The Role and Regulation of Intramuscular Sex Hormones in Skeletal Muscle: A Systematic Review

Abstract Introduction Serum concentrations of androgens and oestrogens, the main male and female sex hormones, respectively, naturally fluctuate across the lifespan. Sex hormones are mainly produced in the gonads, but evidence suggests that they can also be locally synthesised in skeletal muscle. However, little is known about the purpose of intramuscular sex hormones and their role in skeletal muscle. This systematic review aimed to investigate 1) how intramuscular sex hormone concentrations vary across the lifespan, 2) whether exercise affects intramuscular sex hormone concentrations, and 3) whether intramuscular sex hormones are associated with skeletal muscle mass and function. Methods Four databases were searched, and studies were included if they contained measurements of intramuscular sex hormones from healthy males and females free from any hormonal treatment or from rodents. Results Thirteen studies were included. Intramuscular testosterone was reduced in older males compared to their younger counterparts, but comparison of intramuscular sex hormone concentrations between pre- and post-menopausal females yielded inconclusive findings. Chronic exercise decreased androgens and oestradiol in females, but increased androgens in males. Acute exercise did not change intramuscular hormone concentrations in humans but increased them in rodents. Intramuscular androgens were positively associated with muscle mass and strength in males. In females, conflicting findings were reported for both oestradiol and androgens, and measures of muscle mass and function. Conclusion Current evidence suggests that ageing and exercise differentially modulate intramuscular sex hormone concentrations, and their association with muscle mass and function, between males and females, and model systems.