Gut Check is a periodic look at health claims made by studies, newsmakers, or conventional wisdom. We ask: Should you believe this?

The claim: Exercise reduces the risk of 13 kinds of cancer, researchers reported on Monday.

Tell me more:

The research, in JAMA Internal Medicine, pooled data from 12 earlier studies, the oldest of which began in 1982, of 1.44 million people. That made it the largest-ever analysis of exercise and cancer. Volunteers were asked how much physical activity they’d engaged in over the past year. Half exercised less than about 150 minutes per week, half exercised more. Their health was tracked for seven to 28 years.

Compared to the bottom 10 percent of exercisers (measured by minutes of leisure activities, such as walking, running, and swimming each week), the top 10 percent had 42 percent less esophageal cancer, 27 percent less liver cancer, 26 percent less lung cancer (but only for current and ex-smokers), 21 percent less endometrial cancer, 16 percent less colon cancer, and 10 percent less breast cancer. Overall, they had 7 percent less cancer. (They also had a 27 percent higher risk of malignant melanoma, but only in places with higher levels of UV, probably because exercising exposed them to cancer-causing sunlight.) The researchers found no association between exercising and 13 other cancers, including pancreatic, ovarian, and brain.

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“This study highlights that physical activity has benefits for cancer prevention,” said Alpa Patel of the American Cancer Society, who led the study with Steven Moore of the National Cancer Institute.

In the United States, men have a 42 percent chance of developing cancer; women, a 38 percent chance. A 7 percent reduction lowers those risks to 39 percent and 35 percent, respectively. Patel called that a “significant” drop, adding that “the more interesting finding is the vast number of cancer sites associated with inactivity.”

Hundreds of studies have examined whether exercise is linked to a lower chance of developing cancer. They have been inconclusive for virtually every kind except colon, breast, and endometrium. And even for those three, the studies did not prove causation. Neither did the new one.

Really?

An observational study like this — ask volunteers how much they exercised, then see who develops cancer — can’t show that exercise caused a lower risk of cancer. It can only show that the two go together. Proving causality requires assigning some volunteers to exercise, others to be sedentary, and then comparing their cancer incidence. Only that type of study can make sure some third factor is not at work.

Smokers, for instance, are less physically active (it’s hard to use a treadmill when you’re hacking or gasping for breath) and more likely to develop lung and other cancers. The researchers acknowledge that they “cannot fully exclude the possibility that diet, smoking, and other factors may affect the results.”

One of those “other factors” is weight. Like smoking, obesity is associated with less exercise and more cancer. Finding that more sedentary people have more cancer could be a roundabout way of seeing that obesity raises the risk of some cancers. When the scientists used standard statistical techniques to factor out body mass index, most of the reductions in cancer risk held up, though the benefits of exercise shrank. Unfortunately, a 2016 paper has cast doubt on whether statistical techniques to control for such confounding factors work as advertised.

Even more worrisome are the confounders you don’t even think to control for. The study did not include genetics, which might influence both exercise and cancer risk, said statistician Rebecca Goldin, a professor of mathematics at George Mason University who was not involved in the research. “Other health factors might influence both, too. Imagine you have something simmering before cancer has reared its ugly head, which might cause you to both not exercise and develop cancer.” That would show up as the association the study found, undermining the idea that exercise helps prevent cancer.

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“If people have precancerous illness, there might be latent effects that cause them both to have poor health and not to exercise much,” coauthor Moore agreed.

Another concern is that the study compared cancer incidence among the bottom 10 percent of exercisers to the top 10 percent. Those extremes could well differ on many more attributes than just exercise. Someone who works out more than 90 percent of the population might have other habits that avert cancer, including eating a healthy diet and undergoing regular cancer screening (especially for cervical or colon cancer) to detect abnormalities before they become tumors. Exercise enthusiasts and couch potatoes could also differ in socioeconomic characteristics that affect risk of cancer. The study authors tried to control for these, too.

The verdict:

We all hope that healthy living will keep cancer away, but the evidence that physical activity does that is far from definitive. And even if this study is correct, the benefit is small.

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