Endurance health: what all aspiring dads should know!

Andrew Hamilton looks at research on endurance training loads and male fertility, and discusses the implications for would-be dads

Endurance training is not only fun, it’s great for your fitness and health, improving your cardiovascular fitness, muscular strength and shedding or keeping the excess pounds off. For male readers who are trying for a baby, this makes encouraging reading because doctors and health professionals have long known that being fit and healthy can help increase the chances of conception – and not just because you might be more appealing to your partner! In short, higher levels of fitness are associated with favourable biochemical and hormonal changes in the body, which can help fertility. Like most things in life however, it seems you can have too much of a good thing because some research suggests that men who train too intensively on the bike are likely to become less fertile – not more.

Intensive training and fertility

In one study, Iranian scientists studied the effects of 16 weeks of intensive training on the health of semen in twenty-four healthy non-professional male road cyclists(1). In particular, they were interested to see how the training affected markers of free-radical damage to sperm. Free radical damage to cells and molecules within cells occurs all the time but is increased during very intense exercise when a lot of oxygen is being processed. Free radicals are continually being mopped up or ‘quenched’ in the body by our antioxidant defences. However, at times of physiological stress, this process is less efficient and more free radical damage can occur. Sperm cells are particularly vulnerable because even slight damage can make the sperm less viable, thus reducing fertility. The cyclists’ semen samples were collected at the start of the study then at 8 weeks, 16 weeks (end of the training period), 17 weeks and 20 weeks later. All the samples were analysed for free radical activity, markers of damage and also how active the cyclists’ antioxidant defence systems were.

The results showed that during the 16-week intensive training period, levels of free radical activity rose and so did levels of a marker of free radical damage (malondialdehyde). This was mirrored by a decline in the activity of the antioxidant defences. Even 4 weeks after the intensive training had ceased, thee markers of free radical damage in the semen remained elevated and antioxidant defences depressed.

Scientists have long known that intense exercise can result in hormonal changes in the body. So were these reductions in fertility due to the training intensity? In a follow-up study, the same group of scientists looked at the effects of a 16-week period of cycling on fertility – but this time with a low-moderate training intensity(2). Rather surprisingly perhaps, there were still negative effects; semen volume, sperm motility, sperm morphology, sperm concentration, and number of spermatozoa decreased, indicating reduced fertility. This led the researchers to conclude that ‘male cyclists and other athletes wishing to conceive should routinely employ sufficient rest after their training sessions to ensure that the sperm healthy parameters and seminal immunological profiles have properly recovered from the last training sessions.’

The bigger picture

An increase in free radical production during exercise is completely normal – indeed, many scientists now believe that this increase in free radical activity is part of the process whereby muscles adapt to training and become more efficient. However, when it comes to the nether regions and sperm quality, it seems that prolonged periods of intense training on the bike could harm the quality of sperm, in turn reducing male fertility. This fits with what we already know; studies have shown that moderately physically active men tend to have better sperm health than men who play competitive sport or those who are elite athletes(3). And studies on cyclists have also shown that cycling more than five hours per week reduces the motility and concentration of sperm, both of which lead to lower levels of fertility(4). Add in the fact that pregnancy and live birth rates are higher in couples where men take antioxidant supplements(5), and it’s perhaps not surprising the results showed that too much cycling can damage sperm health!

Practical implications for male cyclists and other athletes

If you’re a male cyclist, runner, triathlete etc and you and your partner are trying for a baby here are some tips to help maximise your fertility:

  • Limit your total training volume to no more than 5 hours per week;
  • Don’t do too many intense sessions and allow plenty of recovery after those that are;
  • Boost your antioxidant intake naturally by eating plenty of brightly coloured fresh fruits and vegetables – at least five portions a day – and ensure your diet is as nutritious as possible, both of which can improve sperm quality.


  1. Clin J Sport Med. 2014 Jul;24(4):302-7
  2. Clin J Sport Med. 2015 Nov;25(6):535-4
  3. Fertil Steril 2009, 92:1941–1946
  4. Fertil Steril 2011, 95:1025–1030
  5. Cochrane Database of Systematic Reviews (Online) 2011,1:CD007411

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