According to controversial new regulations, female athletes with as much testosterone in their blood as some men are not allowed to compete in women’s sporting events.

In 2011 and 2012, the International Association of Athletics Federations (IAAF) and the International Olympic Committee (IOC) implemented new rules barring a female from competing in the women’s category if her blood testosterone level is above 10 nanomoles of testosterone per liter (nmol/L).

That’s the bottom end of what’s considered a normal range for men. Women usually have less than three nmol/L, according to Dr. Peter Sonksen of Kings College London. Men tend to have 10 to 30 nmol/L.

But 2.5 percent of women have hyperandrogenism, meaning more testosterone in their blood than the reference range for females, usually due to genetics, Sonksen told Reuters Health by email.

“In most cases it is of no consequence to health but in some cases there may be a tendency to greasy skin and excess body hair and this may be associated with menstrual irregularity and infertility,” and in rare cases can lead to adrenal tumors, he said.

Experts disagree about whether testosterone actually enhances athletic performance, and if this should be a determinant of sports eligibility. That’s one major sticking point in dueling commentaries published in the Journal of Clinical Endocrinology and Metabolism.

“Sports, athleticism in general is inherently unfair,” said Dr. Eric Vilain of the UCLA School of Medicine, who coauthored a commentary in support of the current regulations.

Sporting ability depends on a variety of genetic and environmental factors, including height, proportion and access to high quality training facilities, Vilain told Reuters Health.

“We could have men and women compete against each other in the same categories, that’s a model we would be fine with, but for the exception that women in most events would systematically lose,” he said.

He allowed that segregation of male and female sports is an artificial social construct, but since the construct exists it does need to be enforced somehow.

One option is to have athletes simply identify themselves as male or female.

“All women who have been functioning as such in their everyday lives should be allowed to compete on the field of sport,” said Sonksen, who coauthored the commentary opposing the current regulations.

“The notion that female athletes are in need of protection reflects paternalistic and heterosexist assumptions about what 'real' women should look like and how they ought to perform,” he said.

But allowing women to self-identify would open the door to men competing as women and committing fraud, Vilain said.

“Sports authorities accept the fact that there are huge variabilites between women in terms of their characteristics, but is there a parameter, is there something that we can easily measure that is very different between men and women and relevant to sport performance?” Vilain said.

In the past, the Y chromosome was the determinant of women’s sports eligibility, but the Y chromosome per se is not very relevant to athletic performance, he said. Some women do carry a Y chromosome but are insensitive to male hormones and develop physiologically as a woman. They would be wrongly disqualified by a chromosome rule, he said.

Shifting from the Y chromosome to testosterone takes the emphasis off of male or female identity, Vilain said.

“There is no good evidence that naturally occurring hyperandrogenism enhances athletic performance,” Sonksen said.

Many athletes who “dope” do so with testosterone or related products, Vilain said, noting that if the hormone were not improving performance, athletes would not use it to cheat.

Testosterone is linked to bone and muscle development, and it is not far-fetched to say that it is related to athletic performance, Vilain said.

“It is a natural ‘in-built’ and ‘genetically-determined’ attribute akin to height, hand and foot size, body build etc.,” Sonksen said. “In addition, it is unfair to exclude any athlete based on a genetic condition that they have no control over,” he said.

Depending on the international federation governing the particular sport, T testing is mandatory for some women but not others, he noted. If a female athlete tests high for testosterone, she would need to undergo surgical or hormonal procedures to be eligible for the women’s competition, he said.

However, no one is banned from competing altogether based on testosterone, Vilain said. A woman with T in the male range could compete in the men’s category.

The situation is even more complicated for transgender athletes. According to the IOC Stockholm Consensus, issued in 2003, a male to female transgender athlete may compete if she has had her gonads removed at least two years previously, has taken female hormones and has a legal “change of sex,” which is not an option in many countries and in several U.S. states, Vilain said.

The IOM is looking into revisiting this guideline, he noted.

“At the end of the day, these kinds of biological measures will always be imperfect,” he said.