Body composition and age in African American and Caucasian women: relationship to plasma leptin levels and gonadal steroids

Abstract

Leptin is a recently isolated peptide hormone released from adipocytes that has been postulated to play a role in appetite regulation and energy metabolism. Aging affects both food intake and body composition. Body composition is also affected by ethnicity. We have evaluated the relationships between serum leptin levels, age, body composition (by dual-energy x-ray absorptiometry), and hormonal parameters in a cross-sectional study of 94 women, 53 African-American (AAF) and 41 Caucasian (CF). Our hypotheses were as follows: (1) changes in body composition would be related to age in a sinusoidal pattern, (2) changes in serum leptin would parallel changes in body fat, (3) serum leptin levels would be influenced by body fat distribution, and (4) serum leptin would be related to serum concentrations of sex hormones. Serum leptin paralleled changes in body fat and body mass index (BMI) with age. In the entire group, serum leptin correlated closely with measures of body fat, including BMI and total fat mass, and there was no difference in leptin levels between the two ethnic groups. In simple regression analysis, serum leptin was related to both serum estradiol and testosterone. The relationship between serum leptin and trunk fat was linear in both groups, but significantly different in AAF and CF (P = .014). Serum leptin was associated with the trunk to lower-extremity fat ratio in CF (r = .67, P = .001) but not in AAF. Body fat was increased with advancing age until about 65 years and then declined. Measures of lean body mass declined linearly with age in the entire group, as well as both subgroups. In the entire group, total lean body mass and lean body mass corrected for BMI (lean body mass/BMI) were inversely related to age. In subjects aged less than 60 years AAF were stronger (P < .05) and had both a larger BMI and fat mass (P < .05) than CF. However, the patterns of age-related changes in fat body mass, lean body mass, and BMI were similar in both groups. In the entire group, multiple regression analysis indicated that the age, free thyroxine index (FTI), and leptin concentration were predictors of the body composition and distribution of trunk to lower-body fat. These observations indicate that there is a sinusoidal relationship between body fat and age, with a decline in body fat in extreme old age in both AAF and CF, and that serum leptin concentrations are more closely related to body fat and BMI than to age or ethnicity.