It may come as no surprise to hear that exercise is good for you, but it’s interesting to see it measured so clearly. Plus the finding that men with good cardiorespiratory fitness – aerobic fitness developed by cycling – is associated with an increased risk of two specific types of cancer is an interesting finding.
Let’s first look at how this study, published in the British Journal of Sports Medicine, was conducted, to better understand the context.
The group, made up of Swedish men who were conscripted into the military between 1968 and 2005, numbered more than one million traced for an average of 33 years. As part of their mandatory military service, the men are given a detailed medical assessment, measuring – among other metrics – their aerobic fitness using a variant of the ramp test.
This provides a very large, detailed data set that researchers can drill into and pull out important trends. As Aron Onerup et al notes: “There is a paucity of studies with large enough sample sizes and long enough follow-ups to assess the relationship between CRF [cardiorespiratory fitness] and site-specific cancer development.”
Sure enough, this Swedish population has been used regularly in the past, including studies related to prostate cancer diagnosis and total cancer incidence.
The relationship between cardiorespiratory fitness and cancer is site specific
Coming back to the details of this study, the aim was to “assess the relationship between cardiorespiratory fitness (CRF) in young men and site-specific cancer incidence.”
After military service, one of the assessments Swedes have to do is test their maximal aerobic workload on an “ergometer cycle” – that is, an exercise bike. After five minutes of warming up, endurance “increases by 25 W per minute until interrupted by fatigue” – a procedure very familiar to cyclists who have performed a ‘ramp test’ as part of their structured cycling training plan.
Based on the test results, the researchers divided the cohort into three groups: low, moderate, and high cardiorespiratory fitness. The researchers then looked at the incidence rates of “18 site-specific cancers” across these groups – and in most cases there was a strong association between cancer and low cardiorespiratory fitness.
“[A] Higher CRF is associated with a lower risk of cancers in the head and neck, esophagus, stomach, pancreas, liver, colon, rectum and kidney.” However, the incidence rates of “prostate cancer and malignant skin cancer” were both higher in the higher cardiorespiratory fitness group compared to the low fitness group.
Aron Onerup et al speculate that: “the higher CRF increase may be due to higher UV exposure for those with higher CRF,” but they also point out that “their data does not allow adjustment for UV exposure.”
Regarding the increase in deaths from prostate cancer, Aron Onerup et al point out that this increased association “may be explained by increased prostate cancer screening”, as there is no demonstrable association between cardiorespiratory fitness and “aggressive prostate cancer or death from prostate cancer.”
What might be happening here is that fitter individuals are more proactive about getting tested, so their medical records will show a diagnosis of prostate cancer. Individuals who are less fit are less likely to get tested, so may not have a diagnosis of prostate cancer on their record – but that doesn’t mean they’re less likely to get prostate cancer, just that they’re not being screened for it (on average).
Aron Onerup et al‘s study indicates that “public health efforts aimed at reducing cancer should focus on aerobic PA with sufficient relative intensity to improve cardiorespiratory fitness.” Additionally, they note that this conclusion is also suggested by studies looking at all-cause mortality.
So, even though Aron Onerup et al have demonstrated an association 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 sunscreens we’ve used here.
So there you have it. It may not be a surprising conclusion to find, but another brick in the wall of knowledge is always worth its place.
Find out more about the relationship between cycling and prostate cancer in our guide here.
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