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Men who use steroids to gain muscle could be damaging their chances of having children, according to scientists.
Researchers have revealed that men who use anabolic steroids, which mimic the effect of the male hormone testosterone in the body and are used as performance-enhancing drugs to increase muscle growth, have a 90 per cent chance of becoming sterile.
The academics state that so many people are inadvertently sacrificing their fertility that men are at risk of an evolutionary paradox.
Dubbed the Mossman-Pacey paradox, the theory is named after the two researchers who first described it: Dr James Mossman of Brown University and Professor Allan Pacey, from the University of Sheffield.
Speaking to the BBC, Dr Mossman said that taking steroids might make men look more “attractive” but could turn them into “an evolutionary dud”.
“I noticed some men coming in to have their fertility tested and these guys were huge,” said Dr Mossman.
“They are trying to look really big, to look like the pinnacles of evolution. But they are making themselves very unfit in an evolutionary sense, because without exception they had no sperm in their ejaculation at all.”
According to the NHS, the use of anabolic steroids has become one of the main causes of preventable male factor infertility.
Phil Harris, consultant gynaecologist and clinical lead at Fertility Fusion said: “I am seeing an increasing number of men at the clinic who simply aren’t aware of the damage that taking anabolic steroids can do to their long-term health and fertility.
"We advise anyone who wishes to start a family now and in the future to avoid taking anabolic steroids where possible”.
But, what effect does it really have?
The researchers state that steroids trick the brain’s pituitary gland into thinking the testes are going into overdrive.
As such, the glands react by shutting down the production of two hormones – called FSH and LH – which are the key hormones behind the production of sperm.
Mossman and Pacey add that there is a similar theme in men using medication to prevent male pattern baldness, and that while the damage to fertility from steroid use is well known, this is the first time scientists have coined a specific term due to the vast numbers of people now affected.
“I would say more anabolic steroid users are likely to become sterile than you would think - 90 per cent probably,” Pacey said.
"Baldness is a bit more hit-and-miss, but sales are going through the roof and that makes it an increasingly common problem.
“Isn’t it ironic that men go to the gym to look wonderful, for the most part to attract women, and inadvertently decrease their fertility.”
The NHS states that regularly taking anabolic steroids can lead to a number of physical and psychological changes in both men and women, as well as potentially dangerous medical conditions.
The physical effects in men can include reduced sperm count, shrunken testicles, breast development, increased risk of prostate cancer and severe acne.
While the physical effects in women include facial hair growth, loss of breasts, deepened voice and hair loss.
The findings follow a recent study which revealed that, like women, men may also have a time limit on having children.
According to a new study published in the journal Maturitas, men should consider banking their sperm before reaching “advanced paternal age”, which has been variably defined as above the age of 35 or 45 in medical fields.
This is because men older than this may experience decreased fertility, the study states, and put their female partners at higher risk of a number of pregnancy and birthing complications, such as gestational diabetes, pre-eclampsia and preterm birth.
Additionally, infants born to older fathers may also be at higher risk of a number of conditions, such as congenital heart disease, newborn seizures and low birth weight.