Kickboxing can cause damage to the part of the brain which controls hormone production, a study has shown.
Around a million people around the world take part in the sport.
The Turkish study found head injuries in kickboxing can cause damage to the pituitary gland, which affects the body's metabolism and stress response.
In Clinical Endocrinology, researchers say amateurs with head injuries should be screened. But kickboxers say they are unaware of such injuries.
"People who take part in combative sports and are exposed to repeated head trauma should be screened to ensure their pituitary is working properly"
The pituitary is a pea-sized gland, weighing no more than a gram, which is found at the base of the brain, just behind the bridge of the nose.
It produces a range of hormones which control, among other things, the body's regulation of metabolism, coping with daily stress, general wellbeing and sex drive amongst other areas.
The team at Erciyes University Medical School in Turkey measured the levels of these hormones in 22 amateur kickboxers (16 men and six women) and compared these to healthy people of the same age and sex.
It was found that 27% - six - of the kickboxers were deficient in at least one hormone compared with the healthy group.
The researchers say the head is one of the most common sites of injury for both amateur and professional kickboxers.
They said more research was needed to understand how the pituitary gland is damaged and to develop more effective head protection gear for kickboxers.
'Critical role'
Professor Fahrettin Kelestimur, who led the research, said: "This is the first time that amateur kickboxing has been shown to cause damage to the pituitary, resulting in insufficient hormone production.
"Our study shows that kickboxers experience an increased risk of suffering from hypopituitarism, a condition where the pituitary fails to produce enough hormones.
"In healthy people, hormones produced by the pituitary fulfil a critical role in helping the body maintain a healthy metabolism and cope with daily stress."
He suggested that, if the results of the study were extrapolated out to everyone who kickboxed, a quarter of a million people worldwide could be producing decreased amounts of hormones as a direct result of head injuries sustained during kickboxing.
Professor Kelestimur added: "We recommend that people who take part in combative sports, like boxing or kickboxing, and are exposed to repeated head trauma should be screened to ensure their pituitary is working properly."
A spokesman for the World Kickboxing Association said: "I have never heard of any such damage."
He added that amateurs, though not professionals, already wore head protection.
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