Cycling linked to prostate cancer, but not infertility

"Men who cycle more than nine hours a week are … more likely to develop prostate cancer," the Mail Online inaccurately reports. The story comes from the publication of an online survey into cycling in the UK and its effects on health outcomes.

Researchers were particularly interested in whether frequent cycling was linked to an increased risk of prostate cancer, infertility and erectile dysfunction (impotence).

Fears have been raised regarding the effect cycling has on these conditions. These concerns have been attributed to a wide range of factors, such as repetitive trauma.

This study found no association between the amount of time spent cycling and erectile dysfunction or infertility.

But it did find a dose-response relationship between cycling time and the risk of prostate cancer in men aged over 50, with risk increasing as the hours a week spent cycling increased.

Despite these seemingly alarming results, regular cyclists do not need to panic – this type of study cannot prove increased cycling time leads to prostate cancer; it can only prove an association.

Also, the prostate cancer analyses were only carried out on fewer than 42 men, which is only a relatively small sample of men. With such a small sample, it increases the possibility that any association is the result of chance.

Most experts would agree that the health benefits of frequent cycling outweigh the risks.

For more on the pros and cons of the sport, read our special report on cycling.

Where did the story come from?

The study was carried out by researchers from University College London. Funding is not explicitly stated in the study, but the authors acknowledge the support of the national cycling charity CTC, British Cycling, Sky Ride and Cycling Weekly magazine. They state the funders had no role in the study other than providing funding.

The study was published in the peer-reviewed Journal of Men's Health.

The story was picked up by both the Mail Online and The Daily Telegraph, with each choosing to report the findings differently.

The Mail was keen to emphasise the link to prostate cancer, as seen in the inaccurate headline stating that, "Men who cycle more than nine hours a week are six times more likely to develop prostate cancer". The rest of the coverage of the study is appropriate and goes on to report that no causal relationship has been proven. 

The Telegraph took a more cheerful approach, emphasising that the study found no link between frequent cycling and infertility or erectile dysfunction. Its reporting was also balanced and accurate.

What kind of research was this?

This was a cross-sectional study looking at associations between erectile dysfunction, infertility and prostate cancer in a group of regular male cyclists from the UK. 

A cross-sectional type of study looks at the characteristics of a population at a given point in time. This type of study is useful for finding out how common a particular condition is in a population. Often, the relationships between the characteristics or factors assessed are then analysed.

Importantly, because this study only looks at one point in time, it cannot establish cause and effect between factors, as it does not show which factor came first.

What did the research involve?

The research included 5,282 male cyclists from the Cycling for Health UK Study. This study aimed to examine cycling habits and the health of men from a variety of cycling backgrounds, ranging from commuters to amateur racing cyclists.

In the current study, an online survey was advertised from October 2012 to July 2013 through cycling magazines and UK cycling bodies. The survey included questions on:

• demographics – age, sex and education
• cycling history – number of years cycling, average weekly total and commuting cycling distances, as well as time, speed and best times for various standard distance competitive cycling races
• body measurements – weight, height and resting heart rate, which was self-recorded from the wrist

The survey also captured additional information on:

• alcohol intake
• current and past cigarette use
• history of cardiovascular events, such as stroke or heart attack
• any cancers
• diagnosis or treatment of hypertension (high blood pressure), high cholesterol or diabetes
• participation in non-cycling physical activity

The men were then asked whether they had experienced erectile dysfunction in the past five years and if they had been diagnosed with infertility or prostate cancer by a doctor.

The researchers used statistical techniques to determine the association between the number of hours a week spent cycling and erectile dysfunction, infertility or prostate cancer.

They did this by categorising the amount of time a week spent cycling into four groups:

• fewer than 3.75 hours per week
• between 3.76 hours and 5.75 hours per week
• between 5.76 and 8.5 hours per week
• more than 8.5 hours per week

The researchers only analysed the data of men aged over 50 (2,027 men) because prostate cancer diagnosed under the age of 50 accounts for less than 1% of diagnoses.

They then adjusted the results for potential confounders, including:

• age
• smoking status
• weekly alcohol intake
• body mass index (BMI)
• diagnosed hypertension (high blood pressure)
• other physical activity

What were the basic results?

The average age of the men was 48.2 (range 16 to 88 years), the average BMI was 25.3 kg, and 3.8% of the cyclists were current smokers. On average, they cycled 4.2 days a week for 6.5 hours a week.

Of the 5,282 men, 8.4% (444 men) self-reported erectile dysfunction in the past five years, 1.2% (63 men) reported a diagnosis of infertility, and 0.8% (42 men) reported a diagnosis of prostate cancer. Prostate cancer was reported in 1.8% of men aged over 50 (figure not reported).

The main results of this study were:

• there was no association found between cycling time and erectile dysfunction
• there was no linear association found between cycling time and infertility – in fact, cycling for between 3.76 and 5.75 hours per week was actually associated with a decreased risk of infertility (odds ratio [OR] 0.44, 95% confidence interval [CI] 0.21 to 0.94)
• after adjusting for confounders, a graded increase in the association between cycling time and the risk of prostate cancer in men aged over 50 was found – the risk increased as the hours a week spent cycling increased

Compared with cycling less than 3.75 hours a week:

• cycling between 3.76 and 5.75 hours had an OR for prostate cancer of 2.94
• cycling between 5.76 and 8.5 hours had an OR of 2.89 
• cycling for more than 8.5 hours had an OR of 6.14

To determine if the results for prostate cancer were related to health-seeking behaviour (the idea that more active people are more likely to be aware of their health and seek treatment if necessary), associations between the time spent cycling and yearly consultations with doctors were also analysed. No association was found.

How did the researchers interpret the results?

The researchers concluded weekly cycling duration was not associated with erectile dysfunction or infertility. They also conclude there is a dose-response association between cycling time and prostate cancer in men aged over 50, particularly for men who cycle more than 8.5 hours a week.

One of the researchers, Dr Mark Hamer from University College London, is reported in the media as saying: "These results are not straightforward. It may be that these men are more health aware and therefore more likely to get a diagnosis."

Those who are cycling the most did not make up a huge sample, so more research is needed. Hamer went on to say that, "We are talking about very keen cyclists who are on their bikes for nine hours a week – not people who are just commuting to work."

He also made the point that cycling leads to health benefits in other areas, such as reducing the risk of type 2 diabetes, heart disease and stroke.

Conclusion

This study has looked at the associations between the number of hours spent cycling a week and erectile dysfunction, infertility and prostate cancer in men over the age of 50 who cycle regularly.

It found no association between the time spent cycling and erectile dysfunction or infertility, but did find a dose-response association with prostate cancer for men over the age of 50, with risk increasing as the time a week spent cycling increased.

As the researchers point out, this type of study cannot prove causality (that increased cycling time leads to prostate cancer), only an association. Different study designs, such as randomised controlled trials, are required before we can draw these kinds of conclusions.

There are several other limitations of this study worth noting:

• The survey was only sent to current cyclists. There was therefore no control group to compare the results with, and the results would have missed men who no longer cycle because of ill health.
• The study was carried out at only one point in time, so self-reported cycling may have differed if the men were surveyed at a different point in time. Factors such as the time of year when they answered the survey (whether it was winter or summer) may have affected their responses.
• Only self-reported data was required for classifying men as having erectile dysfunction in the past five years. It is possible that either men who reported having erectile dysfunction in fact didn't have this, or that some men with the condition were potentially missed. The researchers note the prevalence of erectile dysfunction was lower in this study than in previous studies. But they also acknowledge that because the survey was anonymous and online, this may have reduced any embarrassment of reporting erectile dysfunction and infertility.
• Only self-reported data was used to categorise the number of hours a week spent cycling. It is possible men either over or underestimated the amount of cycling they do a week.
• The study was internet-based, so there was no opportunity for interviews or physical examination, which limited the ability to explore the reported diagnoses further.
• Overall, there were only about 42 men out of 5,282 in the study who had prostate cancer, and fewer than 42 cases among the men aged over 50 (figure not reported). This means analyses were only performed on a relatively small sample of men.

Cyclists need not panic, as this research has not proved a cause and effect relationship between prostate cancer and the amount of time a week a man spends cycling.