The Science Behind The Unfair Advantage Trans ‘Women’ Have Over Biological Women In Sports

Given that biological males experience a substantial performance advantage over females in most sports, there is currently a debate whether inclusion of transgender women (born male) in the female category of sports would compromise the objective of fair and safe competition.

Sex differences determined by sex steroids during puberty and preserved throughout reproductive life promote diverse physical performance between males and females⁽¹⁾.

The major male hormone testosterone prompts changes in muscle mass, strength, anthropometric variables, and hemoglobin levels⁽¹⁾.
 
In addition, VO2 peak, a measure of maximal cardiopulmonary oxygen consumption is 25-30% or even 50% lower in women than in age-matched men⁽¹⁾.

This is quite a drastic difference between men and women.

Physiologically speaking, these differences are due to lower arteriovenous oxygen difference (a-vO2diff) and lower cardiac output in females⁽²⁾.  

Even truly elite women have VO2max values ~10% lower than those seen in men of similar elite status when expressed as mL/kg/min.

Transgender women (born male) are treated with estrogen therapy with or without antiandrogenic agents to diminish endogenous levels of testosterone. Hormone treatment in younger transgender women aims to mirror estradiol levels in premenopausal women to encourage the development of female characteristics and reduce androgen-related features⁽³⁾.

The performance of transgender women (born male) during physical effort is not well known. There are no data on cardiopulmonary capacity in transgender women. A recent study aimed to elucidate aspects of the sporting abilities of transgender women (born male) after long-term gender-affirming hormone therapy by utilizing gold standard methodologies for analyzing cardiopulmonary and strength capacities⁽⁴⁾.

Major Findings of Cardiometabolic Aspects

Physical performance is determined directly by anthropometric characteristics, strength, and cardiopulmonary capacity⁽⁵⁾. Biological men have greater performance, whether at an amateur or elite level, which is due to having greater muscle mass, strength and VO2 [5].

Thus, current evidence indicates the biological advantage, most notably in terms of muscle mass and strength, granted by male puberty and thus enjoyed by most transgender women (born male) is only minimally reduced when testosterone is suppressed as per current sporting guidelines for transgender athletes⁽⁴⁾. This evidence is relevant for policies regarding participation of transgender women (born male) in the female category of sport.

The most recent data describes the first in the literature on VO2 peak in non-athlete transgender women (born male) undergoing estrogen hormone therapy.

Results indicate similarity between parameters of men and transgender women (born male) undergoing gender-affirming hormone therapy at rest and difference between those of women and transgender women (born male). These data indicate that the cardiopulmonary physiology of transgender women was partially maintained in the male pattern at rest.

However, during physical activity, there was a progressive distancing of the transgender women (born male) parameters from mens performance and an approximation of womens characteristics.

Studies in sports physiology demonstrate an average difference of 25%–35% VO2peak between men and women⁽³⁾, data that were corroborated by our results. For the first time in literature, we present the absolute VO2peak value of transgender women (born male), which was intermediate between the values of the women and men groups.

Transgender women (born male) without a history of pulmonary and/or cardiac diseases on prolonged use of hormone therapy have a expiratory volume with an intermediate value between the values of cisgender men and cisgender women. One explanation for this different expiratory volume pattern identified in transgender women (born male) would be a possible reducing effect on the diameter of the airways by estrogen, described as a potential bronchoconstrictor in the literature⁽⁶⁾.

Major Findings of Body Composition

Analysis indicated that transgender women (born male) after gender-affirming hormone therapy exhibited an intermediate muscle mass value between those of women and men.

Hence, long-term estrogen exposure and testosterone suppression were not enough to completely shift body composition of transgender women (born male) to the female pattern, despite their direct and indirect effect on fat and lean mass⁽⁷⁾.

After starting gender-affirming hormone therapy, increased body weight among transgender women (born male) is often reported as an adverse effect⁽⁵⁾.

The increases in gross weight, fat mass and % fat mass as well as the decrease in skeletal muscle mass among transgender women (born male) on estrogen therapy alone or in association with antiandrogens have been described in several studies in the literature⁽⁸⁾⁽⁹⁾.

Limitations of This Research

Despite the known limitations of cross-sectional studies, these data are useful for establishing preliminary evidence and planning a future advanced study. Larger study is needed, ideally with multiple time points, for more definitive results. This study also did not examine athletes, and therefore these findings may not be applicable to populations that engage in regular intensive exercise.

Summary

These are the first scientific data on the cardiopulmonary capacity of transgender women (born male) undergoing long-term gender-affirming hormone therapy. The absolute mean VO2 peak of non-athlete transgender women (born male) while performing physical exertion was higher than that of non-athlete women and lower than that of men, but there were no differences in relative VO2 peak when adjusted for fat-free mass.

These findings add new insights to the scarce information available on a highly controversial topic about the participation of transgender women in physical activities.

Future research involving transgender athletes that account for and measure variable exposure times to pubertal development and assess muscle cell metabolism are needed to elucidate the effects of long-term gender-affirming hormone therapy on transgender women sports performance.

You can learn more about the role estrogen plays between men and women here.


References:
1.    Handelsman, D.J., A.L. Hirschberg, and S. Bermon, Circulating Testosterone as the Hormonal Basis of Sex Differences in Athletic Performance. Endocr Rev, 2018. 39(5): p. 803-829.
2.    Wilmore, J.H., et al., Cardiac output and stroke volume changes with endurance training: the HERITAGE Family Study. Med Sci Sports Exerc, 2001. 33(1): p. 99-106.
3.    Hembree, W.C., et al., Endocrine Treatment of Gender-Dysphoric/Gender-Incongruent Persons: An Endocrine Society Clinical Practice Guideline. J Clin Endocrinol Metab, 2017. 102(11): p. 3869-3903.
4.    Alvares, L.A.M., et al., Cardiopulmonary capacity and muscle strength in transgender women on long-term gender-affirming hormone therapy: a cross-sectional study. Br J Sports Med, 2022. 56(22): p. 1292-1298.
5.    Hilton, E.N. and T.R. Lundberg, Transgender Women in the Female Category of Sport: Perspectives on Testosterone Suppression and Performance Advantage. Sports Med, 2021. 51(2): p. 199-214.
6.    Richardson, R.S. and B. Saltin, Human muscle blood flow and metabolism studied in the isolated quadriceps muscles. Med Sci Sports Exerc, 1998. 30(1): p. 28-33.
7.    Card, J.W. and D.C. Zeldin, Hormonal influences on lung function and response to environmental agents: lessons from animal models of respiratory disease. Proc Am Thorac Soc, 2009. 6(7): p. 588-95.
8.    Klaver, M., et al., Cross-sex hormone therapy in transgender persons affects total body weight, body fat and lean body mass: a meta-analysis. Andrologia, 2017. 49(5).
9.    Harper, J., et al., How does hormone transition in transgender women change body composition, muscle strength and haemoglobin? Systematic review with a focus on the implications for sport participation. Br J Sports Med, 2021. 55(15): p. 865-872.