The Myth of Equal Odds

Photo by Sabri Tuzcu on Unsplash

What hormone therapy does to physical performance — a year of transition, seen through the lens of science.

People are not equal. They are not equally tall, are not born with identical genetics, do not grow up in the same circumstances, and do not share the same access to food. I could go on listing the ways two people differ, even if they’re similar in sex, body weight, height, hair, and eye color. So who dares to claim that an athlete — even within a strictly defined age group — enters a competition against an opponent with truly equal odds?

This piece isn’t trying to argue that biological differences don’t matter. It’s trying to do the opposite: to show how complex and how far from black-and-white this question actually is — based on my own year of hormone therapy and on the scientific literature currently available.

What I feel in my body

I started estrogen therapy a year ago. The change hasn’t been small: in terms of physical strength, I’m a shadow of my former, male self. My shoulders are narrower, more sloped. I tire faster. This isn’t imagination — it’s exactly the process the scientific literature documents.

A systematic review published by Harper and colleagues (2021) — which appeared in the British Journal of Sports Medicine — reviewed four scientific databases and found that hemoglobin and hematocrit levels drop relatively quickly, within a few months, into the range typical of cisgender women following testosterone suppression and estrogen replacement in transgender women.[¹]

Unfortunately, the studies reviewed observed general clinical populations rather than elite athletes, and covered too short a time span. This leaves an important open question: the effect of hormone therapy isn’t a sprint — it’s more like a decade-long, potentially lifelong process — and the research doesn’t tell us whether further decline, stagnation, or some kind of equilibrium follows after the 36-month mark.

A narrower study, a broader question

A newer, 2025 study moves past both of these limitations — though, as we’ll see, it trades them for a different one entirely. This study specifically examined competitive transgender women runners and swimmers before and after hormone therapy, and among the participants were people who had been on HRT for years — some for as long as 86 months, nearly 7 years. The results showed that running performance declined by an average of 14.6% after starting HRT — ranging from 6% to 24% among participants — with the decline larger in middle- and long-distance events than in sprints.[²]

This study represents an important methodological advance, because it specifically used the athletes’ own pre-HRT results as the point of comparison, rather than a general cisgender female average. At the same time, the sample size is extremely small — just 9 runners and 1 swimmer — which severely limits how much can be generalized from it. This is more of an important signal for future research than a definitive answer, and it points to the continued need for larger, long-term studies.

About bones — what we talk about less, and where a myth persists

Something discussed even less often: bone density. Bone isn’t a static structure — it’s a living tissue that’s constantly being remodeled, regulated by sex hormones. Many people assume — and honestly, I used to think this too — that HRT automatically increases the risk of osteoporosis. The available long-term data doesn’t support this.

A comprehensive review by T’Sjoen and colleagues, published in Endocrine Reviews, describes how sex hormones play a key role in bone growth and maintenance.[³] A 10-year follow-up study by Wiepjes and colleagues, published in the Journal of Bone and Mineral Research (involving 711 transgender women and 543 transgender men), found that hormone therapy did not decrease bone density in either transgender women or transgender men — the researchers’ explicit conclusion was that HRT has no negative effect on bone density, and that regular bone density monitoring during HRT is therefore unnecessary.[⁴]

Both studies were published in 2019. The one genuine warning sign: nearly 22% of the transgender women in the study already had low bone density before starting HRT — but this isn’t a consequence of hormone therapy; it’s a pre-existing condition that a baseline assessment can screen for.

Where I need to be even more careful

The popular claim about ultra-endurance sports — that the performance gap between men and women “virtually disappears” — is an overstatement. The reality is more nuanced. Several studies confirm that the performance gap between the sexes narrows as distance increases — from roughly 10–14% at marathon distances to as little as 4% at ultramarathon distances[⁵] — but it doesn’t disappear, and it doesn’t reverse as a general rule.

Several physiological mechanisms have been proposed in the literature that could theoretically narrow the gap between the sexes at extreme distances — a higher proportion of fatigue-resistant Type I muscle fibers, more efficient fat metabolism, more even pacing. However, analysis of actual competition results and world records doesn’t support the idea that these mechanisms translate, in practice, into a large advantage favoring women — the gap narrows, but it doesn’t disappear, and it doesn’t reverse.

A 2025 meta-analysis examining data from 15 studies across swimming, cycling, running, and triathlon still found an average 22% performance gap between the sexes in ultra-distance sports.[⁶]

The question of competitive advantage for transgender women in sport is itself an actively contested scientific field, not a settled consensus. While Harper and colleagues’ (2021) review argues that hormone therapy significantly reduces physical advantages, another serious, widely-cited study — Hilton and Lundberg (2021), in Sports Medicine — concluded that testosterone suppression does not fully eliminate the muscle mass and strength advantage originating from male puberty, even years after starting hormone therapy.[⁷]

These two research groups continue to debate each other in the literature to this day — this is an open, ongoing scientific discourse.

What this means

I don’t believe this debate has a single, simple answer. What I can state with confidence — because I live it in my own body, and because the science confirms it — is that hormone therapy causes a real, measurable, and significant physiological transformation: muscle strength, oxygen-carrying capacity, and bone density all change, and this happens over months, not years. What I cannot state with confidence — because science itself doesn’t yet know for certain — is whether this transformation fully levels the competitive advantage across every sport, every distance, every athlete.

This debate isn’t about whether transgender people are “cheating,” or whether they’re “not really” women or men. It’s about how sport — which organizes people into strict categories, age groups, and weight classes precisely to approximate fairness — can remain just alongside a biological reality that is far more complex and individual than any single category could ever fully capture.

On framing the debate — before anyone passes judgment

There’s an important distinction I need to draw between two things that are frequently conflated.

The first: there is a good-faith, scientifically grounded debate about how fairness in sport can be reconciled with the inclusion of transgender athletes. This debate — as the disagreement between Harper and Hilton & Lundberg above demonstrates — takes place among genuine, serious researchers, and there’s no clear winner. This debate shouldn’t be suppressed or demonized; it’s science functioning as it normally does.

The second, and this needs to be sharply separated: the kind of public discourse that begins by pathologizing transgender existence itself, and derives the case for exclusion from sport from that premise. This isn’t scientific debate — it’s prejudice — and there’s a very specific, verifiable fact that supports this distinction. In 2019, the World Health Organization, in the newest edition of the International Classification of Diseases (ICD-11), removed transgender identity (“gender incongruence”) from the mental disorders chapter and moved it into a separate chapter on “conditions related to sexual health” — acknowledging that it is not a disorder.[⁸] This marks a fundamental difference from a category like transvestism, which — in the previous classification (ICD-10, code F64.1) — was a distinct classification tied to sexual preference, and which the new classification no longer maintains in the same form. Being transgender, then, is not a sexual deviance or disorder — according to the WHO’s official position, it is not a disorder at all.

When someone frames the debate about sport by first characterizing transgender existence as degenerate, disordered, or deviant, that isn’t scientific reasoning — it’s prejudice that decides the conclusion in advance, before looking at any data. This is the same logical structure as claiming that women are inherently kind, soft, nurturing, and caring, while men are inherently hard, rough, and violent. These aren’t biological laws — they’re social templates that individual reality continually disproves — exactly as I wrote at the start of this piece: people are not equal, but they can’t be sorted into a single group identity either.

The scientific debate, then, can and should continue — but only when it’s genuinely about the physiological data, not about treating someone’s existence as inherently a disturbance or a threat.

References:

[¹] Harper, J., O’Donnell, E., Sorouri Khorashad, B., McDermott, H., & Witcomb, G. L. (2021). “How does hormone transition in transgender women change body composition, muscle strength and haemoglobin?” British Journal of Sports Medicine, 55(15), 865–872. https://doi.org/10.1136/bjsports-2020-103106

[²] Harper, J., Blagrove, R. C., Hunsicker, E., Witcomb, G. L., Ferguson, R. A., & O’Donnell, E. (2025). “Longitudinal Performance Changes in Transgender Women Athletes Pre and Post Gender Affirming Hormone Therapy.” European Journal of Sport Science, e70036. https://doi.org/10.1002/ejsc.70036

[³] T’Sjoen, G., Arcelus, J., Gooren, L., Klink, D. T., & Tangpricha, V. (2019). “Endocrinology of Transgender Medicine.” Endocrine Reviews, 40(1), 97–117. https://doi.org/10.1210/er.2018-00011

[⁴] Wiepjes, C. M., de Jongh, R. T., de Blok, C. J., et al. (2019). “Bone Safety During the First Ten Years of Gender-Affirming Hormonal Treatment in Transwomen and Transmen.” Journal of Bone and Mineral Research, 34(3), 447–454. https://doi.org/10.1002/jbmr.3612

[⁵] Besson, T., Macchi, R., Rossi, J., et al. (2022). “Sex Differences in Endurance Running.” Sports Medicine, 52(6), 1235–1257. https://doi.org/10.1007/s40279-022-01651-w

[⁶] Sitko, S., López-Laval, I., Cirer-Sastre, R., & Valenzuela, P. L. (2025). “Ultra-endurance performance: Are women closing the gap? A systematic review and meta-analysis.” https://doi.org/10.1177/17479541251354909

[⁷] Hilton, E. N., & Lundberg, T. R. (2021). “Transgender Women in the Female Category of Sport: Perspectives on Testosterone Suppression and Performance Advantage.” Sports Medicine, 51(2), 199–214. https://doi.org/10.1007/s40279-020-01389-3

[⁸] World Health Organization, ICD-11 (2019) — classification of “gender incongruence.” https://icd.who.int/browse11/l-m/en