Does Exercise Reduce Cancer Risk? It’s Just Not That Simple

“Exercise is medicine” has become something of a mantra, with good reason. There’s no doubt that regular physical activity has a broad range of health benefits. Exercise can improve circulation, help control weight, reduce stress, and boost mood — take your pick.

Lower cancer risk is also on the list — with exercise promoted as a risk-cutting strategy in government guidelines and in recommendations from professional groups such as the American Cancer Society.

Despite confidently worded recommendations, the relationship between exercise and cancer risk is much less certain than the guidelines would suggest. The bulk of the data hangs on less rigorous, observational studies that have linked physical activity to lower risks for certain cancers, but plenty of questions remain.

What are the cancer types where exercise makes a difference? How significant is that impact? And what, exactly, defines a physical activity pattern powerful enough to move the needle on cancer risk?

Here’s an overview of the state of the evidence.

Exercise and Cancer Types: A Mixed Bag

When it comes to cancer prevention strategies, guidelines uniformly endorse less couch time and more movement. But a deeper look at the science reveals a complex and often poorly understood connection between exercise and cancer risk.

For certain cancer types, the benefits of exercise on cancer risk seem fairly well-established.

The latest edition of the Physical Activity Guidelines for Americans, published in 2018, cites “strong evidence” that regular exercise might curb the risks for breast and colon cancers as well as bladder, endometrial, esophageal, kidney, and gastric cancers. These guidelines also point to “moderate”-strength evidence of a protective association with lung cancer.

The evidence of a protective effect, however, is strongest for breast and colon cancers, said Jennifer Ligibel, MD, senior physician in the Breast Oncology Center at Dana-Farber Cancer Institute, Boston, Massachusetts. “But,” she pointed out, “that may be because they’re some of the most common cancers, and it’s been easier to detect an association.”

Guidelines from the American Cancer Society, published in 2020, align with the 2018 recommendations. 

“We believe there’s strong evidence to suggest at least eight different types of cancer are associated with physical activity,” said Erika Rees-Punia, PhD, MPH, senior principal scientist, epidemiology and behavioral research at the American Cancer Society.

That view is not universal, however. Current recommendations from the World Cancer Research Fund and American Institute for Cancer Research, for example, are more circumspect, citing only three cancers with good evidence of a protective effect from exercise: Breast (postmenopausal), colon, and endometrial.

“We definitely can’t say exercise reduces the risk of all cancers,” said Lee Jones, PhD, head of the Exercise Oncology Program at Memorial Sloan Kettering Cancer Center in New York City. “The data suggest it’s just not that simple.”

And it’s challenging to put all the evidence together, Jones added.

The physical activity guidelines are based on published systematic reviews, meta-analyses, and pooled analyses of data from observational studies that examined the relationship between physical activity — aerobic exercise, specifically — and cancer incidence. That means the evidence comes with all the limitations observational studies entail, such as how they collect information on participants’ exercise habits — which, Jones noted, is typically done via “monster questionnaires” that gauge physical activity in broad strokes.

Pooling all those findings into a meta-analysis is tricky, Jones added, because individual studies vary in important ways — from follow-up periods to how they quantify exercise and track cancer incidence.

In a study published in February in Cancer Cell, Jones and his colleagues attempted to address some of those issues by leveraging data from the PLCO screening trial.

The PLCO was a prospective study of over 60,000 US adults that compared the effects of annual screening vs usual care on cancer mortality. At enrollment, participants completed questionnaires that included an assessment of “vigorous” exercise. Based on that, Jones and his colleagues classified 55% as “exercisers” — meaning they reported 2 or more hours of vigorous exercise per week. The remaining 45%, who were in the 0 to 1 hour per week range, were deemed non-exercisers.

Over a median of 18 years, nearly 16,000 first-time invasive cancers were diagnosed, and some interesting differences between exercisers and non-exercisers emerged. The active group had lower risks for three cancers: Head and neck, with a 26% lower risk (hazard ratio [HR], 0.74), lung (a 20% lower risk), and breast (an 11% lower risk).

What was striking, however, was the lack of connection between exercise and many cancers cited in the guidelines, including colon, gastric, bladder, endometrial, and renal cancers.

Perhaps even more surprising — exercisers had higher risks for prostate cancer (12%) and melanoma (20%). This finding, Jones said, is in line with a previous pooled analysis of data from 12 US and European prospective cohorts. In this study, the most physically active participants (90th percentile) had higher risks for melanoma and prostate cancer, compared with the least active group (10th percentile).

The melanoma findings do make sense, Jones said, given that highly active people may spend a lot of time in the sun. “My advice,” Jones said, “is, if you’re exercising outside, wear sunscreen.” The prostate cancer findings, however, are more puzzling and warrant further research, he noted.

But the bottom line is that the relationship between exercise and cancer types is mixed and far from nailed down.

How Big Is the Effect?

Even if exercise reduces the risk for only certain cancers, that’s still important, particularly when those links appear strongest for common cancer types, such as breast and colon.

But how much of a difference can exercise make?

Based on the evidence, it may only be a modest one. A 2019 systematic review by the Physical Activity Guidelines Advisory Committee provided a rough estimate: Across hundreds of epidemiological studies, people with the highest physical activity levels had a 10%-20% lower risk for the cancers cited in the 2018 exercise guidelines compared with people who were least active.

These figures, however, are probably an underestimate, said Anne McTiernan, MD, PhD, a member of the advisory committee and professor of epidemiology, at Fred Hutchinson Cancer Center, Seattle, Washington.

“This is what we usually see when a factor is not measured very well,” said McTiernan, explaining that the individual studies differed in their categories of “highest” and “lowest” physical activity, such that one study’s “highest” could be another’s mid-range.

“In other words, the effects of physical activity are likely larger” than the review found, McTiernan said.

The next logical question is whether a bigger exercise “dose” — more time or higher intensity — would have a greater impact on cancer risk. A 2019 study published in the Journal of Clinical Oncology tried to clarify that by pooling data on over 750,000 participants from nine prospective cohorts.

Overall, people meeting government recommendations for exercise — equivalent to about 2.5-5 hours of weekly moderate activity, such as a brisk walk, or about 1.25-2.5 hours of more vigorous activities, like running — had lower risks for seven of 15 cancer types studied compared with less active people.

For cancers with positive findings, being on the higher end of the recommended 2.5- to 5-hour weekly range was better. Risk reductions for breast cancer, for instance, were 6% at 2.5 hours of physical activity per week and 10% at 5 hours per week. Similar trends emerged for other cancer types, including colon (8%-14%), endometrial (10%-18%), liver cancer (18%-27%), and non-Hodgkin lymphoma in women (11%-18%).

But there may be an exercise sweet spot that maximizes the cancer risk benefit.

Among people who surpassed the recommendations — exercising for more time or more intensely — the risk reduction benefit did not necessarily improve in a linear fashion. For certain cancer types, such as colon and endometrial, the benefits of more vigorous exercise “eroded at higher levels of activity,” the authors said.

The issue here is that most studies have not dug deeply into aerobic exercise habits. Often, studies present participants with a list of activities — walking, biking, and running — and ask them to estimate how often and for what duration they do each.

Plus, “we’ve usually lumped moderate and vigorous activities together,” Rees-Punia said, which means there’s a lack of “granular data” to say whether certain intensities or frequencies of exercise are optimal and for whom.

Why Exercise May Lower Cancer Risk

Exercise habits do not, of course, exist in a vacuum. Highly active people, Ligibel said, tend to be of higher socioeconomic status, leaner, and have generally healthier lifestyles than sedentary people.

Body weight is a big confounder as well. However, Rees-Punia noted, it’s also probably a reason that exercise is linked to lower cancer risks, particularly by preventing weight gain. Still, studies have found that the association between exercise and many cancers remains significant after adjusting for body mass index.

The why remains unclear, though some studies offer clues.

“There’s been some really interesting mechanistic research, suggesting that exercise may help inhibit tumor growth or upregulate the immune system,” Ligibel said.

That includes not only lab research but small intervention studies. While these studies have largely involved people who already have cancer, some have also focused on healthy individuals.

A 2019 study from Ligibel and her colleagues, which randomly assigned 49 women newly diagnosed with breast cancer to start either an exercise program or mind-body practices ahead of surgery, found exercisers, who had been active for about a month at the time of surgery, showed signs of immune system upregulation in their tumors, while the control group did not.

Among healthy postmenopausal women, a meta-analysis of six clinical trials from McTiernan and her colleagues found that exercise plus calorie reduction can reduce levels of breast cancer-related endogenous hormones, more so than calorie-cutting alone. And a 2023 study found that high-intensity exercise boosted the ranks of certain immune cells and reduced inflammation in the colon among people at high risk for colon and endometrial cancers due to Lynch syndrome.

Defining an Exercise ‘Prescription’

Despite the gaps and uncertainties in the research, government guidelines as well as those from the American Cancer Society and other medical groups are in lockstep in their exercise recommendations: Adults should strive for 150-300 minutes of moderate-intensity aerobic exercise (like brisk walking), 75-150 minutes of vigorous activity (like running), or some combination each week.

The guidelines also encourage strength training twice a week — advice that’s based on research tying those activity levels to lower risks for heart disease, diabetes, and other chronic conditions.

But there’s no “best” exercise prescription for lowering cancer risk specifically. Most epidemiological studies have examined only aerobic activity, Rees-Punia said, and there’s very little known about whether strength conditioning or other moderate heart rate-elevating activities, such as daily household chores, may reduce the risk for cancer.

Given the lack of nuance in the literature, it’s hard to say what intensities, types, or amounts of exercise are best for each individual.

Going forward, device-based measurements of physical activity could “help us sort out the effects of different intensities of exercise and possibly types,” Rees-Punia said.

But overall, McTiernan said, the data do show that the risks for several cancers are lower at the widely recommended activity levels.

“The bottom-line advice is still to exercise at least 150 minutes per week at a moderate-intensity level or greater,” McTiernan said.

Or put another way, moving beats being sedentary. It’s probably wise for everyone to sit less, noted Rees-Punia, for overall health and based on evidence tying sedentary time to the risks for certain cancers, including colon, endometrial, and lung.

There’s a practical element to consider in all of this: What physical activities will people actually do on the regular? In the big epidemiological studies, McTiernan noted, middle-aged and older adults most often report walking, suggesting that’s the preferred, or most accessible activity, for many.

“You can only benefit from the physical activity you’ll actually do,” Rees-Punia said.

Ligibel echoed that sentiment, saying she encourages patients to think about physical activity as a process: “You need to find things you like to do and work them into your daily life, in a sustainable way.”

“People often talk about exercise being medicine,” Ligibel said. “But I think you could take that too far. If we get too prescriptive about it, that could take the joy away.”