Despite greater muscle mass of transgender women 1-3 years after hormone therapy
Evidence is of variable quality, but doesn’t back inherent athletic advantage theories
Transgender women might have more muscle mass than cisgender women 1 to 3 years after hormone therapy, but their physical fitness is comparable, finds a pooled data analysis of the available evidence, published online in the British Journal of Sports Medicine.
While the current body of evidence is of variable quality, and doesn’t look at the potential for any advantage at the elite athlete level, it doesn’t back up prevailing theories about the inherent athletic advantage of transgender women, conclude the researchers.
Whether transgender women should be allowed to compete in female sports, and under what conditions, remains hotly contested, explain the researchers. Policies advocating blanket bans on transgender women’s participation in female sports often cite residual advantages from previous testosterone exposure, they add.
While previously published studies suggest that hormone therapy may alter body composition in transgender people, the evidence on their functional performance and physical advantage is mixed, they point out.
In pursuit of clarification, the researchers scoured research databases for studies comparing the body composition or physical fitness of transgender people before and after hormone therapy with that of cisgender people.
In all, 52 studies involving 6485 people (2943 transgender women, 2309 transgender men, 568 cisgender women and 665 cisgender men; age range 14 to 41) were eligible for review.
The studies varied in design and methodology. Some 45 focused on adults; 7 involved teenagers. Only 16 included any form of physical activity assessment, and most didn’t compare transgender athletes with cisgender athletes. And only 7 adjusted for potentially influential factors, including body composition, hormone levels, and nutrient intake.
In all, 22 studies were prospective; 9 were retrospective; 17 were cross-sectional (observational studies): 3 were randomised controlled trials; and 1 was a quasi-experimental study.
Pooled data analysis of the findings from 46 of the studies showed that transgender women’s body composition differs from that of both cisgender men and cisgender women.
Transgender women have significantly greater amounts of body fat than cisgender men but levels comparable to those of cisgender women.
And while transgender women had more lean mass, a proxy for muscle, there were no observable differences in upper or lower body strength, or in a key measure of cardiorespiratory fitness-maximal oxygen consumption (VO₂ max)-between them and cisgender women.
Transgender women’s upper and lower body strength and their VO₂ max were also all much lower than they were in cisgender men.
While hormone therapy was associated with higher amounts of body fat and lower amounts of muscle and less upper body strength 1-3 years after the start of treatment
in transgender women, transgender men had less fat, more muscle, and greater strength after hormone therapy.
The researchers acknowledge various limitations to their findings, including short study length and an absence of information on elite athletes. Few studies assessed specific outcomes or the impact of puberty suppression.
And reporting on, and adjustment for, potentially influential factors, such as training history, diet, baseline fitness, physical activity and body composition or previous hormone therapy, were inconsistent. Few studies included a broad spectrum of ages, types of sport, and competitive levels.
Nevertheless, the findings prompt the researchers to conclude: “The convergence of transgender women’s functional performance with cisgender women, particularly in strength and aerobic capacity, challenges assumptions about inherent athletic advantages derived solely from [gender affirming hormone therapy] or residual lean mass differences.”
They add: “Although the current data do not justify blanket bans, critical gaps in literature were found, notably the under-representation of transgender athletes who may retain more ‘muscle memory’.
“Ideally, to resolve speculation, future long-term, longitudinal studies should prioritise performance-specific metrics in transgender athletes. However, one should be aware of the scarce number of transgender athletes, particularly in the elite sport, which complicates the feasibility of conducting powered studies involving high-performance transgender athletes within specific sport disciplines.”
And they emphasise: “Considering this context of imperfect evidence and despite the methodological challenges, continued research into physiological as well as psychosocial trajectories among transgender athletes with diverse demographics and clinical characteristics remains essential for developing equitable frameworks that balance justice, inclusion and scientific rigour.”
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