Gender differences in serum leptin in obese people: relationships with testosterone, body fat distribution and insulin sensitivity

Testosterone levels are decreased in obese men but increased in obese women. The interplay between gonadal steroids and leptin is, at present, far from being elucidated. This study was carried out to investigate the relationship between serum leptin, plasma insulin, insulin sensitivity and free testosterone in 46 men (29 obese and 17 lean) and 65 premenopausal women (42 obese and 23 lean). In all subjects, anthropometric parameters and serum levels of insulin, leptin, free testosterone (T), dehydroepiandrosterone sulphate and sex hormone-binding globulin were measured. An oral glucose tolerance test (OGTT) and an insulin tolerance test were also performed to determine the insulin sensitivity index. Our results show a significant difference in serum leptin between lean and obese men (3.19 +/- 0.71 vs. 20.28 +/- 0.26 ng mL-1; P < 0.0005) as well as between lean and obese women (10.78 +/- 2.14 vs. 34.79 +/- 2.26 ng mL-1; P < 0.00001). Basal T concentration in the obese men was significantly lower than in the control group (18.6 +/- 1.3 vs. 23.3 +/- 1.4 ng L-1; P < 0.01), whereas in the obese women it was significantly higher than in the control group (2.0 +/- 0.2 vs. 1.3 +/- 0.1 ng L-1; P < 0.05). When multiple linear regression was performed without body mass index (BMI) in the statistical model, leptin was correlated with basal insulin (P < 0.0001), insulin sensitivity (P < 0.0001) and T (P < 0.0001) in both men and women. When BMI was included in the model as an independent variable, leptin was significantly correlated only with BMI (P < 0.0001), the degree of insulin resistance (P < 0.05) and T (only in men, P < 0.05). This study confirms that serum leptin is strongly correlated with the degree of obesity and female sex. The negative correlation between leptin and T in men, independent of BMI, is consistent with the hypothesis that T may possess an inhibitory effect on adipocyte ob gene transcription.