A new study has found that young men who watch 20 hours or more of television per week have a sperm count nearly half of those who watch very little or none at all.

The study, published in the British Journal of Sports Medicine, surveyed the habits of 189 young men aged between 18 and 22.

The young men were asked to provide a sperm sample and were questioned about their diet, exercise and television-watching habits.

It found those who watched 20 hours or more of television per week had a 44 per cent lower sperm count that those who responded that they watched none or almost none.

Sex therapist Desiree Spiering says while the findings are significant, low sperm counts do not necessarily prevent men from fathering children.

"None of the sperm levels were so low that the men would have been unable to father a child, so when it comes to low sperm count that actually impacts fathering a child, what they then have to go through is probably IVF."

The survey found that a big factor determining sperm counts was exercise.

Men who did 15 or more hours of moderate to vigorous exercise per week were found to have a 73 per cent higher sperm count than those who exercised less than five hours per week.

Ms Spiering says apart from trying to be generally healthier, there is not much young men can do about it.

"Young men are often concerned about it, especially if conception hasn't happened... if they are trying to conceive," she said.

"[They often feel] very, very guilty - that it's because of them that this wish for children isn't working."

No need for despair

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But IVF Australia medical director Dr Peter Illingworth says there is no need for young men to despair just yet.

"This is only a preliminary study," he said. "It's entirely consistent with this whole idea that if men have an unhealthy lifestyle, then their male reproductive health, and in particular their future fertility, could be affected."

Dr Illingworth says there are a number of factors that can affect sperm production.

"There are plenty of men walking down the streets with cigarettes in their hands and pushing a pram in front of them," he said.

"Lifestyle is not the be-all and end-all of having children, but there are a number of different effects that can affect a man's reproductive health.

"If a man exercises to extreme, or if he abuses anabolic steroids, then that undoubtedly can have an effect on his short-term fertility.

"Equally, the other extreme - an overly sedentary lifestyle or an extreme of body weight - undoubtedly affects it the other way."

Semen quality appears to have declined over several decades according to studies conducted in several countries.

But Dr Illingworth says the evidence is far from conclusive that there are long-term trends in sperm production in the Western world.

"That's a very interesting and very controversial question," he said.

"There are some studies that have been done that have shown that sperm counts are falling as the generations go by, but other studies carried out in different countries can't see any effect.

"It's an uncertain, and at the moment still an unknown, area."