Sinha-Hikim I et al. (2006) Effects of testosterone supplementation on skeletal muscle fiber hypertrophy and satellite cells in community dwelling, older men. J Clin Endocrinol Metab [doi: 10.1210/jc.2006-0357]

Testosterone is a potential treatment for physical dysfunction due to age-related decreases in muscle mass. These decreases are largely the result of a loss of type II skeletal muscle fibers, and testosterone therapy has been shown to increase muscle mass. A group of US investigators has found that administration of exogenous testosterone results in dose-dependent increase in the number of both type I and type II fibers.

Endogenous testosterone production was blocked in 36 healthy men (aged 60–75 years), who subsequently received testosterone enanthate injections (25 mg, 50 mg, 125 mg, 300 mg, or 600 mg) weekly for 20 weeks. Analysis of cross-sectional biopsies of the participants' thigh muscles taken before and after treatment showed that the number of type I and type II fibers increased dose-dependently. The number of myonuclei and satellite cells also increased dose-dependently (only with the three highest doses); this finding suggests that testosterone promotes entry of myogenic precursor cells into the cell cycle. Further analysis showed that the increased number of satellite cells was the result of satellite-cell replication and activation.

Although these results are highly encouraging, the use of high doses of testosterone to rebuild skeletal muscle in older men is limited by adverse effects. The authors call for further study of the mechanisms involved, in the hope that new targets for anabolic therapy will be discovered.