Sexual health has long been a taboo subject in the endurance exercise community. Some male cyclists suffer in silence from genital numbness, bruising and erectile dysfunction, while some female riders experience genital chafing, urinary issues and menstrual disruption.
There is rising concern among researchers and doctors that extreme endurance training could be impairing the fertility of cyclists, making it harder for them to have children.
A review in the American Journal of Men’s Health found excessive training at higher intensities and with increased loads can damage a man’s semen quality.
And research published in F&S (Fertility and Sterility) Reports concluded that women trying to conceive should consider how vigorous exercise could potentially damage their fertility.
- Read more: Is cycling bad for your sexual health? Erectile dysfunction, genital numbness and more explained
Should we all start panicking?
The short answer is: no. But science is revealing vital lessons that all cyclists should know about in order to avoid any fertility problems in years to come. As with all health issues, the solution is education, awareness and proactive action.
Here is our step-by-step guide to everything you need to know…
Is endurance cycling good or bad for your fertility?
Here’s the good news. A mountain of scientific evidence proves that regular exercise, such as cycling or running, helps not hinders human fertility.
A 2014 study published in the journal Human Reproduction found men who did regular moderate-to-vigorous exercise had sperm concentrations 43% higher than that of less active men.
A 2012 study in the European Journal of Applied Physiology found that regular exercise improves a man’s semen health, and triggers healthier levels of follicle-stimulating hormone (FSH) and testosterone – two key hormones in the male reproductive system.
So, now you have proof that all that endurance training is not only good for your heart, your lungs and your body composition, it is quietly boosting your fertility, too.
It's the same hugely positive story for female athletes, too. A review published in the Journal of Translational Medicine concluded that regular physical activity is associated with an increased pregnancy rate.
Even among women experiencing infertility issues, regular exercise may lead to the resumption of ovulation by regulating the hypothalamic–pituitary–adrenal (HPA) axis, which supports reproductive function.
The same study found that for both men and women “a moderate to high physical activity level significantly reduced the overall risk of infertility and was a common protective factor”.
This should not be a surprise. We already knew from decades of medical research that exercise improves blood circulation and hormone production, so it was therefore always likely to be positive for your reproductive health, too. So far, so obvious…
Can endurance exercise damage fertility?
The point at which endurance exercise seems to slide from being a positive to a negative influence on human fertility is when athletes push things to extremes, through excessive high-intensity or high-volume training.
Experts have dubbed this strange phenomenon the ‘fitness-fertility paradox’: how can uber-healthy athletes who are so athletically fit and strong be so surprisingly unfit when it comes to fertility measures?
This issue also affects bodybuilders and other athletes who push their bodies to the limit.
This ‘fitness-fertility’ paradox has been highlighted by several studies that have revealed declining sperm health among professional or elite athletes, who train more intensely than amateur athletes.
A study in the International Journal of Sports Medicine found that long-distance cyclists had 20% less healthy sperm than even their sedentary peers.
And a study in the Journal of Endocrinology found subjects who completed five draining high-intensity workouts per week (around 80% VO2 max) had 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).
An extensive review of the scientific research, published in the American Journal of Men’s Health, found it was specifically this kind of “intense” physical activity that affected semen concentration and sperm quality.
The experts came to a simple but powerful conclusion: “In recreational athletes, exercise seems to be mainly associated with positive or neutral effects, while professionals should be aware of potential risks.”
The potential for extreme levels of vigorous exercise to negatively impact fertility in female athletes has also been confirmed by two large studies, in 2009 and 2012.
In the 2009 paper, regularly exercising to exhaustion was found to increase a woman’s chances of infertility by 2.3 times compared to doing lower-intensity exercise.
The researchers concluded that “increased risk of infertility was only found for the small group of women reporting the highest levels of intensity and frequency of physical activity”.
The researchers suggested that “awareness of the possible risks of infertility should be highlighted among non-athletic women who exercise vigorously”.
In the 2012 study, there was clear evidence of a “dose-response” relationship between increasing levels of vigorous physical activity and delayed time to pregnancy.
While regular moderate physical activity was associated with a small increase in fertility, regular high doses of vigorous exercise were shown to delay time to pregnancy.
The researchers suggested that athletic women who substitute vigorous physical activity with moderate physical activity may also improve their fertility.
It seems excessive vigorous exercise is the primary issue here. One research paper found women who completed more than five hours of vigorous exercise per week experienced a 32% reduction in their chances of conception, whereas women who did more moderate exercise increased their odds of pregnancy by 15%.
It seems that athletes, whether male or female, can push their bodies extremely hard before suffering fertility problems.
But there’s a limit. And when an athlete pushes their body into more extreme territory, their fertility begins to suffer.
Why does extreme exercise damage fertility?
The exact mechanisms behind the link between extreme exercise and infertility remain unknown, but there are several theories.
Professor Allan Pacey, a respected expert in fertility at the University of Manchester, believes when the body is pushed to the limit, it may shut down what it deems less critical functions, such as reproduction.
The body is so focused on ‘surviving’ the exercise ordeals it is facing, it cannot sustain reproductive health, so these functions are jettisoned.
To illustrate the theory, he points out that fertility is often temporarily compromised in people with serious illnesses, such as cancer, but when they recover so does their fertility.
“I think what's happening with the extreme end of cycling is metabolically probably similar to that,” he says.
Dr Channa Jayasena, a specialist in reproductive endocrinology at Imperial College London, agrees that extreme training is the problem for endurance athletes.
“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 (which regulates testosterone production) 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.”
A similar mechanism may be affecting elite female athletes. Dr Emma O'Donnell, an exercise physiologist at Loughborough University, has explained how elite female athletes who regularly torch a huge number of calories can become vulnerable to disrupted menstrual cycles.
“The leading idea is that having a baby is so energy-intensive that the brain shuts off reproduction if it thinks there is insufficient spare energy,” she explained.
Fat is also required to make the female sex hormone oestrogen, so if extreme exercise is negatively affecting a woman’s body fat percentage, it will likely affect her oestrogen levels, too.
Nearly two-thirds of female athletes can experience interrupted periods, and that is especially the case among endurance athletes, compared to only 2-5% of the general population.
Ballet dancers, who strive for less body fat in order to achieve the aesthetic visual grace required in their performances, are also at risk of this issue.
Compelling research backs this interesting theory up. It has long been known that the condition RED-S (Relative Energy Deficiency In Sport) – which occurs when a female athlete does not get sufficient energy intake to fuel the exercise she is completing – can lead to menstrual disruption and reduced oestrogen levels.
But research in the American Journal of Physiology, Endocrinology and Metabolism found that even among women who completed vigorous exercise five days a week – a seriously high volume – the detrimental impact on their fertility could be countered by increasing their calorie intake to match the physical demands of their training.
That is why, in an optimistic mini-review for F&S Reports, a team of researchers concluded that while female athletes who suffer from infertility problems may wish to increase their calorie intake or perform more moderate-intensity exercise, “we would advocate against indiscriminately discouraging highly active women from engaging in vigorous exercise while trying to conceive”.
In other words, it seems that the issue can be countered successfully, even among athletes who are training extremely hard, but only if it is managed intelligently.
How can you protect your fertility?
The lesson here for male and female athletes is surprisingly simple: don’t push your body too hard, too often.
Regular endurance exercise is incredibly good for your fertility, but extreme exercise can be detrimental. Enjoy your long endurance rides and runs, but don’t overdo them.
Go hard in high-intensity sessions, but don’t do them every day. Stay trim enough to conquer the mountains and the fells, but don’t cut out calories and leave yourself low.
And, above all else, always fuel for the work required, and don’t routinely leave your body in a calorie deficit.
This appears to be when fertility problems kick in. But if you train sensibly and respect your body, all the evidence confirms that endurance exercise will help, not hinder, your fertility.
Should you be worried?
Despite the troubling fertility problems faced by some cyclists and runners, scientific research confirms most issues can be avoided by intelligently managing your training load and your fuel intake.
So don’t think you are running or cycling into a major problem. Endurance exercise has been shown to improve the menstrual regularity, hormonal health, stress levels and inflammation markers of female athletes, and to improve the semen quality, oxidative stress markers and hormone balance of male athletes.
Together, this confirms that endurance exercise is fantastic for your fertility.
Again and again, the research papers that highlight these disturbing infertility issues among endurance athletes appear to be linked specifically to extreme exercise.
An interesting – and highly revealing – study in the International Journal of Sports Medicine found that men who ran 108km per week, for 12 months, experienced a decline in semen health and sperm concentration.
But runners who covered 40-56 km per week (a still high but less punishing volume) were not affected. So you really do have to go very deep before experiencing the kind of fertility problems these studies are revealing.
Nevertheless, it is important to take these fertility risks seriously. Many athletes push themselves hard in pursuit of success, often at the expense of their general health.
But you don’t want to find out in years to come that you have damaged your procreative powers.
This new wave of research should be taken seriously by the endurance sports community.
However, when managed proactively, with a sensible training balance and a smart nutrition plan to meet the needs of your training, cycling and running remain as positive for your sexual health and fertility as they are for every other area of your health.
