnant rats

Pregnancy is associated with increased glucose-stimulated insulin secretion and increased pancreatic islet-cell proliferation. In the present study it was investigated whether increased food intake, as occurs during pregnancy, is involved in the regulation of these phenomena. From Day 0 of pregnancy, rats received each day the mean amount of food they consumed daily during the estrous cycle prior to conception. This food restriction regime resulted in lower maternal body weight, and in lower fetal weight on Day 20 of gestation, but did not affect fetal survival. Food-restricted rats showed decreased insulin responses to an i.v. glucose challenge on Day 13, and lower islet-cell replication rates on Day 14 of pregnancy than pregnant rats fed ad lib. Plasma lactogenic activity in food-restricted animals was increased on Days 11 and 13; plasma progesterone levels were unchanged, but plasma leptin concentrations declined progressively during food restriction. Glucose tolerance was normal, suggesting that food restriction improved insulin action. On Day 20 of pregnancy, insulin responses were similar in food restricted and ad lib-fed rats; glucose tolerance was still unchanged. It thus seems that the improved insulin action as present on Day 13 had disappeared on Day 20. Also on Day 20, lactogenic activity as well as progesterone concentrations were similar in food-restricted and ad lib-fed rats. It was concluded that increased food intake plays an important role in the stimulation of islet-cell proliferation and insulin secretion, as well as in the diminished insulin action during the second week of rat pregnancy.

Serum leptin levels in males with delayed puberty during short-term pulsatile GnRH administration

Leptin may be a possible trigger for puberty. In normal males, it has been shown that leptin increases from the pre-pubertal to the early pubertal stage, and then declines in the late pubertal stage. We examined leptin levels in six male adolescents (mean age 16.3+/-0.6 yr; range 14.2-17.6 yr) with delayed puberty (constitutional delay of puberty no.=2; idiopathic hypogonadotropic hypogonadism no.=4) during 120 days of subcutaneous pulsatile GnRH administration. A group of subjects in pre-puberty (no.=11), early-puberty (n=10) and mid-puberty (no.=7) were evaluated as controls. Morning blood samples were taken for determination of leptin, testosterone, LH and FSH levels. In delayed puberty subjects blood samples were taken every 30 days after the start of GnRH administration. At each examination BMI and testicular volume were evaluated. A follow-up examination was performed in the 6 patients 1.3-7.5 yr after the end of the 120 days of GnRH therapy. At baseline evaluation in delayed puberty mean leptin levels were 11.3+/-2.0 microg/l (median 11.3 microg/l; range 4.7-17.3 microg/l) and were higher than those found in pre-puberty (p=0.04) and mid-puberty (p=0.001). During GnRH administration there was no change in BMI and leptin levels but there was an increase in gonadotrophin levels, testosterone and testicular volume. One hundred and twenty days after, mean serum leptin were 10.1+/-2.1 microg/l (median 9.1 microg/l; range 3.4-16.8 microg/l). At the end of the study, leptin levels were higher in delayed puberty than in mid-puberty (p=0.002). At the follow-up examination leptin levels were 4.3+/-1.3 microg/l (median 3.4 microg/l; range 1.4-9.1 microg/l) (p=0.03 vs end of 120 days GnRH therapy) while testosterone and BMI were not changed. In conclusion 120-day pulsatile GnRH administration induced in males with delayed puberty physiological-like pubertal changes but not the decline in leptin levels reported during the progression of puberty. Therefore, in males with delayed puberty an impairment in the phenomenon of leptin decline associated with progression of puberty could be suggested. However after retrospective diagnosis of pubertal delay and long-term therapy in subjects with idiopathic hypogonadotropic hypogonadism leptin levels declined. These data seem to indicate that time more than increase in testosterone levels and testicular volume is the determinant of leptin decline at puberty.

Therapeutic controversy: Obesity--a modern-day epidemic

While the hyperleptinemia of obesity is likely to be associated with the metabolic complications of obesity/hyperinsulinemia/insulin resistance, it is not associated with diabetes, with the relative hypercortisolism of upper body obesity, with hypertension in women, (it is in men), or with dyslipidemia. Overall, the correlations between leptin and the metabolic diseases associated with obesity are weak. The equivocal results of an association of leptin with components of the metabolic syndrome make it unlikely that leptin affects these directly. (On the other hand, these correlations, when found, preclude any causal relationship between leptin and metabolic diseases.) There are experimental data showing a definite role for insulin and glucocorticoids in the regulation of leptin, and of leptin in the regulation of insulin. More data are required on the effects of leptin, but it is likely that leptin will not be a major link between obesity and the metabolic syndrome. Certainly, however, when leptin is available for clinical use, its effect on different aspects of the metabolic syndrome will be worth studying.

Serum leptin and longevity

There are changes in the ability to regulate energy balance and caloric intake with aging. Consequently, we investigated whether human aging modifies the levels of serum leptin, a novel hormone implicated in the regulation of energy balance. We studied 268 Caucasian men and women aged 22-85 years, and divided them into groups with mean ages of 30, 40, 50, 65, 75, 80 and 85 years. Fasting serum leptin concentrations were determined by radioimmunoassay. Subjects aged 65 or older were followed for five years after the blood sampling. There were no statistically significant differences in fasting serum leptin concentrations across different age groups in females (p = 0.090). Fasting serum leptin concentrations were also similar in different age groups in males, except that males in the 30-year age group had lower serum leptin levels than males in the 75-year age group (p = 0.042). Leptin levels were 2-3 fold higher in females than in males in each age group (p < 0.005 except p = 0.063 in the 75-year age group). Elderly women, who lived longer, had 47% higher (p < 0.05) serum leptin concentrations, and 17% higher (p < 0.001) BMI than the women who died within five years of blood sampling. Leptin levels were not statistically different in these women after adjusting for BMI. Thus, aging has no apparent effect on serum leptin levels in women or men, and the gender differences in leptin is present also in the older age groups. Higher leptin concentrations in the females who live longer may reflect a better nutritional status, and a greater adipose tissue mass rather than a primary factor for survival.

Renal effects of leptin in normotensive, hypertensive, and obese rats

The hemodynamic, hormonal, and renal excretory effects of intravenous bolus administration of synthetic murine leptin were examined in groups of anesthetized normotensive (Sprague-Dawley), hypertensive (spontaneously hypertensive), and both lean and obese Zucker rats. In the normotensive animals (n = 8) an intravenous bolus of 400 microgram/kg of leptin produced a significant six- to sevenfold elevation in sodium excretion compared with controls (n = 8). The onset of natriuresis was delayed for approximately 30-45 min. Mean arterial pressure (MAP), creatinine clearance, plasma renin activity (PRA), and plasma aldosterone concentration (PAC) remained unchanged. In contrast, the hypertensive rats were refractory to the natriuretic effects of leptin when infused either with 400 (n = 8) or 1,600 (n = 8) microgram/kg. Also in these animals MAP, creatinine clearance, PRA, and PAC were unmodified. Finally, whereas lean Zucker rats (n = 8) responded very similarly to the Sprague-Dawley animals, the natriuretic effect of the hormone was attenuated in the obese Zucker groups. At 400 microgram/kg (n = 8) no natriuresis was elicited, but at 1,600 microgram/kg (n = 8) a modest but significant two- to threefold increment in sodium excretion was observed in the obese rats. In both Zucker groups, MAP, creatinine clearance, PRA, and PAC were unchanged. Collectively, these results demonstrate a significant natriuretic effect of exogenous leptin in the normal rat and a blunted saluretic response in hypertension and obesity. It is suggested that leptin may be a potential salt-excretory factor in normal rats and may function pathophysiologically in obesity and hypertension.

JTT-501, a novel oral antidiabetic agent, improves insulin resistance in genetic and non-genetic insulin-resistant models

1. We investigated whether JTT-501 (4-[4-[2-(5-methyl-2-phenyl-4-oxazolyl)ethoxy]benzyl]-3,5-isoxa zolidinedione) would improve insulin resistance in genetic (Zucker fatty rats) and non-genetic (high-fat fed rats) rodent models of obesity. 2. JTT-501 (10-100 mg kg(-1) day(-1)) was administered orally to Zucker fatty rats for 7-21 days. In the high-fat fed rat model, JTT-501 (100 mg kg(-1) day(-1) was administered orally for 7 days. In both models, JTT-501 improved metabolic abnormalities by enhancing insulin action during the glucose tolerance test and the euglycaemic-hyperinsulinaemic clamp study. In ex vivo assays, JTT-501 ameliorated the impaired insulin-sensitive glucose oxidation and lipid synthesis in peripheral tissues. Furthermore, JTT-501 enhanced insulin receptor autophosphorylation in hindlimb muscle. 3. JTT-501 reduced serum leptin concentrations in both models, but did not affect body weight or epididymal fat weight. 4. Our observations indicate that JTT-501 improves the metabolic abnormalities in both genetic and non-genetic insulin-resistant models by enhancing insulin action in peripheral tissues. These effects of JTT-501 are due, at least in part, to enhanced insulin receptor autophosphorylation. In addition, JTT-501 is able to reduce serum leptin concentrations in hyperleptinaemia of the insulin-resistant model. We expect JTT-501 to show promise for treating non-insulin dependent diabetes mellitus patients with insulin resistance.

The obese gene product, leptin: possible role in obesity-related hypertension in adolescents

OBJECTIVE

The mechanism of the association between obesity and hypertension is not clear. The recently discovered obese gene product, leptin, the levels of which increase in obese subjects, has been shown to reduce food intake and increase sympathetic nervous system activity in animal studies. The present study was undertaken to elucidate the relationship between blood pressure and factors related to obesity, including leptin, in different age groups.

METHODS

The subjects were 348 Japanese male adolescents (15-17 years old) and 165 men (40-59 years old) not taking medication for hypertension, diabetes mellitus or hyperlipidaemia. Height, weight, blood pressure, heart rate, plasma glucose, lipid profiles, serum insulin and leptin levels were measured in the morning after an overnight fast

RESULTS

Body mass index (BMI), serum leptin level and the homeostasis model insulin resistance index increased in the order of blood-pressure category (i.e. normotensive < high normal < hypertensive) in both the male adolescents and the middle-aged men. In addition, simple linear regressions revealed that both systolic and diastolic blood pressure correlated significantly with serum leptin and the insulin resistance index in both groups. Even after adjustment for age and BMI, the correlation of mean blood pressure with leptin remained in the obese adolescents (r2 = 0.390, P = 0.02). The heart rate also correlated with leptin in the adolescents (r = 0.18, P< 0.001), but not in the middle-aged subjects (r = 0.04). Even after adjustment for age and BMI in adolescents, serum leptin correlated significantly with heart rate.

CONCLUSION

These results suggest a role for leptin in obesity-related hypertension, especially in adolescents.

Plasma leptin turnover rates in lean and obese Zucker rats

Conscious female adult lean and obese Zucker rats were injected through the jugular vein with radioactive iodine-labeled murine leptin; in the ensuing 8 min, four blood samples were sequentially extracted from the carotid artery. The samples were used in a modified RIA for leptin, in which paired tubes received the same amount of either labeled or unlabeled leptin, thus allowing us to estimate both leptin levels and specific radioactivity. The data were used to determine the decay curve parameters from which the half-life of leptin (5.46 +/- 0.23 min for lean rats and 6.99 +/- 0.75 min for obese rats) as well as the size of its circulating pool (32 pmol/kg for lean rats and 267 pmol/kg for obese rats) and the overall degradation rate (96 fkat/kg for lean rats and 645 fkat/kg for obese rats) were estimated. These values are consistent with the hormonal role of leptin and the need for speedy changes in its levels in response to metabolic challenge.

Arterial compliance, blood pressure, plasma leptin, and plasma lipids in women are improved with weight reduction equally with a meat-based diet and a plant-based diet

Obesity, strongly associated with the risk for coronary heart disease (CHD), is becoming increasingly prevalent. This study was designed to establish first whether systemic arterial compliance (SAC), an index of arterial function, is improved with weight loss and second, whether cardiovascular risk factors that improve with weight loss are reduced equally with lean meat or with an equivalent amount of plant protein in the diet. Thirty-six women, mostly overweight or obese, aged 40+/-9 years, were allocated nonrandomly to a 16-week parallel-design trial of two equienergetic diets designed to lead to weight loss, with one arm of the study emphasizing red meat and the other soybeans as the major protein source. Body weight, waist and hip circumference, and plasma lipids, glucose, insulin, and leptin levels were measured, and SAC was calculated from ultrasound measurement of aortic flow velocity and aortic root driving pressure. Subjects lost weight (9% of body weight in 16 weeks) and showed decreased plasma total and low-density lipoprotein (LDL) cholesterol (12% and 14%, P < .0001, respectively), triacylglycerol (17%, P < .05), and leptin (24%, P < .01) concentrations. However, lipoprotein(a) [Lp(a)] levels did not change significantly. Mean arterial pressure (MAP) decreased 7% and SAC increased 28% (P < .001 for both). However, only the decrease in arterial pressure correlated significantly with the reduction in the waist to hip ratio (WHR), and the improvement in SAC correlated inversely with the blood pressure reduction (P < .001 for both). Further, weight loss and the metabolic benefits of weight loss occurred equally with the meat-based and plant-based diets. We conclude that moderate weight loss in women leads to a substantial reduction in the cardiovascular risk, including SAC.

Leptin does not affect adipocyte glucose metabolism: studies in fresh and cultured adipocytes

Leptin, the 16-kd hormone produced by white fat cells, regulates energy homeostasis, satiety, and multiple sites in the neuroendocrine system. Leptin receptors have been identified in the central nervous system (CNS) and are widespread in peripheral tissues, including fat. Given the association between insulin resistance and obesity, it is important to establish whether leptin has additional effects on peripheral insulin action and glucose metabolism. This study examined whether leptin has a direct autocrine/paracrine action on glucose metabolism in both freshly isolated and 24-hour cultured rat fat cells. Freshly isolated rat adipocytes were incubated for 30 minutes with 200 ng/mL recombinant murine leptin. Thereafter, basal and insulin-stimulated (10(-8) mol/L) glucose transport, glycolysis-Krebs oxidation and lipogenesis ([6-14C]glucose conversion to [14C]O2 and to [14C]triglyceride), and lipolysis were measured. Upon leptin exposure, no statistical differences were detected in glucose transport or metabolism. Increasing the leptin concentration to 1 to 2 microg/mL or prolonging the duration of preincubation with the fat cells to 60 minutes before the metabolic assays did not alter the results. Finally, using two disparate fat cell culture methods with differing substrate additions (pyruvate and high or low glucose concentrations), there was no effect of 24-hour exposure to leptin (200 ng/mL) on basal and insulin-stimulated glucose transport or lipogenesis. We conclude that leptin does not modulate basal or insulin-stimulated glucose metabolism in isolated and cultured fat cells in vitro. However, in vivo, higher pericellular leptin concentrations, as well as other cellular or soluble serum factors, may exist that might lead to a physiologically relevant autocrine action of leptin.

Leptin in human pregnancy: the relationship with gestational hormones

OBJECTIVES

The aims of the study were (1) to examine the relationship between leptin and placental hormones by measuring serial changes in serum levels of leptin during and after pregnancy and (2) to study the effects of several gestational hormones on leptin release from fully differentiated 3T3-L1 adipocyte cell cultures.

STUDY DESIGN

Serum levels of leptin were measured throughout pregnancy and at 3 months post partum in 29 healthy women and were also measured in 18 healthy women at delivery by cesarean section and on postpartum day 3. In addition, 3T3-L1 mouse adipocytes were incubated for 24 hours in media containing various reproductive hormones and leptin production was measured.

RESULTS

Serum leptin levels increased significantly (8.4 +/- 0.9 vs 13.5 +/- 1.5 ng/mL; P <.001) between the first 2 trimesters of pregnancy but not between the second and third trimesters. These changes in leptin did not correlate significantly with changes in body mass index. Leptin levels dropped significantly during the immediate postpartum period, from 34.1 +/- 4.9 at cesarean delivery to 7.3 +/- 1.4 ng/mL on postpartum day 3 (P <.001). Fasting insulin level did not correlate significantly with leptin level during pregnancy but did so during the postpartum period (r = 0.60; P <.05). Leptin secretion from 3T3-L1 adipocytes was increased significantly when cells were cultured with human chorionic gonadotropin (150%, P <.01) and also when they were cultured with estrogen (120%, P <.03).

CONCLUSION

The data suggest that leptin production by adipose tissue is stimulated by several hormones of pregnancy, which may contribute to the increased leptin levels observed during gestation.

Leptin and the pituitary-thyroid axis: a comparative study in lean, obese, hypothyroid and hyperthyroid subjects

OBJECTIVE

To study interactions between leptin and the pituitary-thyroid axis, both in euthyroid and dysthyroid states.

SUBJECTS AND MEASUREMENTS

We investigated the relationships of plasma leptin to levels of free thyroid hormones and TSH in 18 patients with newly diagnosed hyperthyroidism, 22 with newly diagnosed primary hypothyroidism, and 32 lean (body mass index [BMI] < 30) and 37 obese (BMI > 30 kg/m2) euthyroid subjects. Hypothyroid patients were restudied during thyroxine replacement treatment.

RESULTS

Median [interquartile range] plasma leptin concentrations were highest in obese euthyroid subjects (31.5 [19.0-48.0] and in untreated hypothyroid patients (19.2 [11.5-31.5]), and lowest levels in untreated hyperthyroid patients (8.9 [5.5-11.1]) and lean euthyroid control subjects (6.6 [3.9-14.4] micrograms/l (Kruskall-Wallis one-way analysis of variance; P < 0.0001). In euthyroid subjects, plasma leptin levels were higher in obese than in lean subjects (P < 0.00001). In obese subjects plasma levels of TSH correlated with percentage body fat (r = 0.67; P < 0.001) and plasma leptin (r = 0.61; P < 0.001). In untreated hyperthyroid subjects plasma leptin was unrelated to free T3, and in untreated hypothyroidism plasma leptin was unrelated to either free T3 or TSH concentrations (all P = NS). In untreated hyperthyroid, but not hypothyroid, patients plasma leptin concentrations correlated with BMI (r = 0.57; P = 0.02). Treatment of hypothyroidism with thyroxine resulted in a significant reduction in plasma leptin concentrations from 20.8 (11.8 to 31.6) to 12.9 (4.6-21.2) micrograms/l (P = 0.005), but BMI did not change significantly in the hypothyroid subjects being studied prospectively.

CONCLUSIONS

(i) In euthyroid subjects, plasma leptin and TSH levels correlate, and both are positively correlated with adiposity. (ii) Plasma leptin was significantly elevated in hypothyroid subjects, to levels similar to those seen in obese euthyroid subjects. (iii) Treatment of hypothyroidism resulted in a reduction in the raised plasma leptin levels. The data are consistent with the hypothesis that leptin and the pituitary-thyroid axis interact in the euthyroid state, and that hypothyroidism reversibly increases leptin concentrations.

Plasma leptin levels in trauma patients: effect of adjuvant recombinant human growth hormone in intravenously fed multiple trauma patients

BACKGROUND

Leptin, the newly discovered ob gene product, is synthesized primarily in adipose tissue and circulates to all parts of the body. Injury elicits significant metabolic changes, and it is not known how these changes affect the circulating leptin levels.

METHODS

Plasma leptin levels were measured in postabsorptive normal subjects (n = 14, 5 men and 9 women) and severely injured (injury severity score [ISS], 34+/-2), hypermetabolic (resting energy expenditure [REE]/basal energy expenditure [BEE], 1.31+/-0.04), adult (39+/-4 years; n = 28, 18 men and 10 women) trauma patients within 48 to 60 hours after injury when they were receiving no nitrogen or calories. The nutritional influence on plasma leptin in these patients was monitored during the subsequent 7 days of total parenteral nutrition (TPN). During TPN the patients were randomized to receive or not to receive recombinant human growth hormone (rhGH) supplementation (0.15 mg/kg/d).

RESULTS

Trauma significantly lowered plasma leptin levels, both in women (56%) and in men (68%). Gender dimorphism in plasma leptin levels was seen in normal subjects and in both fasted and fed trauma patients, and in all cases female patients had significantly higher levels. Body mass index showed significantly positive correlations with plasma leptin both in normal and injured subjects. One day of TPN restored normal levels of leptin, both in men and women. Adjuvant rhGH treatment did not show any significant changes over that seen with TPN alone.

CONCLUSIONS

Decreased plasma leptin levels seen due to trauma may be partly related to the fasting conditions, because 1 day of refeeding restored normalcy. Leptin metabolism in trauma patients seemed to be not altered during rhGH supplementation, suggesting a relatively minor metabolic role of leptin.

The relationship between the serum leptin concentrations of thyrotoxic patients during treatment and their total fat mass is different from that of normal subjects

OBJECTIVE

Previous studies of leptin in thyrotoxic human subjects have been short-term and cross-sectional. We have measured serum leptin concentrations in thyrotoxic patients in order to study the influence of endogenous thyroid hormones on the relationship between serum leptin and fat mass. DESIGN PATIENTS, MEASUREMENTS: In 10 fasting thyrotoxic patients (8 females, 2 males, mean age: 51 years) we measured serum leptin (microgram/l), total thyroxine (TT4), total triiodothyronine (TT3), thyrotropin (TSH) and by Dual Energy X-ray Absorptiometry (DEXA) total fat mass (TFM, kg) at time of diagnosis (0 months, baseline) and during 12 months treatment with thiamazole. For comparison 16 fasting thiamazole-treated euthyroid patients (14 females, 2 males, mean age: 38 years) were studied after one year follow-up (26 months (15-48)) and 18 normal controls (12 females, 6 males, mean age: 39 years).

RESULTS

The serum leptin concentration was 9.1 (1.3 micrograms/l (mean (SEM) in the thyrotoxic patients and increased significantly to 16.0 (1.3 micrograms/l (P < 0.0005) after 12 months treatment compared to both normal subjects and their own baseline. There was a significant correlation between serum leptin concentration and TFM in the normal control group (r = 0.79, P < 0.00009), in the thiamazole-treated euthyroid group (r = 0.85, P < 0.00003) and in the baseline thyrotoxic group (r = 0.84, P < 0.002) but the serum leptin/TFM ratio increased significantly during 12 months of antithyroid drug treatment from 0.34 (1.2 micrograms/l/kg to 0.53 (1.2 micrograms/l/kg (P < 0.03).

CONCLUSION

The thiamazole-treated thyrotoxic patients increased their serum leptin concentrations during 12 months antithyroid drug treatment without a significant corresponding degree of changes in TFM as expected from normal controls. It is suggested that the metabolic state in thyrotoxic patients can influence the regulation of serum leptin concentrations without any associated changes in TFM.

The effect of body composition on hexarelin-induced growth hormone release in normal elderly subjects

OBJECTIVE

Growth hormone (GH) release is influenced by several factors including age, gender, physical exercise, nutritional status, sex steroids and body composition. The relationship with body composition is complex. Obesity is accompanied by suppression of spontaneous and stimulated GH release. As increasing body fat reduces stimulated GH secretion following a standard provocative test, the potential clinical uses of GH-releasing peptides (GHRPs), therapeutically or diagnostically, may be dependent on the relationship between body fat and GHRP-stimulated GH release. We have therefore assessed the effect of body composition and gender on the GH releasing capacity of hexarelin.

DESIGN

A single bolus of subcutaneous hexarelin at a dose of 1.5 micrograms per kg of body weight was administered at time 0. Blood samples were taken at -10, 0, 10, 20, 30, 40, 50, 60, 90, 120, 170 and 180 min.

SUBJECTS

Twenty-one (eight male) healthy elderly subjects with a median (range) age of 68 (60-81) years and BMI of 26 (19-30) kg/m2 were studied.

METHODS

Dual-energy X-ray absorptiometry (DEXA) was used to assess body composition.

RESULTS

Peak GH response correlated negatively with fat mass, BMI, percentage body fat, and weight [r = -0.72, P = 0.0001; r = -0.56, P = 0.009; r = -0.63, P = 0.002 and r = -0.48, P = 0.029, respectively,]. AUC GH correlated negatively with fat mass, BMI and percentage fat mass [r = -0.58, P = 0.006; r = -0.51, P = 0.019 and r = -0.66, P = 0.001 respectively]. Using multiple linear regression, fat mass was the most useful predictor for both peak GH response [R2 = 0.61, P < 0.0001] and AUC GH [R2 = 0.38, P = 0.003]. Gender was not a significant variable.

CONCLUSIONS

Increasing total fat mass results in a blunted GH response following subcutaneous hexarelin. Total fat mass appears to be a useful predictor of peak GH response even in normal individuals as none of the subjects in the present study was morbidly obese. This indicates that there is a continuum of effect of fat mass on hexarelin-stimulated GH release. Any impact of gender on the GH response to hexarelin is almost certainly indirect and mediated via differences in body composition. This observation will have an impact on the potential diagnostic and therapeutic uses of hexarelin and related GH secretagogues.

Interrelationship between circulating leptin concentrations, hunger, and energy intake in healthy subjects receiving tube feeding

BACKGROUND

Tube feeding is an unphysiological route of nutrient delivery, and yet there is a lack of controlled trials examining its effects on appetite, food intake, and factors involved in their control. This study aimed to investigate the relationship between diurnal tube feeding, hunger, food intake, and circulating concentrations of leptin (a putative satiety factor).

METHODS

Six healthy lean men received a continuous nasogastric infusion (9:00 AM to 9:00 PM) of colored water (2 days), liquid feeding (4.2 kJ/mL, energy provision 1 x the initial predicted basal metabolic rate; 3 days), and colored water (2 days). Measurements of hunger (visual analog scales), weighed food intake, and fasting circulating leptin concentrations were made while the subjects were allowed free access to isoenergetically dense food items.

RESULTS

Three days of diurnal nasogastric feeding (mean, 6.9 MJ/d) significantly increased total energy intake (to 19.4 MJ/d; p < .001; analysis of variance [ANOVA]), suppressing oral energy intake by only 17%, with no significant effect on mean daily hunger. Higher levels of energy intake led to a universal rise in circulating leptin concentrations (2.82 to 4.23 ng/mL; p < .004; ANOVA) that was not significantly related to subsequent breakfast energy intake, first rated hunger of the day, timing of morning food consumption, or subsequent mean daily oral energy intake or hunger.

CONCLUSIONS

This study suggests that 3 days of diurnal tube feeding (equivalent to basal metabolic rate) failed to suppress hunger and reduced food intake by only 17%. The rise in circulating leptin concentrations, associated with tube feeding and the increase in total energy intake, failed to predict subsequent hunger or oral energy intake.

Role of cytokines in AIDS wasting

There is now a large literature implicating cytokines in the development of wasting and cachexia commonly observed in a variety of pathophysiologic conditions. In the acquired immunodeficiency syndrome (AIDS), cytokines elicited by primary and secondary infections seem to exert subtle and sustained effects on behavioral, hormonal, and metabolic axes, and their combined effects on appetite and metabolism have been postulated to drive wasting. However, correlations of increased blood levels of a particular cytokine with wasting in AIDS have not been consistent observations, perhaps because cytokines act principally as paracrine and autocrine hormones, as well as indirectly by activating other systems. A better understanding of the mechanisms underlying the catabolic effects of cytokines in clearly needed if more efficacious strategies are to be developed for the prevention and treatment of wasting in AIDS. In this review we first examine the interacting factors contributing to the AIDS wasting syndrome. We then analyze the complex and overlapping role of cytokines in the pathophysiology of this condition, and put forward a number of hypotheses to explain some of the most important features of this syndrome.

Plasma leptin levels: interaction of obesity with a common variant of insulin receptor substrate-1

Obesity is associated with insulin resistance and other major cardiovascular risk factors. A common amino acid polymorphism at codon 972 of the insulin receptor substrate-1 (IRS-1) has been shown to interact with obesity in the expression of insulin resistance. The plasma concentration of the adipocyte-specific hormone leptin is increased in obesity and is correlated with adipose tissue mass. Because in vitro studies demonstrated inhibitory effects of leptin on insulin signaling, leptin may be involved in obesity-associated insulin resistance. To gain insight into the relationship between insulin and leptin in obesity, we studied plasma leptin levels and several cardiovascular risk factors, as well as their modification by the IRS-1 codon 972 genotype, in 156 obese individuals and 131 lean control subjects. In both groups, 10% of the subjects were heterozygous for the IRS-1 codon 972 variant. Obese individuals harboring the IRS-1 variant displayed significantly lower plasma concentrations of leptin than obese subjects without the polymorphism (means, 26.7 versus 37.8 ng/mL, P<0.0293). In a subgroup of obese patients, leptin mRNA abundance was measured in the adipose tissue and was significantly lower in carriers of the IRS-1 variant than in subjects with the wild-type variant (P<0.0291). Our data suggest that insulin signaling influences plasma leptin concentrations at the mRNA expression level and argue against leptin as a major causative factor of insulin resistance.

Leptin and the regulation of body weight in mammals

The assimilation, storage and use of energy from nutrients constitute a homeostatic system that is essential for life. In vertebrates, the ability to store sufficient quantities of energy-dense triglyceride in adipose tissue allows survival during the frequent periods of food deprivation encountered during evolution. However, the presence of excess adipose tissue can be maladaptive. A complex physiological system has evolved to regulate fuel stores and energy balance at an optimum level. Leptin, a hormone secreted by adipose tissue, and its receptor are integral components of this system. Leptin also signals nutritional status to several other physiological systems and modulates their function. Here we review the role of leptin in the control of body weight and its relevance to the pathogenesis of obesity.

Circulating leptin did not associate with the development of the hyperglycemia accompanied by insulin insensitivity in spontaneous noninsulin dependent diabetes mellitus model Otsuka-Long-Evans-Tokushima-Fatty rats

Leptin, the product of the ob gene, has been reported to regulate feeding behavior and energy metabolism. Plasma leptin concentration was strongly correlated with body fat content in humans. It is well known that increased body fat content is accompanied by insulin insensitivity. In order to study the relationship between serum leptin level and metabolic variables, we performed caloric restriction on Otsuka-Long-Evans-Tokushima-Fatty (OLETF) rats, a model of noninsulin dependent diabetes mellitus. The male OLETF rats were allocated at random to three groups: 100% group, and 85% and 70% groups (which consumed 85% and 70% of the amount of food consumed by the 100% group, respectively). A significant correlation between serum leptin level and the body fat content, body weight, triglyceride, and fasting plasma glucose was observed. Using a partial correlation analysis to control for body fat content, however, the correlation between serum leptin and these variables disappeared. No significant changes in serum leptin levels were observed before and after a 1 h hyperinsulinemic euglycemic clamp test. In conclusion, serum leptin was significantly correlated with body fat content rather than fasting plasma glucose, serum insulin and insulin sensitivity. This suggests that circulating leptin per se may not result in hyperinsulinemia and insulin insensitivity in the OLETF rat.

Effect of obesity on estradiol level, and its relationship to leptin, bone maturation, and bone mineral density in children

The purpose of this study was to investigate 24-h estradiol and leptin levels in obese and nonobese children to further understand the roles of estradiol and leptin in obesity and puberty. We measured serum estradiol, leptin, insulin, glucose, and GH levels every hour for 24 h in 18 obese (12 females and 6 males) and 30 nonobese (11 females and 19 males) prepubertal and early pubertal (stages 1-2) children. Bone age and dual energy x-ray absortiometry (DEXA) were obtained upon completion of the 24-h study. Obese children were significantly younger than nonobese children, with no difference in pubertal stage, height, or bone age between the 2 groups. Obese children had greater bone age to chronological age ratios than nonobese children, indicating a more advanced rate of bone maturation. Mean 24-h estradiol levels correlated significantly with chronological age and bone age as well as with insulin-like growth factor I, insulin-like growth factor-binding protein-3, dehydroepiandrosterone sulfate, mean 24-h GH, and lean body mass. Mean 24-h estradiol levels did not differ between obese and nonobese children [1.65+/-1.47 us. 2.75+/-3.30 pmol/L (0.45+/-0.40 vs. 0.75+/-0.90 pg/mL), respectively]. Similar mean 24-h estradiol levels in obese and nonobese children are consistent with the increased bone maturation of the obese children. Estradiol did not correlate significantly with DEXA fat mass, body mass index, or arm fat measures of adiposity. Obese children had higher 24-h mean leptin concentrations than nonobese children (28.6+/-17.4 vs. 6.8+/-7.1 ng/mL; P < 0.001). Leptin concentrations positively correlated with DEXA fat mass, body mass index, and arm fat measurement of adiposity. Girls had higher 24-h mean leptin levels than boys when controlling for adiposity. Estradiol and leptin concentrations fluctuated over a 24-h period in both groups, with all children having higher leptin concentrations at night and higher estradiol concentrations in the morning. This diurnal rhythm was of a similar pattern, but at higher levels for leptin and lower levels for estradiol in the obese children compared to nonobese children. There was no significant correlation between estradiol and leptin levels. Bone mineral density, as measured by DEXA, did not differ between obese and nonobese children. Similar bone mineral density values in obese and nonobese children are consistent with the increased bone maturation of the obese children. Bone mineral density was not correlated with estradiol or leptin level in these children. In conclusion, obese children had similar estradiol levels and equivalent bone ages at a younger chronological age than nonobese children. Leptin was higher in these obese children, but did not correlate with estradiol level or bone age. These findings suggest that the role of leptin in both obesity and pubertal development is not directly correlated with the estradiol level.

Leptin is involved in gender-related differences in insulin sensitivity

BACKGROUND

Although the effects of insulin on leptin levels are relatively well characterized, the possible actions of leptin on insulin sensitivity are not so well studied. This study was undertaken to examine whether gender-related differences in insulin sensitivity could be explained partially by leptin levels.

SUBJECTS

The study involved 22 women (13 obese) and 20 (11 obese) fat mass- and age-matched men. All participants were healthy.

MEASUREMENTS

Several anthropometric measures of body fatness were quantified and the percentage of body fat was determined through bioelectric impedance. Oral glucose tolerance test and a frequently sampled intravenous glucose tolerance test was performed in all subjects. Serum leptin was measured by radioimmunoassay.

RESULTS

Nine lean women (BMI 20.4 +/- 2 kg m2 mean +/- SD) showed increased leptin levels (7.8 +/- 2.7 vs. 4.3 +/- 1.3 micrograms/l, P = 0.003), increased insulin sensitivity (5.2 +/- 1.3 vs. 2.9 +/- 0.9 min-1/mU/l, P = 0.001) and similar fat mass (11.1 +/- 3.7 vs. 13.2 +/- 7.8 kg, P = NS) in comparison with 9 age-matched lean men (33.6 +/- 6 vs. 34.5 +/- 6.3 years, P = NS). Thirteen obese women (BMI 32.5 +/- 2.7) kg m2 also showed increased leptin levels (29.6 +/- 8.4 vs. 11.7 +/- 4.8 micrograms/l, P < 0.0001), increased insulin sensitivity (1.7 +/- 0.7 vs. 0.95 +/- 0.9 min-1 mU/l, P = 0.04) and similar fat mass (34.4 +/- 8.0 vs. 30.9 +/- 9.6 kg, P = NS) in comparison with 11 age-matched obese men (34.5 +/- 7.8 vs. 38.7 +/- 8.2 years, P = NS). A strong linear association between leptin levels and insulin sensitivity (Si) was found (r = -0.67, P = 0.001, in men; r = -0.82, P < 0.0001, in women). After controlling for percentage of body fat, this association remained significant only in men (r = -0.56, P = 0.01, in men; r = -0.30, P = NS in women). In stepwise regression analysis models, both gender (P = 0.00001) and leptin (P = 0.00001) contributed to 67% of the variance in Si independently of body fat.

CONCLUSIONS

Leptin levels and gender contribute to the variance of insulin sensitivity, independently of body fat. These results suggest that leptin could affect insulin sensitivity.

Leptin-independent hyperphagia and type 2 diabetes in mice with a mutated serotonin 5-HT2C receptor gene

Brain serotonin and leptin signaling contribute substantially to the regulation of feeding and energy expenditure. Here we show that young adult mice with a targeted mutation of the serotonin 5-HT2C receptor gene consume more food despite normal responses to exogenous leptin administration. Chronic hyperphagia leads to a 'middle-aged'-onset obesity associated with a partial leptin resistance of late onset. In addition, older mice develop insulin resistance and impaired glucose tolerance. Mutant mice also responded more to high-fat feeding, leading to hyperglycemia without hyperlipidemia. These findings demonstrate a dissociation of serotonin and leptin signaling in the regulation of feeding and indicate that a perturbation of brain serotonin systems can predispose to type 2 diabetes.

The effects of a high fat diet on leptin mRNA, serum leptin and the response to leptin are not altered in a rat strain susceptible to high fat diet-induced obesity

Osborne-Mendel (OM) and S5B/Pl rats differ in their sensitivity to develop obesity when fed a high fat (HF) diet; OM rats become obese, whereas S5B/Pl rats remain thin. We have investigated the possibilities that either an impaired leptin response or resistance to leptin action underlies the sensitivity to this form of obesity in OM rats. In Experiment 1, OM and S5B/Pl rats fed a nonpurified diet were killed at d 0 or were fed either a HF (56% fat energy) or a low fat (LF, 10% fat energy) diet for 2 or 7 d. The HF diet increased serum leptin significantly by d 2 to levels that were similar in both rat strains. At 7 d, leptin levels were lower than at d 2 but remained higher than levels in the d 0 control groups. The leptin mRNA:18S RNA ratio in epididymal adipose tissue increased to higher levels in HF-fed OM rats than in S5B/Pl rats fed that diet. However, although the LF diet had no effect in S5B/Pl rats, it increased leptin mRNA levels in epididymal adipose tissue of OM rats compared with the controls fed the nonpurified diet. In Experiment 2, OM and S5B/Pl rats were fed HF or LF diets for 5 wk. At that time, their feeding response to a range of leptin doses (0, 1, 5 or 10 microgram) given intracerebroventricularly was tested after overnight food deprivation. There was a similar dose-dependent reduction in energy intake in response to leptin in both OM and S5B/Pl rats. These responses were independent of the diet. The data suggest that the susceptibility of OM rats to HF diet-induced obesity is not related to either a loss of central sensitivity to leptin or a failure to enhance leptin production acutely, although the failure to maintain chronically increased levels of serum leptin could contribute to the obesity.

Gender-dependent alterations in serum leptin in alcoholic cirrhosis

BACKGROUND & AIMS

Leptin is a peptide that decreases food intake and increases energy expenditure. It is produced in fat cells, is stimulated by cytokines, and its levels in serum are higher in females. Because anorexia, hypermetabolism, and elevated cytokine levels are frequently observed in cirrhosis, we hypothesized that the serum leptin level would be elevated in cirrhosis. The aim of this study was to investigate the relationship of serum leptin to gender, body composition, and tumor necrosis factor (TNF).

METHODS

Male (n = 18) and female (n = 10) abstinent alcoholic cirrhotic patients were studied and compared with control subjects (15 male and 8 female). Fat mass, fat-free body mass, and body cell mass were calculated by using H2[18O] and bromide dilution methodology. Serum leptin and TNF concentrations were measured by immunoassays.

RESULTS

Fat mass was decreased only in male cirrhotics (P < 0.05), whereas body cell mass was decreased in both male and female cirrhotics (P < 0.01). Leptin levels were elevated in female (P < 0. 001) but not male cirrhotics compared with controls. When expressed per kilogram of fat mass, leptin was elevated in both male (P < 0. 01) and female (P < 0.01) cirrhotics. Women in both cirrhotic and control groups had higher leptin levels than men. TNF was elevated in both male and female cirrhotics and did not correlate with leptin levels.

CONCLUSIONS

Cirrhotics have elevated serum leptin levels, which are related to both gender- and gender-dependent alterations in body composition.