Inverse correlation between serum testosterone and leptin in men.

Besides its role in the regulation of energy balance, leptin seems to be involved in linking energy stores to the reproductive system. A gender-dependent difference exists in plasma leptin concentration and leptin messenger ribonucleic acid expression in rodents and humans. This difference does not seem to be explained simply by differences in the amount of body fat between genders. To elucidate the relationship of endogenous testosterone and leptin, we studied the serum leptin concentrations in 269 elderly nondiabetic men. In addition, to assess whether exogenously administered testosterone could influence leptin production, we followed the serum levels of leptin in 10 healthy men during a 12-month treatment with 200 mg testosterone enanthate, i.m., weekly for contraceptive purposes. We found that the serum leptin concentration correlated inversely (r = -0.39; P < 0.001) with that of testosterone in elderly men. This inverse correlation was still present when body mass index and plasma insulin were included in the analysis. The administration of testosterone to young men suppressed serum leptin from the pretreatment level of 3.4 +/- 1.4 to 1.9 +/- 0.6 micrograms/L during the therapy. After cessation of testosterone injections, serum leptin concentration returned back to the pretreatment level. It is concluded that testosterone has a suppressive effect on leptin production, as reflected by circulating levels of this hormone.