Leptin released from adipose tissue is believed to participate in a negative feedback loop regulating appetite, and malfunction of this mechanism could lead to obesity. We measured plasma leptin and body composition (dual energy x-ray absorptiometry) in 70 healthy subjects, divided into 3 age groups (young, 25 ± 1 yr; middle-aged, 53± 1 yr; old, 70 ± 1 yr), while on a 5-day weight-maintaining diet. Pairwise correlations were assessed using product-moment correlation, and regression analysis was used to evaluate relationships between leptin and other variables.

Leptin concentrations and relative body fat content were correlated in young females (r = 0.71; P = 0.009) and in young males (r = 0.76; P = 0.007), but not in the combined middle-aged and elderly groups (r = 0.19; P = 0.36 and r = 0.19; P = 0.38 in females and males, respectively). Regression analysis showed a clear correlation between circulating leptin and relative fat mass in the young subjects (P = 0.0001), but not in the older subjects (P = 0.199).

We conclude that body fat content in young subjects correlates with plasma leptin in both genders, whereas this relationship is disrupted in elderly subjects, thus possibly contributing to the obesity occurring with age.