Reduced total testosterone concentrations in young healthy South Asian men are partly explained by increased insulin resistance but not by altered adiposity

Abstract

To compare ethnic differences in total, free and bioavailable testosterone amongst young healthy South Asian and Caucasian men. Cross-sectional study of 134 healthy men (age 20-40 years) of South Asian (n = 67) or Caucasian (n = 67) origin, recruited from hospital staff and students working in Newport, UK. Subjects were excluded if they had a fasting plasma glucose >5.9 mmol/l, central obesity [waist circumference ≥ 94 cm (Caucasian) or ≥ 90 cm (South Asian)] or significant other disease. Fasting plasma glucose, total testosterone (determined by immunoassay and mass spectrometry), albumin, sex hormone-binding globulin (SHBG) and insulin were measured. Free and bioavailable testosterone were calculated using Vermeulen's formula, and insulin resistance was estimated by HOMA-IR. The South Asians were slightly older (P = 0.04), shorter (P < 0.001), lighter (P < 0.001), more insulin resistant (P = 0.006), and had a lower body mass index BMI (P = 0.012), waist circumference (P = 0.043) and SHBG (P = 0.001) than the Caucasians. Total testosterone was significantly lower in South Asians (mass spectrometry: geometric mean 16.3 nmol/l; 95% reference interval 9.3-28.6 nmol/l) compared with Caucasians (mass spectrometry: geometric mean 18.4 nmol/l; 95% reference interval 10.6-31.9 nmol/l; P = 0.015), but calculated free and bioavailable testosterone were not different between groups. Adjusting for HOMA-IR, but not BMI or waist circumference, partly attenuated the differences in total testosterone. Total, but not free, testosterone concentrations are lower in healthy South Asian men than in Caucasians. These differences are apparent at a young age and may be partly attributable to alterations in insulin sensitivity.