Serum Leptin in Elderly People: Associations with Sex Hormones, Insulin, and Adipose Tissue Volumes Richard N. Baumgartner, * Robert R. Ross, f Debra L. Waters,* William M. Brooks,$ John E. Morley,§ George D. Montoya," and Philip J. Garry* Abstract BAUMGARTNER, RICHARD N., ROBERT R. ROSS, DEBRA L. WATERS, WILLIAM M. BROOKS, JOHN E. MORLEY, GEORGE D. MONTOYA, AND PHILIP J. GARRY. Serum leptin in elderly people: associations with sex hormones, insulin, and adipose tissue volumes. Obes Res. 1999;7:141-149. Objective: There are few data for associations of serum leptin with body fat, fat distribution, sex hormones, or fasting insulin in elderly adults. We hypothesized that the sex difference in serum leptin concentrations would disappear after adjustment for subcutaneous, but not visceral body fat. Serum leptin would not be associated with sex hormone concentrations or serum fasting insulin after adjusting for body fat and fat distribution. Research Methods and Procedures: Subcutaneous adipose tissue (SAT) and visceral adipose tissue (VAT) volumes were measured using magnetic resonance imaging in a cross-sectional sample of 56 nondiabetic, elderly men and women aged 64 years to 94 years. Serum leptin, sex hormones (testosterone and estrone), sex hormone-binding globulin, and fasting insulin were also measured. Nine women were taking hormone replacement, and five men were clinically hypogonadal. Results: Leptin was significantly associated with both SAT and VAT in each sex. Adjustment for SAT reduced the sex
Received for review February I I. 1998. Accepted for publication in final form October I , 1998. From the *Clinical Nutrition Program, Center for Population Health, Departments of Internal Medicine and Pathology, University of New Mexico School of Medicine, Albuquerque, NM 87131; ?Department of Physical and Health Education, Queen's University, Kingston, Ontario, Canada K7L 3N6; +Clinical and Magnetic Resonance Research Center, University of New Mexico School of Medicine. Albuquerque, NM 87131; and the $Geriatric Research. Education, and Clinical Center, St. Louis VA Medical Center, St. Louis, MO 63104. Reprint requests to Dr. Richard N. Baumgartner. Clinical Nutrition Program, 215 Surge Building, University of New Mexico School of Medicine, Albuquerque, NM 87131. FAX: (505) 272-9135. E-mail:
[email protected] Copyright 0 1999 NAASO.
difference in leptin by 56%, but adjustment for VAT increased the difference by 25%. Leptin was not associated with serum estrone or hormone replacement therapy in the women, but had a significant, negative association with testosterone in the men that was independent of SAT, but not VAT. Leptin was significantly associated with fasting insulin in both sexes independent of age, sex hormones, sex hormone-binding globulin, VAT and SAT. Discussion: Sex difference in serum leptin is partly explained by different amounts of SAT. Studies including both men and women should adjust for SAT rather than total body fat that includes VAT. The sex difference in serum leptin is not due to estrogen, but may be partly explained by testosterone. Testosterone is negatively associated with leptin in men, but the association is confounded with VAT. Leptin is associated with fasting insulin in nondiabetic elderly men and women independent of body fat, fat distribution. or sex hormones. Key words: leptin, body composition, sex hormones, insulin, elderly
Introduction The discovery of leptin, a hormone-like protein produced and secreted by adipose tissue, has led to considerable excitement and research on its possible regulatory role in metabolism (1). In rat and mouse models, leptin has been shown to produce an anorexigenic effect by inhibiting secretion of neuropeptide Y in the hypothalamus and to increase energy expenditure, possible by stimulating sympathetic nervous system activity and resting metabolic rate (2,3). Leptin receptors are distributed in liver, pancreas, adipose, and muscle tissues, as well as the hypothalamus (4). In vitro studies indicate that leptin may modulate insulin action in these peripheral tissues, suggesting a role for leptin in hyperinsulinemia and insulin resistance in obesity (5-8). In vivo, leptin is significantly, positively correlated with OBESITY RESEARCH Vol. 7 No. 2 Mar. 1999 141
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fasting serum insulin independent of body fatness (9-1 3). Insulin administration, however, does not acutely affect leptin levels, although long-term hyperinsulinemia does seem to result in increased leptin concentrations (14-16). Reported associations with measures of insulin sensitivity and resistance are inconsistent (12,14,17-22). Thus, the importance of leptin in human metabolism and obesity remains controversial. Serum leptin concentrations are correlated strongly with body fatness, suggesting that humans are relatively “leptin resistant,” although a basis for this resistance has not been clearly established (23-25). Substantial interindividual variability in serum leptin levels remains after adjusting for measures of body fatness, indicating that other environmental and genetic factors additionally influence leptin concentrations. The majority of studies indicate that women have higher leptin concentrations than men, regardless of body fatness (13,24-27). This has led to the suggestion that sex hormones may play a role in regulating the production or secretion of leptin by adipose tissues. Studies of associations of endogenous and exogenous sex hormones with serum leptin, however, have been inconsistent (28-36). Previous studies may not have adequately controlled for sex differences in body fat distribution, in particular the amounts of subcutaneous versus intra-abdominal or visceral adipose tissues (VATS). Some studies have shown that the production of leptin is greater in subcutaneous adipose tissue (SAT) than VAT, which could explain the higher leptin levels in women who have more SAT and less VAT than men (37). There are few data for serum leptin and its associations with body composition, fasting insulin or insulin resistance, or hormones in elderly people. Body fat is increased and its distribution is more centralized on and within the trunk, whereas glucose tolerance and sex hormone levels are lower in elderly, compared with young or middle-aged men and women. It is not clear whether there is any effect of age on serum leptin concentrations that is independent of these differences in body fat, fat distribution, glucose metabolism, or sex hormones. The purposes of this study were to determine in elderly men and women the associations of serum leptin with: (1) subcutaneous and visceral body fat; (2) sex hormone concentrations and the effect of hormone replacement therapy (HRT) in postmenopausal women; and (3) fasting serum insulin independent of body fatness and fat distribution. We hypothesized that the sex difference in serum leptin concentrations would disappear after adjustment for subcutaneous, but not visceral, adiposity. In addition, serum leptin would not be associated with serum testosterone in men or either serum estrone or HRT in women or serum fasting insulin concentrations in either sex after adjusting for measures of body fat and fat distribution. To test these hypotheses, serum leptin, sex hormone, fasting insulin concentra142 OBESITY RESEARCH Vol. 7 No. 2 Mar. 1999
tions, and total and VAT volumes were measured in 56 elderly, nondiabetic men and women selected from the New Mexico Aging Process Study (NMAPS).
Subjects and Methods The subjects were 28 men and 28 women, 64 to 94 years of age, from the NMAPS cohort, a longitudinal study of elderly men and women that began in 1980. A detailed description of the NMAPS can be found elsewhere (38). Body composition data have only been collected since 1993, and the present study is limited to participants who volunteered for magnetic resonance imaging (MRI) body composition studies that were conducted during 1995. These volunteers were selected randomly from the study cohort without regard for either body composition or health status; however, those with metallic, ferromagnetic clips, implants, or prostheses were excluded, because these may interfere with MRI. Nine of the 28 women were currently taking HRT (estrogen or estrogedprogesterone), as determined from medical histories and a questionnaire administered by the study nurse. All women had taken HRT continuously for >1 year at the time of the study. None of the subjects were current smokers, and alcohol intake was reported to be light to moderate. All subjects were weight stable at the time of examination, and did not report significant weight loss or gain in the 6 months before the study. None reported recent acute illness or current use of significant medications other than HRT in the women. All participants gave informed consent. This study was approved by the Human Research Review Committee of the University of New Mexico School of Medicine. Anthropometric measurements (weight, height, waist, and hip circumferences) were made using standardized procedures (39). Weight was measured to the nearest 0.1 kg on a beam-balance scale. Height was measured to the nearest 0.1 cm using a wall-mounted stadiometer. Waist circumference was measured to the nearest 0.1 cm at a level 1 cm superior to the iliac crests using a flexible steel tape. Hip circumference was measured at the level of the maximum posterior protrusion of the buttocks. Body mass index (BMI) was calculated as weight (kg) divided by height (m) squared. The waisthip circumference ratio was calculated as an index of fat distribution. VAT and SAT volumes were measured using a modification of the whole-body MRI procedure described by Ross et al. (40). Magnetic resonance images were obtained using GE 1.5 T whole-body scanner at the University of New Mexico Center for Non-Invasive Diagnosis. A T1weighted, spin-echo pulse sequence with a repetition time of 150 msec and TE of 17 msec was used to acquire all image data. The protocol acquired 39 to forty-two 10 mm thick transverse image slices, depending on body size, at 50 mm intervals along the length of the body from the tips of the outstretched arms to the feet. Two exams were used to
Serum Leptin in Elderly People, Baurngartner et at.
acquire axial images from the whole body. A sagittal scout scan was used to locate the L4 to L5 vertebral space, which was used as the point of origin for aligning the lower and upper body series. Images through the chest and upper abdomen were acquired using two series of images with single 13-second breath-holds to minimize breathing motion artifacts. The total acquisition time to obtain all images ranged from 25 to 35 minutes. The images were analyzed using specially designed image analysis software (Tomovision, Montreal, Quebec, Canada) on Indigo2 computers (Silicon Graphics, Mountain View, CA). This software allows the automated segmentation of tissue-areas on each slice based on image morphology using a combination of edge detection filters and watershed techniques as described in detail elsewhere (40). Each segmented image was reviewed using an interactive editing program to correct misclassified pixels caused by radiofrequency inhomogeneity of the scanner body coil. This was facilitated by superimposing the original graylevel image on the binary segmented image using a transparency mode. Muscle, adipose, and nonmuscle lean soft tissue (e.g., organ) and bone areas were measured on each axial image by summing the given tissues' pixels and multiplying by the pixel surface area. The volumes (cm3) of VATS and SATs in each slice were calculated by multiplying the tissue area (cm2) by the slice thickness (10 mm). The slice volumes were then summed to estimate total tissue volumes using the formula of Ross (41). The coefficients of variation for the measurements of VAT and SAT volumes were estimated to be 5% and 3%, respectively, based on two repeated analyses of five sets of MRI images by two independent observers in our laboratory. We did not estimate the reliability of repeated scans of the same subjects due to the high cost of MRI imaging. Ross et al. who used the same imaging protocol, estimated this interscan reliability for total adipose tissue volume to range from 1% and 4% in a previous report (40). Abate et al. (42) validated MRI estimates of SAT and VAT volumes, compared with those obtained by dissection in cadavers: the mean difference was
