The Olympic 800m champion Caster Semenya could run up to seven seconds slower under new rules requiring her to lower her natural testosterone levels to race internationally, a prominent sports scientist has predicted.

Under rules due to be announced on Thursday morning by the IAAF, the world athletics governing body, a separate female classification for an athlete with differences of sexual development (or DSDs) will be introduced. Such athletes, including Semenya, will have to reduce and then maintain their testosterone levels to no greater than 5nmol/L by 1 November if they want to compete in events ranging from 400 metres to a mile.

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The International Association of Athletics Federations believes its new rules will “preserve fair and meaningful competition in the female classification” because women athletes with high testosterone have an advantage of up to 9% over women with normal levels of testosterone.

The move, which is sure to divide opinion, was unanimously approved by the IAAF council last month. According to the world-renowned sports scientist Ross Tucker, the effect will be significant on athletes such as Semenya. “If this policy passes, then I would predict that Semenya will be five to seven seconds slower over 800 metres,” he wrote.

“The other really interesting thing is that, guided by the new IOC transgender policy that lowers the upper limit for testosterone to 5nmol/L, the IAAF are setting their limit at 5nmol/L. It used to be 10. That will slow the times of these athletes down even more than it did in the past.”

He told the Guardian: “History suggests Semenya will lose about four seconds to five seconds. Because in 2013 and 2014, when the IAAF were diligent about ensuring her compliance with the upper limit at that time (10nM), she was running 2:00. I don’t think she even qualified for the Commonwealth Games. Now, at 5nM, that effect will be even larger, I suspect. She will go from a 1:54 to 2:01-2:03, is my estimate.”

Semenya burst on to the scene in 2009 when she won the world 800m title but was then required to take testosterone-suppressing medicine by the IAAF in order to compete. However, in July 2015 the court of arbitration for sport reversed that rule when it examined the Dutee Chand case – allowing inter-sex athletes to compete without taking testosterone-suppressing medication.

The decisive factor for the Cas panel in 2015 was whether testosterone above the 10nmol/L threshold set by the IAAF gave female athletes a competitive advantage over their fellow competitors. However research by the IAAF since then shows that in certain events testosterone does make an enormous difference. An IAAF report notes that “most females have low levels of testosterone circulating naturally in their bodies (0.12 to 1.79 nmol/L in blood); while after puberty the normal male range is much higher (7.7 – 29.4 nmol/L). Absent a DSD or a tumour, no female would have serum levels of testosterone approaching 5 nmol/L, but individuals with DSDs can have very high levels of natural testosterone, extending into and even beyond the normal male range.”

The IAAF also makes it clear that science is on its side. In its report it writes: “There is a broad medical and scientific consensus that if these individuals are sensitive to androgens (ie, they have properly-functioning androgen receptors), such very high levels of natural testosterone can increase their muscle mass and strength, as well as their levels of circulating haemoglobin, and so significantly enhance their sporting potential.

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“This evidence shows clearly that (at least in certain events) DSD athletes with levels of circulating testosterone in the normal male range have a very significant competitive advantage over female athletes with testosterone levels in the normal female range.”

It also uses numbers to back up its case. As it notes: “In particular, increasing testosterone levels in women from 0.9 nmol/L to just 7.3 nmol/L increases muscle mass by 4% and muscle strength by 12-26%; while increasing it to 5, 7, 10 and 19 nmol/L respectively increases circulating haemoglobin by 6.5%, 7.8%, 8.9% and 11% respectively. Taking all available knowledge and data into account, the experts estimate that the ergogenic advantage in having circulating testosterone levels in the normal male range rather than in the normal female range is greater than 9%.”

“To the best of our knowledge, there is no other genetic or biological trait encountered in female athletics that confers such a huge performance advantage.”

In explaining its decision, the IAAF said it was trying to create “a level playing field and ensure that success is determined by talent, dedication, hard work, and the other values and characteristics that the sport embodies and celebrates”.

It added: “The IAAF wants athletes to be incentivised to make the huge commitment and sacrifice required to excel in the sport, and so to inspire new generations to join the sport and aspire to the same excellence. It does not want to risk discouraging those aspirations by having unfair competition conditions that deny athletes a fair opportunity to succeed.



“Because of the significant advantages in size, strength and power enjoyed (on average) by men over women from puberty onwards, due in large part to men’s much higher levels of circulating testosterone, and the impact that such advantages can have on sporting performance, it is generally accepted that competition between male and female athletes would not be fair and meaningful, and would risk discouraging women from participation in the sport. Therefore, in addition to separate competition categories based on age, the IAAF has also created separate competition categories for male and female athletes.



It added: “The IAAF respects the dignity of all individuals, including individuals with DSDs. It also wishes the sport of athletics to be as inclusive as possible, and to encourage and provide a clear path to participation in the sport for all. The IAAF therefore seeks to place conditions on such participation only to the extent necessary to ensure fair and meaningful competition.”

In a Q&A sent out with its new rules, the IAAF also anticipated a frequent argument made in this debate – namely that all elite athletes have natural genetic and/or biological advantages (such as height or lung capacity), so why deny DSD athletes the benefit of their natural levels of circulating testosterone?

In response the IAAF wrote: “The IAAF divides competition into male and female classifications because male athletes have clear performance advantages in terms of size, strength and power, as a result (in particular) of increased lean body mass and increased serum haemoglobin, which in turn is due mainly to the fact that, starting from puberty, they produce 10‐30 times more testosterone than women. As noted above, the CAS has found that this difference justifies protecting female athletes from competition from male athletes.”