Case Studies in Physiology: Male to female transgender swimmer in college athletics.

There is current scientific and legal controversy about sports competition eligibility regulations for transgender athletes. In this case study, we quantified performances by an elite, transgender woman (male sex, female gender identity) college swimmer who competed in both the men's and women's National Collegiate Athletic Association (NCAA) categories. We also contextualized her performances with respect to world-record performances and contemporary elite college swimmers. These data demonstrate that the declines in freestyle swimming performances of a transgender woman after about 2 yr of reported feminizing gender-affirming hormone treatment (0.5% for the 100 to 7.3% for the 1,650 yard distance) are smaller than the observed sex-related differences in performance of top 200 world record performances in metric distances of similar durations (11.4% for the 100 m to 9.3% for the 1,500 m distance). Despite slower performances, the transgender woman swimmer experienced improvements in performance for each freestyle event (100 to 1,650 yards) relative to sex-specific NCAA rankings, including producing the best swimming time in the NCAA for the 500-yard distance (65 ^th in the men's category in 2018-2019 to 1 ^st in the women's, 2022). Similarly, NCAA-ranked male swimmers had no improvements in rank in the men's category during the same time frame. Our findings suggest that the performance times of the transgender woman swimmer in the women's NCAA category were outliers for each event distance and suggest that the transgender woman swimmer had superior performances relative to rank-matched swimmers. Our analysis may be useful as a framework for regulators considering participation guidelines, which promote fair competition for all athletes-irrespective of gender identity. NEW & NOTEWORTHY This case study, longitudinal analysis of freestyle swimming performances before and after 2 yr of feminizing gender-affirming hormone therapy of an elite transgender woman (male sex, female gender identity), demonstrates superior performance relative to rank-matched female swimmers and a lower performance gap than previously observed between elite male and female swimmers.