“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.
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, . “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, Dr. 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, Dr. 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, Dr. 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, Dr. 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, Dr. 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, Dr. 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, Dr. Jones said, given that highly active people may spend a lot of time in the sun. “My advice,” Dr. 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.