Transgender women exhibit strength and fitness similar to cisgender women months after hormone therapy, according to a comprehensive review of studies that challenges claims about trans athletes reaping advantages in women’s sports.
The inclusion of trans athletes in women’s sports remains hotly debated, with fairness concerns raised due to biological differences.
While initial research suggests that gender-affirming hormone therapy can alter body composition in trans individuals, how this links to functional performance remains unclear.
Current policies in some sports advocate for a blanket ban on trans athletes in women’s sports, citing advantages from early testosterone exposure since birth.
Now, a landmark study challenges this notion, finding that despite larger muscle mass about one to three years after hormone therapy, trans women have physical fitness levels comparable to those of cis women.
In the new research, Brazilian scientists scoured research databases for studies comparing the body composition or physical fitness of trans people before and after hormone therapy with that of cis people.
Overall, about 50 studies were analysed, involving 6485 people – 2943 trans women, 2309 trans men, 568 cisg women and 665 cis men, falling in an age range of 14 to 41.
The pooled data analysis from these studies showed that trans women’s body composition differs from that of both cis men and cis women.
Trans women in the studies were found to have significantly greater amounts of body fat than cis men, but levels comparable to those of cis women.
However, while trans women appeared to have more muscle mass, there were no observable differences in upper or lower body strength, the study found.
Scientists suspect there could be residual musculoskeletal differences that may not confer an athletic edge.
There was also no observable difference between trans women and cis women in their maximal oxygen consumption (VO₂ max), a key fitness measure.
In the case of trans men, after one to three years of starting hormone treatment, they had less fat, more muscle, and greater strength.
Citing a key limitation of the research, researchers said studies included in the analysis mainly focused on physiological outcomes, “with little consideration of the social, psychological and cultural factors”, which also shape sport performance.
The concept of “muscle memory”, or the capacity for previous exercise-adapted muscles to retain some advantages even after hormonal alterations, remains unexplored, according to scientists.
Scientists call for future long-term studies to prioritise performance-specific metrics in trans athletes.
“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,” they wrote.
However, they say the latest findings still indicate that physical performance in trans women converges toward that of cis women over time.
“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,” researchers wrote in the study.
Experts, who were not involved in the study, urge caution in interpreting the findings, though many appreciate it as a step in the right direction.
“These findings are interesting, but they need to be interpreted carefully,” said Kim Meredith-Jones, the director of the Bone and Body Composition Research Unit at the University of Otago.
“Although this review included a large number of transgender participants overall, most were adults. This means the results cannot tell us what happens for young people who use puberty blockers or begin medical transition during adolescence. That remains an important evidence gap,” Dr Metedith-Jones said.
Endocrinologist Ada Cheung from The University of Melbourne said there needs to be evidence-based and sport-specific guidelines instead of “one-size-fits-all rules”.
“Blanket bans on transgender women in sport are not supported by the best available evidence,” Dr Cheung said, adding that “fairness and inclusion can coexist”.
“Instead of blanket bans, we need eligibility criteria for elite sport, and we should focus on the real work of supporting women’s sport: improving visibility and pay, reducing sexual harassment and assault, expanding access to facilities and coaching, and ensuring fair media coverage,” she said.
“These changes actually advance equity, while building sporting systems that are safe, inclusive, and grounded in evidence,” the endocrinologist added.
