Endurance cyclists are supremely healthy athletes, but scientific studies now suggest that many male cyclists have surprisingly low sperm counts. Some ultra-fit marathon runners are discovering a similar decline in their reproductive health. In fact, research suggests that many super-fit professional athletes often have lower sperm counts than “ordinary” men. Why are so many ultra-fit men discovering that their sperm counts are declining? And what should athletic men do – or not do – to safeguard their fertility?

Also in our Male Infertility special investigation

• Sperm counts in freefall: is the male infertility crisis an existential threat to humanity?
• “Are men facing a terminal decline in testosterone?”
• 10 ways men can boost their fertility
• How to protect sexual health and fertility during intense training

Fitness and fertility

Let’s start with the good news. A wealth of scientific evidence confirms that regular exercise is a wonderful way for men to protect their fertility. A 2014 study, published in the journal Human Reproduction, found that men who did regular moderate-to-vigorous exercise had sperm concentrations which were 43% higher than those of less active men.

And these benefits are apparent even among virile young men: a 2015 study in the British Journal of Sports Medicine, which analysed men aged 18-22, found that those who enjoyed regular moderate-to-vigorous exercise had sperm concentrations which were 73% higher than those of their less active peers. The same study warned that sedentary men who watched more than 20 hours of TV per week had 44% lower sperm concentrations than guys who know how to locate their TV’s off button

But if we dig a little deeper, we find even better news. The fertility benefits of regular exercise seem to extend beyond sperm concentration to general sperm health and hormonal health. A 2012 study in the European Journal of Applied Physiology found that regular physical exercise delivers “statistically significant differences” to a man’s semen health, including sperm motility (its ability to swim efficiently) and sperm morphology (its size and shape). It also creates an “improved hormonal environment,” with healthier levels of follicle-stimulating hormone (FSH) and testosterone – two key hormones in the male reproductive system.

This shouldn’t be a big surprise. Exercise is known to aid improve blood circulation and hormone production, so the same activity which helps to protect your general health was always likely to aid your reproductive health too. There are also long-established links between male obesity and infertility. “We know that men who are sedentary and overweight – one of the two or both – have poor sperm,” says Allan Pacey, deputy dean of the faculty of Biology, Medicine and Health at the University of Manchester. “We also know that moderate exercise is good. There’s a nice randomised control trial out there showing that if men just do moderate exercise on a bike in the lounge, for example, you can improve your sperm production over the course of about three months.”

Too much, too often

The point at which things seem to take a turn for the worse is when men plunge into the world of extreme exercise, through an excessive volume of training, or an excessive intensity of training. This may explain why studies are discovering declining sperm health among professional male athletes, who train more intensely than amateur athletes. And it may also be why recreational endurance cyclists and marathon runners, who also train at a high volume, seem to be particularly vulnerable.

An extensive review of the scientific research on this subject, which was published in the American Journal of Men’s Health, confirmed that “training at higher intensities and with increased loads seems to be associated with more profound changes in semen quality.” Certain types, intensities and durations of exercise can affect a man’s semen concentration, as well as his number of motile sperm. The researchers concluded with a simple message: “In recreational athletes, exercise seems to be mainly associated with positive or neutral effects, while professionals should be aware of potential risks.”

Endurance cycling, in particular, seems to negatively affect sperm health. A study published in the journal Fertility and Sterility found that men who cycled at least 5 hours a week were more likely to have a low sperm count and fewer motile sperm. Another study, published in the International Journal of Sports Medicine, found that long-distance cyclists had a significantly lower proportion of healthy sperm.

But there is similarly worrying evidence for male endurance runners. A study in the International Journal of Sports Medicine found that men who ran 108km per week, for 12 months, experienced reductions in several semen health parameters, as well as in their sperm concentration and motility. But in a crucial twist, runners who covered 40-56 km per week (a still seriously impressive but nevertheless less extreme volume) were not negatively affected. It’s extreme exercise that’s the problem here.

An excessive amount of high-intensity training can also be problematic, according to a study in the Journal of Endocrinology. Subjects who completed five treadmill workouts per week at a high intensity (around 80% VO2 max) demonstrated significantly reduced semen parameters and lower testosterone levels than those who did the same volume of training but at a more moderate intensity (around 60% VO2 max).

Heat, stress and sperm

There are several theories behind this troubling link between extreme exercise and reproductive health. It is known that sperm can be affected by testicular heat stress and tight clothing – and endurance sports, which may involve elevated scrotal temperatures or the wearing of tight or sweaty clothing, like Lycra or running shorts, may not help.

But extreme exercise – whether high volume or high intensity – can also place the body under duress, which can negatively affect hormone production and sperm health. “Where we get into interesting territories is with people who are really pushing their bodies to the extreme,” says Professor Pacey. “And I don’t know the mechanism. But there are enough studies out there to raise a flag. What we’re talking about here are, say, triathletes – men who are doing lots of physical cardio stuff. And I suspect the reason is because they’re putting their bodies on the edge. And when the body is on the edge, it begins to look at those critical functions it needs to maintain, and those things that it can jettison. This is not a million miles away to what happens when someone is ill. Often fertility is compromised in people with cancer. And when they’re treated, and they recover, it comes back. And I think what’s happening with the extreme end of cycling is metabolically probably similar to that.”

Body fat problems

Dr Channa Jayasena, a specialist in reproductive endocrinology at Imperial College London, believes that the low body fat of endurance athletes may be the problem. And that means gymgoers striving for a ‘cut’ physique may also be at risk. “When you are pushing your body to the limit, and you’re really going for that low fat mass, then we are seeing an increasing phenomenon of men paradoxically switching off their reproductive system,” he explains.

“That’s because you need a minimum amount of fat in your body to signal to the hypothalamus in the brain to ‘keep the pilot light on’. And as a safety mechanism, if you don’t have enough fat, your body thinks you’re starving and at risk of dying. Therefore, it thinks it’s not a good idea for you to be procreating at that time. It’s an emergency measure. So, unfortunately, men who really try to go for the ultra-defined body with zero fat are at risk of this condition.”

Accepting that exercise is incredibly good for fertility, but extreme exercise can be detrimental, it is perfectly possible for active men to enjoy all the benefits of regular exercise, without enduring the fertility problems associated with pushing your body to the edge. Enjoy endurance training, but don’t overdo it. Stick with your high-intensity sessions, but don’t repeat them every day. And stay lean, but not obsessively lean.