Sex differences in athletic performance emerge coinciding with the onset of male puberty

Although testosterone levels and muscle mass decline with age, many older men have serum testosterone level in the normal range, leading to speculation about whether older men are less sensitive to testosterone. We determined the responsiveness of androgen-dependent outcomes to graded testosterone doses in older men and compared it to that in young men. The participants in this randomized, double-blind trial were 60 ambulatory, healthy, older men, 60-75 yr of age, who had normal serum testosterone levels. Their responses to graded doses of testosterone were compared with previous data in 61 men, 19-35 yr old. The participants received a long-acting GnRH agonist to suppress endogenous testosterone production and 25, 50, 125, 300, or 600 mg testosterone enanthate weekly for 20 wk. Fat-free mass, fat mass, muscle strength, sexual function, mood, visuospatial cognition, hormone levels, and safety measures were evaluated before, during, and after treatment. Of 60 older men who were randomized, 52 completed the study. After adjusting for testosterone dose, changes in serum total testosterone (change, -6.8, -1.9, +16.1, +49.5, and +101.9 nmol/liter at 25, 50, 125, 300, and 600 mg/wk, respectively) and hemoglobin (change, -3.6, +9.9, +20.9, +12.6, and +29.4 g/liter at 25, 50, 125, 300, and 600 mg/wk, respectively) levels were dose-related in older men and significantly greater in older men than young men (each P < 0.0001). The changes in FFM (-0.3, +1.7, +4.2, +5.6, and +7.3 kg, respectively, in five ascending dose groups) and muscle strength in older men were correlated with testosterone dose and concentrations and were not significantly different in young and older men. Changes in fat mass correlated inversely with testosterone dose (r = -0.54; P < 0.001) and were significantly different in young vs. older men (P < 0.0001); young men receiving 25- and 50-mg doses gained more fat mass than older men (P < 0.0001). Mood and visuospatial cognition did not change significantly in either group. Frequency of hematocrit greater than 54%, leg edema, and prostate events were numerically higher in older men than in young men. Older men are as responsive as young men to testosterone's anabolic effects; however, older men have lower testosterone clearance rates, higher increments in hemoglobin, and a higher frequency of adverse effects. Although substantial gains in muscle mass and strength can be realized in older men with supraphysiological testosterone doses, these high doses are associated with a high frequency of adverse effects. The best trade-off was achieved with a testosterone dose (125 mg) that was associated with high normal testosterone levels, low frequency of adverse events, and significant gains in fat-free mass and muscle strength.

Sex steroids are chief regulators of gender differences in the skeleton, and male gender is one of the strongest protective factors against osteoporotic fractures. This advantage in bone strength relies mainly on greater cortical bone expansion during pubertal peak bone mass acquisition and superior skeletal maintenance during aging. During both these phases, estrogens acting via estrogen receptor-α in osteoblast lineage cells are crucial for male cortical and trabecular bone, as evident from conditional genetic mouse models, epidemiological studies, rare genetic conditions, genome-wide meta-analyses, and recent interventional trials. Genetic mouse models have also demonstrated a direct role for androgens independent of aromatization on trabecular bone via the androgen receptor in osteoblasts and osteocytes, although the target cell for their key effects on periosteal bone formation remains elusive. Low serum estradiol predicts incident fractures, but the highest risk occurs in men with additionally low T and high SHBG. Still, the possible clinical utility of serum sex steroids for fracture prediction is unknown. It is likely that sex steroid actions on male bone metabolism rely also on extraskeletal mechanisms and cross talk with other signaling pathways. We propose that estrogens influence fracture risk in aging men via direct effects on bone, whereas androgens exert an additional antifracture effect mainly via extraskeletal parameters such as muscle mass and propensity to fall. Given the demographic trends of increased longevity and consequent rise of osteoporosis, an increased understanding of how sex steroids influence male bone health remains a high research priority.

Erythrocytosis is a dose-limiting adverse effect of testosterone therapy, especially in older men. Our objective was to compare the dose-related changes in hemoglobin and hematocrit in young and older men and determine whether age-related differences in erythropoietic response to testosterone can be explained by changes in erythropoietin and soluble transferrin receptor (sTfR) levels. We conducted a secondary analysis of data from a testosterone dose-response study in young and older men who received long-acting GnRH agonist monthly plus one of five weekly doses of testosterone enanthate (25, 50, 125, 300, or 600 mg im) for 20 wk. The study took place at a General Clinical Research Center. Participants included 60 older men aged 60-75 yr and 61 young men aged 19-35 yr. Outcome measures included hematocrit and hemoglobin and serum erythropoietin and sTfR levels. Hemoglobin and hematocrit increased significantly in a linear, dose-dependent fashion in both young and older men in response to graded doses of testosterone (P<0.0001). The increases in hemoglobin and hematocrit were significantly greater in older than young men. There was no significant difference in percent change from baseline in erythropoietin or sTfR levels across groups in either young or older men. Changes in erythropoietin or sTfR levels were not significantly correlated with changes in total or free testosterone levels. Testosterone has a dose-dependent stimulatory effect on erythropoiesis in men that is more pronounced in older men. The testosterone-induced rise in hemoglobin and hematocrit and age-related differences in response to testosterone therapy may be mediated by factors other than erythropoietin and sTfR.