A few years ago, transgender athlete Lia Thompson was the 462nd-ranked men’s swimmer in the country. Today, Thompson is No. 1 nationally in the women’s division. Does Lia have an unfair advantage? What does the science say?
Let’s save you and me much time. There is no scientific study on the hormonal effects of hormone replacement therapy on biologic male athletes transitioning to transwomen athletes.
“As trans women have fought for inclusion in women’s sports, various governing bodies have implemented rules for mitigating any physical advantages that they might have. But just what those advantages are and how to counteract them—and whether that is necessary or even possible—has been fiercely debated.” The New Yorker
And to level the playing field of definitions, gender reflects a range of characteristics, defining and differentiating what we might also term feminine or masculine. Those characteristics may be our social roles, perceptions, or the presence or absence of biological chromosomes, organs, or hormones. Our culture, writ large, has successively redefined gender based on external appearance (phenotype), chromosomes (genotype), and hormonal levels, most recently, in the case of runner Caster Semenya.
Our scientific thinking has sought to isolate the characteristic of gender; in doing so, we ignore many other biologic considerations, including the relationship of one hormone to another and how the actions of hormones may wax and wane over more extended periods than just a few weeks or months. A systemic review in the British Journal of Sports Medicine explores these equally important considerations.
In a meta-analysis, researchers reviewed 24 studies on “non-athletic transwomen” undergoing gender-affirming hormone therapy (GAHT). That therapy generally includes an anti-testosterone agent and estrogens to create a hormonal environment reducing masculinization and promoting feminization. There is no evidence that it creates a cis female's hormonal milieu and cyclicity.
The research studies all considered measures of muscle strength and oxygen delivery, arguably necessary but not sufficient factors in athletic performance.  They focused on three measures of muscle, lean body mass, muscle strength, and cross-sectional area, and two measurements of endurance, hemoglobin, and hematocrit, indirect measures of efficacy of oxygenation transportation and delivery . Testosterone increases all these measures, so presumably, and this is a crucial disclaimer, anti-testosterone therapy will reverse those effects.
As with all meta-analyses, there was a hodgepodge of participants, GAHT therapies, study intervals, and types. More importantly,
- All of the studies involved small numbers of participants, the largest involved 277 participants.
- While most transwomen “achieved testosterone levels within the reference range for cisgender women,” several included studies had levels greater than generally accepted, which may muddy the results.
Let's focus on the qualitative findings rather than argue the meaning of the quantitative measures.
- All of the parameters of interest were diminished to varying degrees in the transwomen by GAHT therapy.
- At 12 months, both lean body mass, how much muscle is present, and cross-sectional muscle size were significantly reduced.
- But the loss of muscle strength ranged from non-significant to 7%. “…after 12-36 months of GAHT suggests that transwomen likely retain a strength advantage over cisgender women.”
- The hemoglobin and hematocrit, the oxygen delivery essential to endurance, of transwomen were identical to ciswomen at four months.
What to conclude:
It is, once again, more complex than we anticipated. Attempts to “mimic” a female hormonal environment failed to convert men to women with respect to muscle mass and strength. Transwomen may be more readily comparable to ciswomen in isolated aspects (hemoglobin and hematocrit) of oxygen delivery’s physiology. But the effect of additional muscle strength throws a wrench in the theory that hormonal treatments can be "great equalizers." This, as the author suggests, means that athletic activities requiring endurance may have a different equipoise between transwomen and ciswomen than athletic activities requiring strength.
“Although a level playing field in sport is illusory, it is important that opportunities for women to engage in meaningful competition within the female category exist. Whether transgender and cisgender women can engage in meaningful sport, even after GAHT, is a highly debated question.”
 Additional factors include upper and lower body proportion, coordination, training, perceptual abilities, and experience, among others
 Hemoglobin is the molecule that carries oxygen throughout the body, and hematocrit refers to the ratio of red blood cells (that contain the hemoglobin) to serum.
Source: 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. British Journal of Sports Medicine DOI: 10.1136/bjsports-2020-103106