AbstractTo assess the effect of testosterone treatment on bone remodelling and density in dieting obese men, 100 obese men aged 53 years (interquartile range 47–60) with a total testosterone level <12 nmol/L receiving 10 weeks of a very low energy diet (VLED) followed by 46 weeks of weight maintenance were randomly assigned at baseline to 56 weeks of intramuscular testosterone undecanoate (n = 49, cases) or matching placebo (n = 51, controls). Pre-specified outcomes were between-group differences (mean adjusted difference, MAD) in serum c-telopeptide (CTx), N-terminal propeptide of type 1 procollagen (P1NP) and bone mineral density (BMD). At trial end, CTx was significantly reduced in men receiving testosterone compared to placebo, MAD −66 ng/L (95% CI −113, −18), p = 0.018, and this was apparent already after the 10 week VLED phase, MAD −63 ng/L (95% CI −108, −18), p = 0.018. P1NP was marginally increased after VLED, MAD +4.2 ug/L (95% CI −0.01, +8.4), p = 0.05 but lower at study end, MAD −5.6 ug/L (95% CI −10.1, −1.1), p = 0.03. No significant changes in sclerostin, lumbar spine BMD or femoral BMD were seen. We conclude that in obese men with low testosterone levels undergoing weight loss, bone remodelling markers are modulated in a way that may have favourable effects on bone mass.