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

Abstract

Introduction: Serum concentrations of androgens and estrogens, 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 synthesized 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 rodents or healthy males and females free from any hormonal treatment.

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 postmenopausal females yielded inconclusive findings. Chronic exercise decreased androgens and estradiol 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 estradiol and androgens and measures of muscle mass and function.

Conclusion: Current evidence suggests that aging and exercise differentially modulate intramuscular sex hormone concentrations, and their association with muscle mass and function, between males and females and model systems.

Keywords: estrogen; sex hormones; skeletal muscle; testosterone.

Figure 1.

PRISMA diagram outlining the systematic review search and assessment. Abbreviations: PRISMA, Preferred Reporting Items for Systematic Reviews and Meta-Analysis.

Figure 2.

The effects of aging and acute and chronic exercise on intramuscular sex hormones. ↑ = significant positive findings, ↓ = significant negative findings, ↗ = mixed positive and null findings, ↘ = mixed negative and null findings, ↔ = no significant findings,? = to be investigated.