Transgender women are stronger and have better heart and lung capacity than women, even 14 years after taking hormone therapy, a study suggests.
New research from Brazilian scientists is likely to increase pressure for transwomen to be excluded from female sports because they have an unfair advantage.
The study found transgender women were around 20 per cent stronger, and their ability to use oxygen during exercise was 20 per cent greater than women who exercised at the same levels.
Researchers concluded that even long-term oestrogen exposure and testosterone suppression were not enough to shift male bodies to those of females.
Writing in the British Journal of Sports Medicine, Prof Leonardo Alvares, of the University of Sao Paulo Hospital Clinics, said: “These findings add new insights to the sparse information available on a highly controversial topic about the participation of transgender women in physical activities.
“It could inform policy and help in decisions about the participation of transgender women in sporting activities.”
Several sports governing bodies have banned transgender women from competing in recent months including the International Rugby League, the International Swimming Federation and the International Cycling Union.
For the study researchers assessed heart-lung capacity and strength in 15 transgender women, 13 men, and 14 women. All the volunteers were in their mid-thirties and took part in similar levels of physical activity.
The transgender women had been on hormone therapy for an average of 14 years, which they had started when they were 17, on average.
Exposure to testosterone induces changes in muscle mass, strength, body fat and red blood cell capacity, and researchers were keen to find out if that effect was long-lasting even after suppressing the hormone.
To find out they measured how efficiently the body transports and uses oxygen - known as VO2 - which can be 50 per cent lower in men than women. VO2 is believed to be the best indicator of aerobic capacity.
The study showed that in transgender women, while VO2 level had dropped significantly compared to men, it was still higher than in women.
While the VO2 peak in transgender women was around 2,606, in women it was 2,167. Men scored 3,358.
‘Doesn’t support eligibility of transwomen for female categories’
Likewise, when grip strength was measured - which is a proxy for general muscle condition and overall strength - transgender women achieved around 35kg, compared to women around, who managed 29kg on average.
Commenting on the research, Alun Williams, Professor of Sport and Exercise Genomics, at Manchester Metropolitan University, said: “This new evidence doesn’t support the eligibility of transwomen for the female categories of most sports.”
But he warned that the research had not been carried out on athletes.
“Studies of well-trained athletes before and during their treatment, using tests that are more accurate and more relevant to competitive sport, are required to better inform sport eligibility policies,” he added.
Dr Channa Jayasena, a clinical senior lecturer and consultant in reproductive endocrinology and andrology, at Imperial College London, said: “This suggests that there are likely to be small differences in how the muscles work between trans and cisgender women. However, none of the participants were athletes.
“It is possible that there are similar differences in muscle function found among cis-gender athletes.”