Strong association between serum levels of leptin and testosterone in men

OBJECTIVE: Leptin serves as a hormonal signal linking food intake and energy expenditure to fat mass. As significant effects of testosterone administration on body composition and adipose tissue have been described recently, we examined a possible association between serum levels of leptin and testosterone which has not been reported so far. SUBJECTS: Three groups comprising a total of 58 adult age-matched males were included in this cross- sectional analysis by computer-assisted clinical database selection. Group 1 (n=22) consisted of untreated hypogonadal patients with testosterone serum levels lower than 7 nmol/l. The inclusion criterion for Groups II (n=20) and III (n=16) was a serum testosterone level higher than 30 nmol/l. Group II involved hypogonadal patients under effective androgen substitution therapy; Group Ill comprised males without any endocrine disorder attending our clinic. MEASUREMENTS: Morning blood samples were taken for determination of serum levels of leptin, testosterone, oestradiol and sex-hormone binding globulin (SHBG). Serum levels of leptin were measured by homologous radioimmunoassay, the other hormones by standard immunoassays. RESULTS: No significant differences in serum leptin, serum testosterone, and body mass index (BMI) were detected between Group II (3.7 ± 1.5 μg/l, 40.3 ± 7.6 nmol/l, 24.4 ± 4.9 kg/m2, respectively (mean ± SD)), and Group III (4.0 ± 2.0 μg/l, 35.9 ± 6.5 nmol/l, 24.8 ± 2.6 kg/m2, respectively). However, hypogonadal patients of Group I who were selected for low testosterone serum levels (4.3 ± 1.7 nmol/l) had significantly higher leptin serum levels of 18.8 ± 9.7 μg/l (P < 0.001) and significantly higher BMI of 30.0 ± 6.6 kg/m2 (P < 0.001). Multiple linear regression analysis revealed a significant independent association of leptin with testosterone serum levels (partial correlation coefficient =-0.84; P < 0.001) and with BMI (partial correlation coefficient = 0.44; P = 0.001), whereas serum levels of oestradiol and SHBG had no additional influence. CONCLUSIONS: This study demonstrates a close association between serum levels of testosterone and leptin in males which has not been described previously. Serum testosterone levels could be an important contributor to the known gender difference in serum leptin levels which can be found even after correction for body composition. These findings might be of clinical relevance for testosterone substitution therapy of hypogonadal as well as ageing men.