What are the biological differences in muscle mass, lung capacity, and testosterone levels between biological males and females?

Biological Sex Differences in Muscle Mass, Lung Capacity, and Testosterone

Biological males have approximately 10-fold higher circulating testosterone levels than females, which directly drives substantially greater muscle mass, strength, and power, particularly after puberty. 1

Testosterone Level Differences

Circulating testosterone concentrations in women are roughly 10-fold lower than those in men. 1 This fundamental hormonal difference is the primary driver of sex-based physiological differences in body composition and physical performance.

• The FDA confirms that endogenous androgens, particularly testosterone, are responsible for male sex organ development, male hair distribution patterns, laryngeal enlargement, vocal cord thickening, and alterations in body musculature and fat distribution. 2
• Testosterone promotes protein anabolism and decreases protein catabolism, improving nitrogen balance when adequate calories and protein are available. 2

Muscle Mass Differences

Males demonstrate profoundly greater absolute muscle mass, strength, and power compared to females, with these differences becoming most pronounced after puberty when male testosterone levels increase dramatically. 3

Mechanisms of Testosterone's Effects on Muscle

• Testosterone increases muscle protein synthesis and net muscle protein balance, resulting in increased muscle mass through both anabolic (protein synthesis promotion) and anti-catabolic (protein degradation inhibition) effects. 4, 5
• In healthy young and older men, testosterone increases muscle mass and strength in a dose-dependent fashion, with appreciable hypertrophic response occurring within the physiologic range of circulating testosterone levels. 1
• Males and females have similar testosterone amounts until puberty, after which testosterone levels increase dramatically in males, as does muscle mass. 5

Quantitative Differences

• Transgender men receiving testosterone therapy demonstrate a 15% increase in quadriceps cross-sectional area and thigh muscle volume, with 6% increase in radiological density, along with improved performance on knee extensions and flexion. 1
• After 1 year of testosterone treatment, transgender men were able to equal or outperform cisgender men in numbers of pushups, situps, and run times. 1
• At physiologic levels, men with total testosterone in the highest quartile have significantly more lower-body lean mass (22.1% more) and upper-body lean mass (5.6% more) compared to those in the lowest quartile. 6

Body Composition Differences

Higher testosterone levels are associated with greater lean mass and reduced fat mass in a dose-dependent manner. 6, 7

• Men with higher physiologic testosterone levels have less lower-body fat mass (9.9% less) and upper-body fat mass (6.1% less) than those with lower levels. 6
• Testosterone deficiency is associated with increased fat mass (particularly central adiposity), while testosterone therapy decreases fat mass and increases muscle mass. 7
• Testosterone at the molecular level controls expression of regulatory proteins involved in glycolysis, glycogen synthesis, and lipid and cholesterol metabolism. 7

Lung Capacity and Respiratory Differences

While the provided evidence focuses primarily on muscle mass and testosterone, the guideline literature addresses respiratory parameters in the context of pulmonary rehabilitation studies. The evidence does not provide specific quantitative data on baseline sex differences in lung capacity between healthy males and females, though respiratory muscle strength differences would be expected given the overall muscle mass differences driven by testosterone. 1

Clinical Implications

The profound sex-based differences in physical performance involving strength, power, speed, and endurance are largely attributable to the direct and indirect effects of sex-steroid hormones, sex chromosomes, and epigenetics. 3

Important Caveats

• High testosterone doses in women are inevitably associated with virilization, making it difficult to identify doses that yield muscle hypertrophy without masculinizing effects. 1
• Testosterone administration in men carries dose-dependent side effects including increased hemoglobin, decreased HDL cholesterol, and potential enhancement of prostate cancer growth. 1
• In female athletes, energy deficiency can disrupt normal hormonal function, potentially affecting testosterone levels and other endocrine markers. 8