Cycling and infertility

“According to a new study” is a phrase that should set your bullshit alarm ringing. Journalists don’t in general understand either the methodology or conclusions of published studies and instead seize on the most lurid parts of the accompanying press release. So, with that in mind, here we go:

According to a new study, cycling a lot can affect your fertility if you are a man. The latest study to suggest this is reported quite well here: Elite cyclists ‘risk infertility’.

The conclusions are sensible: “the average man cycling to work would be unlikely to suffer fertility problems because of their time in the saddle.” and “40 years ago cycling was much more common but there is no evidence men then were less fertile”. So there’s nothing to worry about, right?

Perhaps. Perhaps not. My first son was conceived with the aid of a lot of expensive medical help (IVF to be precise), the reason: poor sperm motility and morphology. My second son was conceived the regular way. The only difference between the two pregnancies was that I’d stopped cycling after the first sperm tests.

My anecdotal evidence has no scientific weight at all but I would suggest the following things may be true:

  1. 40 years ago saddles were a lot different to now. Lightweight racing saddles give you little room for manoeuvre (men, you know what I mean). A poor posture can lead to numbness and discomfort that lasts most of the day as I have found myself. If this is happening to you then consider the other long-term effects that might also be happening. Sort out your posture.
  2. A short ride over bad roads (e.g. central London) might inflict damage equivalent to many more miles over a triathlon course. I believe that a modest amount of inner-city commuting might be the equivalent of 186 miles a week of triathlon training.
  3. If your fertility is borderline anyway then it might take relatively little additional damage to make it noticeably more difficult to conceive.

The conclusions of the BBC article are sensible but do consider the personal risks to you rather than the general statistical results.

Also: none of the studies I’ve seen have ever investigated the effects of stopping cycling. From my own experience this had a positive effect on my fertility, but that’s not science. Please could somebody do a follow-up and investigate this aspect too?

Annoying journalistic habit note

How come the study talks about training amounts above and below 186 miles? Isn’t this a rather arbitrary figure to use for the study?

Yes, but of course the study was from a more enlightened country that uses OSI units for measuring distance. 186 miles is 300 kikometres. So the study drew the perfectly acceptable conclusion that around 300 kilometres a week was an amount that might contribute to infertility.

Converting this to exactly 186 miles is ridiculous for two reasons:

  1. The original figure is stated to one significant figure. The equivalent imperial distance would be, say, 200 miles a week.
  2. I am relatively aged, and I remember being taught metric units at school. Why oh why do we have to assume that people only understand our grandparents’ units?

Come on, journalists of Britain, most of us are not geriatric or imbeciles.


9 Responses to “Cycling and infertility”

  1. 1 Dr. V February 10, 2010 at 06:18

    Something I have empirically noticed is that nearly all of my cycling friends have girls. I ride 100-150 miles per week and have 3 girls. Of the nearly 15 other riders I’ve interviewed, there have only been 2 boys and about 30 girls? Has anyone else noticed a correlation?
    Dr. V

    • 2 Dominic Sayers February 10, 2010 at 09:25

      My two sons don’t necessarily harm your thesis! Any study in this area would be helpful. If we knew the physiological mechanisms that caused fertility issues for cyclists then we could probably do something about saddle and seat pillar design to combat it.

  2. 3 TimG July 1, 2009 at 10:23

    There is some research from the US on the ECD Sports website – looking at the effects of prolonged riding among a small group of cycle patrol officers, which seems to show that fertility was not impaired but there was evidence of erectile dysfunction, which increased the longer the officers spent in the saddle.

  3. 5 laggleton June 29, 2009 at 18:57

    But the study was only of elite high-mileage cyclists, so anything else is pure supposition. The journalist didn’t dismiss their effects on social/commuting cyclists, it was a British ‘expert’. I take your point about the unbased assumption that any morphology changes are permanent though.

    Anyway, I’ve got to go on holiday and learn to sail now, so maybe a MOP once I’m back?

    • 6 Dominic Sayers June 29, 2009 at 21:16

      Lawrence, you clearly need a holiday. Have a lovely time.

      For my other reader, the word “might” in each of my three suggestions was an attempt to warn pedants that nothing in my article was to be taken as evidence-based cartesian discourse.

      Ready about? Lee-oh!

  4. 7 laggleton June 29, 2009 at 17:16

    “My anecdotal evidence has no scientific weight at all but I would suggest the following things may be true”

    I’m not sure whether I’m more amused by this sentence or the fact that you’re comparing yourself to an elite cyclist! If you saw a journalist write the above wouldn’t they be a prime target for abuse? (and/or forwarding the column to Ben Goldacre)

    But, to respond to your points in turn:

    1. While I’d agree that modern seats can potentially be more constricting, I’ve found since I got my road bike that my (cycling) posture has actually improved because of having a higer seat. In addition, I’d say seat posts and bike frames are much more shock absorbant these days – and that’s just in road bikes. Lest we forget, far more people ride mountain bikes and hybrids now than they did 40 years ago – and these tend not to have the lightweight racing saddles.
    2. Actually, I find London roads aren’t that bad – it’s not like they’re cobbled which would really suck. And triathlons in general are done on normal roads, so I don’t think the comparison is entirely fair.
    3. Well, yes, but this is really just a truism.

    I suppose my general point is that fertility (amongst other things) is an extremely powerful and emotive issue – cutting to our underlying animal procreation instinct. Making non-scientific guesses about that and then complaining about decimal place rounding in a kilometre conversion jar a little I guess.

    But I’d happily dicuss it further over a beer ;)

    • 8 Dominic Sayers June 29, 2009 at 18:35

      I tried to make it clear my guesses were not rooted in double-blind peer-reviewed statistical granite, just that there might be grounds for not dismissing these results as only applying to high-mileage cyclists. I am happy with what I wrote although I take your point about overall bike technology.

      Rereading the BBC article I also take issue with the study leader saying “it was unclear whether sperm quality would improve if men retired from the sport but that after years of wear and tear this was unlikely.” She has absolutely no basis for saying this and I am immediately suspicious of her motives for doing so.

      ps Lucky you if you find London roads nice to ride on. Come and try the East End.

    • 9 Dominic Sayers June 29, 2009 at 18:41

      I gladly accept your offer of beer, by the way.

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