Female athletes with high levels of testosterone can run for longer

Olympic gold medallist runner Caster Semenya DOES have an advantage over other female athletes because of her high testosterone levels, study claims

  • Women lasted 21 seconds longer on a treadmill after a dose of testosterone
  • Hormone boosts endurance, muscles and leanness without changing weight
  • Olympic gold medallist Caster Semenya battled officials over her high levels

Olympic gold medallist Caster Semenya really does have an advantage over other female athletes because of her high testosterone levels, according to a new study.

Semenya, a double Olympic champion at 800 metres, has fought a long-running battle with the authorities over the impact of her genetic condition.

Unusually, she has been told by the International Association of Athletics Federations (IAFF) that she must take drugs in order to compete fairly.

Now a study has confirmed testosterone increases young women’s running ability dramatically.

It boosts endurance, muscles and leanness – even though their weight remains the same, say scientists.

A study suggests Olympic gold medallist Caster Semenya really does have an advantage over other female athletes because of her high testosterone levels

The sprinter (centre with a gold medal at the World Athletics Championships in Londonin 2017) has fought a long-running battle with the authorities over the impact of her genetic condition

The discovery based on dozens of 18 to 35 year-olds could be a blow to Semenya’s hopes of overturning the decision that was implemented for London 2012.

The women in the study lasted over 21 seconds longer on a treadmill after receiving a dose of the male sex hormone.

The average rise in levels to 4.3 nmol/l (nanomoles per litre of blood) was below a normal man’s.

Yet it still significantly lengthened the time they could keep going before reaching exhaustion.

Lead author Professor Angelica Linden Hirschberg said: ‘Our results are therefore of great importance for the ongoing discussion of whether it is fair to allow athletes with naturally high testosterone to compete in the female category without reducing their hormonal concentration to the female range.’

In her case, South African-born Semenya would have to take medication to reduce her naturally-occurring high testosterone.

Her condition is said to be due to DSD – Differences in Sexual Development – and she says other athletes are just jealous and should suck it up.

The 29-year-old did not defend her World Championships 800m title last month in Doha because of the judgement. Her latest appeal was dismissed by the Swiss Federal Tribunal in July.

Semenya, who won gold at London 2012, lives as a woman but was born intersex – making her biologically both male and female.

Professor Hirschberg, of the Department of Women’s and Children’s Health, Karolinska Institute, Stockholm, said: ‘Ten weeks of exposure to a moderate increase in circulating testosterone caused a significant increase in aerobic running time in comparison with placebo.’


Testosterone is the male sex hormone and is mostly made in the testicles, but also in adrenal glands, which are near the kidneys.

It causes the voice to deepen, body hair to grow and the genitals to become larger during puberty.

As well as affecting sex drive and sperm production, it also plays a role in developing strong bones and muscles, and how the body distributes fat.

Women also create small amounts of the hormone in the ovaries and adrenal glands, and it affects their fertility and bones and muscles.

Testosterone levels which are too high or too low can cause various problems.

Low testosterone in men can cause erection problems, low sex drive, infertility, weakened muscles and bones, body fat gain and hair loss.

Too much testosterone, however, can trigger puberty in boys under the age of nine, is linked to aggression, and can increase the risk of prostate problems, including cancer.

Male testosterone levels tend to be highest when he is around 20 years old, and decline naturally with age. 

There was no such improvement in anaerobic exercise like throwing that lasts only a few seconds – or jumping and muscle strength.

Professor Hirschberg said: ‘But lean body mass increased significantly following testosterone exposure.’ The impact of the hormone has prompted controversy over whether it’s fair to allow female athletes with levels in the male range to compete against women with normal amounts.

The prevalence of elite female athletes with these rare inborn conditions is believed to be around 140 times higher than in the general population.

The IAAF has stipulated they must lower levels to below 5 nmol/l to be eligible to compete in middle distance races of 400 metres to 1 mile. This policy has been widely criticised and legally challenged.

In the study published in the British Journal of Sports Medicine participants were randomly assigned to 10 mg of testosterone cream or an inactive placebo for ten weeks.

The hormone’s effect on aerobic performance was measured by how long they could run on a treadmill before reaching the point of exhaustion.

Anaerobic performance requiring short bursts of energy was assessed in leg power on cycling machines. Muscle strength was estimated through jumps and knee power.

Hormone levels and body composition – the percentage of body fat and lean muscle mass – were measured at the beginning and end of the trial.

Average circulating levels of testosterone rose from 0.9 nmol/litre of blood to 4.3 nmol/l among the women given the hormone cream. No increase occurred in the group given the inactive substance.

Among those given testosterone, running time to exhaustion increased by 21.17 seconds (8.5 per cent) compared with the others.

They also had much larger changes in lean muscle mass – 923g compared to 135g overall.

Professor Hirschberg said: ‘In sports that rely on strength and endurance, male athletes have, in general, an advantage of 10-15 per cent in comparison with female athletes, which most likely is explained by men having, on average, more than 15 times higher circulating concentrations of testosterone than women.

‘The study supports a causal effect of testosterone in the increase in aerobic running time as well as lean mass in young, physically active women.’ Semenya was subjected to a sex test in 2009 when people claimed she was a man after testing found higher than usual levels of testosterone in her blood.

A normal level for healthy women is up to 1.7nmol/L, while men’s is between 7.7nmol/L and 29.4nmol/L.

So athletes with what the IAAF calls ‘differences of sexual development’ which make their testosterone extraordinarily high must take medication to keep their testosterone down.

The world of sport is wrestling with the debate over whether to allow transgender competitors born as men to compete as women as it is claimed they have an unfair advantage.

In some fields transgender women, who were born as men, are dominating the sport.

Examples include the Canadian Rachel McKinnon, who was born a man, but is now the UCI Women’s Masters Track World Championship for the 35-44 age bracket.

In cricket, Maxine Blythin, Kent’s first trans woman player is causing controversy, while she had a batting average of 15 on the men’s team, in the woman’s team she averages 124. 


The South African runner Mokgadi Caster Semenya, 28, was banned from running for almost a year in 2009 after being found to have higher than usual levels of testosterone in her body. 

Critics claimed she was a man because of her outstanding performance which led to her being subjected to a sex test.

The results were never published but are believed to have shown she was born with both female and male biology.

 Caster Semenya has been at the centre of a row over a new rule by the IAAF that restricts testosterone levels in female runners. Pictured during the women’s 800m race at the IAAF Diamon League competition, May 2019

She was allowed to return to competition as a woman and has since won Commonwealth and Olympic gold medals. 

Ms Semenya tried to take legal action against the rule, but The Court of Arbitration for Sport (CAS) rejected Ms Semenya’s challenge in May 2019.

They said the decision was ‘necessary, reasonable and proportionate’ to make sure competition was fair, but added it had ‘serious concerns as to the future practical application’ of the new rules. 

Ms Semenya has expressed how unfair she felt the rules were and that all she wanted was to ‘run naturally, the way I was born’.

To adhere to the regulations, Ms Semenya would either have to start taking medication in order to defend her world 800m title later this year or to compete in 5,000m, which is not covered by the IAAF’s regulations. 

Athletics South Africa (ASA) said it was ‘reeling in shock’, and South Africa’s sports minister Tokozile Xasa said that Caster had been ‘targeted’ and called the proposed IAAF rule a human rights violation. 

Athletics South Africa, the country’s track and field body, will appeal the decision by the Court of Arbitration for Sport.

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