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

Objective: To compare ethnic differences in total, free and bioavailable testosterone amongst young healthy South Asian and Caucasian men. Design and subjects: 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. Measurements:  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. Results:  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. Conclusions: 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.