(CNN)The president of World Athletics, Sebastian Coe, reportedly said that the integrity and future of women's sport is "fragile" and defended his governing body's rules on testosterone. His comments came days after American swimmer Lia Thomas became the first transgender woman to win an NCAA swimming championship last week.
World Athletics president calls future of women's sport 'fragile,' defends testosterone regulations -- Times of London
"The integrity of women's sport if we don't get this right, and actually the future of women's sport, is very fragile," Coe said in remarks at the World Indoor Athletics Championships in Belgrade, according to The Times of London.
University of Pennsylvania swimmer Thomas last week became the first out transgender athlete to win an NCAA Division I title after finishing first in the women's 500-yard freestyle event.
Thomas, who previously swam for the men's team at UPenn, has been the target of intense scrutiny in the US as many states have moved to limit the participation of transgender women in women's sports. Her success has sparked questions, especially in right-wing media and among Republican politicians, about what makes for fair competition and who gets to be counted as a woman.
According to World Athletics rules, which govern track and field events -- and do not apply to swimming -- a transgender woman must demonstrate that she has had testosterone levels continuously below 5 nmol per liter for a period of at least 12 months to be allowed to compete, and must keep those levels "for so long as she wishes to maintain her eligibility to compete in the female category of competition."
But some athletes do not fit neatly into these rules. The governing body considers an athlete to be transgender if the person's "gender identity ... is different from the sex designated to them at birth." But the rules do not specify how, and when, an athlete should prove the sex they were assigned at birth. It is also unclear whether the rules apply to athletes whose gender is nonbinary. The organization has a separate set of rules for athletes with differences of sex development (who are sometimes known as intersex).
"There is no question to me that testosterone is the key determinant in performance," Coe said.
"Look at the nature of 12 or 13-year-old girls. I remember my daughters would regularly outrun male counterparts in their class but as soon as puberty kicks in that gap opens and it remains. Gender cannot trump biology," Coe said in an interview with the Daily Telegraph.
But a 2017 study in the journal Sports Medicine found "no direct or consistent research" of trans people having an athletic advantage over their cisgender peers.
"You can't be oblivious to public sentiment, of course not," he said. "But science is important."
"If I wasn't satisfied with the science that we have and the experts that have been working on this for a long time, this would be a very different landscape," Coe said, according to The Telegraph.
CNN has reached out to World Athletics for comment on Coe's statements but has yet to receive a reply.
Thomas has not spoken publicly since an interview with the SwimSwam podcast in December. In that interview, she nodded in the direction of the controversy but did not engage.
"We expected there would be some measure of pushback by some people. Quite to the extent that it has blown up, we weren't fully expecting," she said. "I just don't engage with it. It's not healthy for me to read it and engage with it at all, and so I don't."
There are several different hormones that are naturally produced at a range of levels in people of all genders and sexes. Sensitivity to hormones can also affect the development of anatomy and may cause anatomical variations that are not associated with typical binary categories of male or female. There is debate in the scientific community as to whether androgenic hormones like testosterone are useful markers of athletic advantage.
For over a decade, the NCAA has required transgender women to be on testosterone suppression treatment for a year before they are allowed to compete on a women's team.
But in January, the NCAA said it would take a sport-by-sport approach to its rules on transgender athletes' participation and defer to each sport's national governing body.
USA Swimming then released a set of stricter guidelines that require elite trans women athletes to have at least three continuous years of testosterone levels below 5 nmol per liter, and to prove to a panel of medical experts that they do not have a competitive advantage over cisgender women.
The new rule threatened to make Thomas ineligible to compete at the NCAA championships. However, the NCAA said those rules will be instituted in a phased approach over the coming seasons rather than in the middle of the current season.
Last week, Reka Gyorgy of Virginia Tech wrote in an open letter published on the website of Swimming World magazine and shared by Republican Rep. Dan Crenshaw on Twitter that it was "disrespectful" for the NCAA to allow Thomas to compete against what Gygory referred to as "biologically female (swimmers)," using a term that does not have one standard medical definition, and is sometimes used to suggest -- contrary to science -- that there are characteristics shared by all cisgender women that differentiate them from all transgender women.
"I'd like to point out that I respect and fully stand with Lia Thomas; I am convinced that she is no different than me or any other D1 swimmer who has woken up at 5am her entire life for morning practice. She has sacrificed family vacations and holidays for a competition," Gyorgy wrote.
"She has pushed herself to the limit to be the best athlete she could be. She is doing what she is passionate about and deserves that right. On the other hand, I would like to critique the NCAA rules that allow her to compete against us," she added.
After Thomas' victory last week, several news sites had alleged that the three college swimmers beaten by Thomas staged a protest on the podium.
However, Erica Sullivan, Thomas' closest competitor, clarified in an Instagram post that she had been subjected to "false claims on Right Wing media," and that the image, where she was posing with fellow Tokyo Olympic competitors, had been taken out of context. In the same post, she also published an image of her shaking Thomas' hand.
In an opinion piece for Newsweek, she wrote: "I have been given a platform to advocate for my community, and I can't sit silently by as I see a fellow swimmer's fundamental rights be put up for debate. All swimmers embody a diverse set of identities and characteristics. What makes us each unique also contributes to our success in the pool.
"Yet no one questions the validity of how cisgender athletes' unique traits and skills, or w