Some female athletes have no or irregular periods
A common cause of menstrual disorders in women may offer a significant advantage to some athletes, according to a Swedish scientist.
Some leading female athletes have either no periods or irregular periods, normally thought to be due to tough training regimes and restricted diets.
But in some cases polycystic ovary syndrome may be the cause, said Magnus Hagmar, of the Karolinska Institute.
This raises male sex hormone levels and may help sufferers in sport, he said.
Many women have polycystic ovaries without any outward symptoms, although it can lower fertility and cause problems such as excess body hair and acne.
While the precise cause is unknown, there seems to be a genetic link to the condition.
Mr Hagmar, from the Karolinska Institute, believes that having polycystic ovaries could explain some of the menstrual disorders found in "elite athletes".
He found polycystic ovaries were much more common in athletes training for the Olympics compared with the average woman - 37% of the athletes had them, compared with one in five women in the general population.
Polycystic ovaries were also more common in "power sports" such as ice hockey and wrestling, compared with technical sports such as archery or curling.
This could mean that the slight increase in production of the male sex hormone testosterone which accompanies polycystic ovaries is offering a competitive advantage.
He said: "What we're dealing with is just a tiny increase in levels, which can make it easier for the women to build muscle mass and absorb oxygen.
"This means that they might have got quicker results from their training and therefore been encouraged to train harder and more often."
He said the results suggested a smaller influence for the conventional view that links eating disorders and heavy training with the loss of periods and an increase in brittle bone risk caused by the suppression of female hormones, as none of the athletes he tested had weak bones.
However, Professor Stephen Franks, an expert on reproductive biology from Imperial College London, said that while the research was "interesting", it did not offer proof that polycystic ovaries were a significant factor in menstrual disorders in athletes.
"It's pretty well established that, at least in endurance athletes, such as marathon runners, menstrual disorders are related to the effect of heavy exercise on the pituitary gland.
"It is possible that in 'power' sports that women who generally have slightly higher levels of testosterone may be better off."