It’s probably not a surprise to hear that exercise is good for you, but it is interesting to see it clearly quantified. Plus the result that men with good cardiorespiratory fitness - the aerobic fitness which is developed by cycling - are associated with an increased risk of two specific types of cancers is a curious finding itself.
First of all, let’s go over how this study, published in the British Journal of Sports Medicine, was conducted, to better understand the context.
The cohort consists of Swedish men who were conscripted into the military between 1968 and 2005, totalling just over one million individuals being tracked for an average of 33 years. As part of their conscription, the men were given a detailed medical assessment, measuring - amongst other metrics - their aerobic fitness using a variant of a ramp test.
This provides a huge and detailed data set which researchers can delve into and pull out notable trends. As Aron Onerup et al notes: “There is a paucity of studies with a sufficiently large sample size and sufficiently long follow-up to assess the associations between CRF [cardiorespiratory fitness] and the development of site-specific cancers.”
Sure enough, this Swedish population has been used regularly in the past, including studies relating to prostate cancer diagnosis and total cancer incidence.
The link between cardiorespiratory fitness and site-specific cancer
Coming back to the particulars of this study, the objective was to “assess the associations between cardiorespiratory fitness (CRF) in young men and the incidence of site-specific cancer.”
Upon their conscription into the military, one of the assessments the Swedes had to perform was a test of their maximal aerobic workload on a “cycle ergometer” - i.e. an exercise bike. After five minutes of warming up, the resistance was “increased by 25 W per minute until interrupted by exhaustion” - a procedure very familiar to cyclists who have performed a ‘ramp test’ as part of their structured cycling training plans.
From the results of those tests, the researchers split the cohort into three groups: low, moderate and high cardiorespiratory fitness. The researchers then looked at the incident rate of “18 of site-specific cancers” across these groups - and in many cases there was a strong association between cancer and low cardiorespiratory fitness.
“[A] higher CRF is associated with a lower hazard for cancer in the head and neck, oesophagus, stomach, pancreas, liver, colon, rectum and kidney.” However, the incident rate of “prostate cancer and malignant skin cancer” were both higher for those in the higher cardiorespiratory fitness group compared to the low fitness group.
Aron Onerup et al speculate that: “the increase with higher CRF could possibly be due to a higher UV exposure for those with higher CRF,” but they also point out that their “data did not allow adjustment for UV exposure.”
Regarding the increase in prostate cancer mortality, Aron Onerup et al point out that this increase in association is “probably explained by increased prostate cancer screening”, as there isn’t a demonstrable link between cardiorespiratory fitness and “aggressive prostate cancer nor prostate cancer mortality.”
What is likely happening here is that fitter individuals are more proactive in getting tested, and so their medical records will show a prostate cancer diagnosis. Less fit individuals are less likely to get tested, so may not have a prostate cancer diagnosis on their record - but that doesn’t mean they are less likely to get prostate cancer, only that they aren’t getting checked for it (on average).
Aron Onerup et al’s study indicates that “public health efforts aimed at reducing cancer should focus on aerobic PA of sufficient relative intensity to increase cardiorespiratory fitness.” Moreover, they note that this conclusion has also been suggested by studies looking at all-cause mortality.
So, even though Aron Onerup et al has shown a link between skin cancer and cardiorespiratory fitness, the benefits still outweigh the negatives. Plus you can significantly reduce your risk of skin cancer by simply using sunscreen - you can find our guide to the best cycling sunscreen we've used over here.
So there we have it. Potentially not a surprising conclusion to have found, but another brick in the wall of knowledge is always worth its place.
Find out more about the links between cycling and prostate cancer in our guide over here.
