Background: Trials of testosterone therapy in aging men have demonstrated increases in fat free mass and skeletal muscle, and decreases in fat mass, but have not reported the impact of baseline body composition.

Objective: To determine the effect, in non-obese aging men with symptoms of androgen deficiency and low-normal serum testosterone levels, of testosterone therapy on total and regional body composition, and hormonal and metabolic indices.

Methods: 60 healthy but symptomatic, non-obese men aged 55 years with TT levels <15nM were randomized to transdermal testosterone patches or placebo for 12 months. Body composition, by DEXA (fat mass, fat free mass, skeletal muscle) and MRI (abdominal subcutaneous and visceral adipose tissue, thigh skeletal muscle and intermuscular fat) and hormonal and metabolic parameters were measured at Weeks 0 and 52.

Results: Serum TT increased by 30% (P=0.01) LH decreased by 50% (P<0.001). Relative to placebo, total body fat free mass (P=0.03) and skeletal muscle (P=0.008) were increased and thigh skeletal muscle loss was prevented (P=0.045) with testosterone therapy while visceral fat accumulation decreased (P=0.001) without change in total body or abdominal subcutaneous fat mass; change in visceral fat was correlated with change in TT levels (r²=0.36; P=0.014). There was a trend to increasing total and LDL cholesterol with placebo.

Conclusion: Testosterone therapy, relative to placebo, selectively lessened visceral fat accumulation without change in total body fat mass, and increased total body fat free mass and total body and thigh skeletal muscle mass. Further studies are needed to determine the impact of these body compositional changes on markers of metabolic and cardiovascular risk.