Effects of testosterone treatment on body fat and lean mass in obese men on a hypocaloric diet: a randomised controlled trial

Background: Whether testosterone treatment has benefits on body composition over and above caloric restriction in men is unknown. We hypothesised that testosterone treatment augments diet-induced loss of fat mass and prevents loss of muscle mass. Methods: We conducted a randomised double-blind, parallel, placebo controlled trial at a tertiary referral centre. A total of 100 obese men (body mass index ≥ 30kg/m2) with a total testosterone level of or below 12nmol/L and a median age of 53years (interquartile range 47-60) receiving 10weeks of a very low energy diet (VLED) followed by 46weeks of weight maintenance were randomly assigned at baseline to 56weeks of 10-weekly intramuscular testosterone undecanoate (n = 49, cases) or matching placebo (n = 51, controls). The main outcome measures were the between-group difference in fat and lean mass by dual-energy X-ray absorptiometry, and visceral fat area (computed tomography). Results: A total of 82 men completed the study. At study end, compared to controls, cases had greater reductions in fat mass, with a mean adjusted between-group difference (MAD) of -2.9kg (-5.7 to -0.2; P = 0.04), and in visceral fat (MAD -2678mm2; -5180 to -176; P = 0.04). Although both groups lost the same lean mass following VLED (cases -3.9kg (-5.3 to -2.6); controls -4.8kg (-6.2 to -3.5), P = 0.36), cases regained lean mass (3.3kg (1.9 to 4.7), P < 0.001) during weight maintenance, in contrast to controls (0.8kg (-0.7 to 2.3), P = 0.29) so that, at study end, cases had an attenuated reduction in lean mass compared to controls (MAD 3.4kg (1.3 to 5.5), P = 0.002). Conclusions: While dieting men receiving placebo lost both fat and lean mass, the weight loss with testosterone treatment was almost exclusively due to loss of body fat.