Why reducing cancer risk is more than ‘just a walk in the park’

This week, researchers at the US National Cancer Institute reported that higher levels of physical activity were associated with a lower risk of 13 types of cancers.

This, predictably, led some national newspapers to claim that the secret to preventing these types of cancer was, quite literally, a walk in the park.

There’s already lots of good evidence that being physically active can reduce the risk of three types of cancer – bowel cancer, breast cancer after the menopause, and womb cancer.

So the new study potentially puts more types of cancer in the running, including oesophageal, liver, and lung cancers. But as we’ll explore below, we’re not completely convinced the evidence is strong enough to add these cancers to the list just yet.

And while walking is a great way to keep active, with a range of tangible health benefits, when it comes to reducing cancer risk, the occasional short walk or cycle every now and then probably won’t make much difference. It’s all about being active, and keeping active.

The new study also highlights why we must tread carefully when it comes to research into how physical activity affects cancer risk. We explain why, on both counts, a little later.

What has the new research found?

The study, published in the journal JAMA Internal Medicine, brings together data from 12 previous US and European studies that looked at ‘leisure-time physical activity’, which covers any activity you might do outside work that improves or maintains health. And could explain why the media reports focused on walking and cycling.

These studies collected information on how active people were, then used follow-up questionnaires and medical data to see how many participants went on to develop any of 26 kinds of cancer. With a combined 1.44 million participants, and over 185,000 recorded cases of cancer, the study is the largest analysis on physical activity and cancer risk to date.

The US team found that leisure-time activity was not only linked to a reduced risk of bowel, postmenopausal breast and womb cancers (the three that were already known), but also nine other types of cancer too: oesophageal (food pipe or gullet), gastric cardia (the top part of the stomach where the oesophagus joins), liver, lung, kidney, stomach, myeloid leukaemia, head and neck, bladder, and myeloma.

They also found that people who do lots of physical activity might actually have a higher risk of certain cancers, such as malignant melanoma. This makes sense, because people often exercise outdoors and may not adequately cover their skin (so if you are exercising outside, be sure to do it safely).

So why must we tread carefully?

While the study is encouraging, and reinforces the importance of being physically active, it leaves a number of questions unanswered.

Those people doing the most physical activity were found to have a lower risk of certain cancers compared with people doing the least, but this doesn’t tell us at what level of activity we might start to see a benefit, or whether the benefits increase as people do more activity. And there may be other differences between these groups that also affect their chance of being diagnosed with cancer – such as how healthy they are more generally.

The researchers did take body size and smoking into account, but it’s still possible that differences that weren’t accounted for might have affected the findings. This is notoriously difficult to untangle in cancer studies, and even the authors themselves recognise that there could be differences that may have affected the results.

Also, not all of the studies the authors include reported how much time participants spent being active, and whether they were moderately or intensely active. These inconsistencies mean we can’t be sure how reliable the results are.

And the researchers relied on studies that had asked participants to remember how active they had been, which could lead to inaccuracies. Plus some people have physically active jobs, which this study couldn’t account for. What’s more, the reported activity levels are only likely to give a snapshot of someone’s behaviour at that point in time.

The results also contradict those of some previous analyses looking at studies of physical activity and cancer risk. For example, a previous large analysis of physical activity and risk of myeloid leukaemia found no significant association.

Not just a walk in the park

This new study may suggest physical activity can reduce the risk of 13 types of cancer but it’s not definitive. Perhaps more importantly, it doesn’t mean you can reduce your cancer risk by a one-off cycle ride or brisk walk this weekend, which is what several media reports suggested. We’re not sure where the claim came from, but while walking may be a legitimate leisure-time activity and a great way to increase the amount of physical activity you do – you can slip it into your daily routine, you don’t need any fancy clothing or equipment, and it’s free – when it comes to reducing cancer risk, we need to be active and keep active.

If the thought of the gym brings you out in a cold sweat, don’t fret. Moderately intense activity includes anything that makes you warmer and slightly out of breath, so alongside walking and cycling, activities like dancing, gardening and even household chores can all count. And you don’t have to do the Government’s recommended 2.5 hours a week all in one go – you can add up the bits you do over the week.

So while a brisk walk won’t necessarily prevent 13 types of cancer, it’s certainly a good place to start.

Thea Cunningham is a health information officer at Cancer Research UK