Circadian variation of serum leptin in healthy and diabetic men

Leptin, from the Greek leptos, meaning thin (in reference to its ability to reduce body fat stores), is a hormone secreted primarily by adipocytes. At one time, leptin was portrayed as a potential means of combating obesity. Recently, leptin has been identified as a potent inhibitor of bone formatio n, acting through the central nervous system. Since numerous studies clearl y show that bone remodeling is circadian rhythmic with peak activity during sleep, it is of interest to explore circadian variability in serum leptin. Accordingly, circadian characteristics of serum leptin were examined in 7 clinically healthy men and 4 obese men with type II diabetes. Blood samples were collected for 24h at 3h intervals beginning at 19:00. The dark (sleep ) phase of the light-dark cycle extended from 22:30 to 06:30, with brief aw akening for sampling at 01:00 and 04:00. Subjects consumed general hospital meals (2400 calories) at 16:30, 07:30, and 13.30. Serum leptin levels were determined by a R&D Systems enzyme immunoassay technique. Data were analyz ed by linear least-squares estimation using the population multiple compone nts method. A statistically significant (P < .018) circadian rhythm modeled by a single 24h cosine curve characterized the data of each group. The 24h mean leptin level was statistically greater (P < .001) in the obese diabet ic men than in the healthy men (9.47 +/- 0.66 ng/mL vs. 24.07 +/- 1.71 ng/m L, respectively). Higher leptin levels occurred between midnight and roughl y 02:30, and lowest leptin levels occurred between noon and the early after noon. The phasing of this rhythm is similar to the circadian rhythm in bone remodeling previously described. Our results suggest the findings from a s ingle morning blood sampling for leptin may be misleading since it may unde restimate the mean 24h and peak concentrations of the hormone.