Exercise is promoted for lowering the risk for certain gastrointestinal cancers, but recommendations lack specifics on how much is enough. Now, a large, decades-long study suggests that relatively moderate amounts of exercise, done consistently, may be all that is needed.

The study, of over 200,000 US health professionals, found that those who consistently exercised at moderate levels over three decades were 17% less likely to develop any of 10 digestive system cancers compared with those who were minimally active.

The exercise sweet spot: about 17 metabolic equivalent task (MET) hours per week — roughly equivalent to 5 hours of brisk walking or 2 hours of running each week.

Activity levels above 17 MET hours per week were not associated with any further reduction in the risk for cancer, said study author Edward Giovannucci, MD, ScD, professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health in Boston.

The bottom line for patients, Giovannucci told Medscape Medical News, is that “consistency over time is very important.”

The study, published in JAMA Oncology, is among the latest to dig into the complex relationship between physical activity and the risk for cancer.

Government guidelines and recommendations from medical societies have long promoted physical activity as a strategy for reducing the risk for cancer. Guidelines from the American Cancer Society (ACS), for example, cite data linking physical activity to the risks for at least eight cancers, including colon, esophageal, gastric, and pancreatic cancers.

The evidence, however, is largely from observational research, and it’s a mixed bag. Among the broad unanswered questions: What amount and intensity of exercise may be necessary for lowering the risk for any given cancer type?

Currently, ACS guidelines recommend the same levels of exercise advised by the CDC for general health: at least 150 minutes of moderate-intensity or 75 minutes of vigorous-intensity exercise (equivalent to at least 7.5 MET hours) per week. But whether that’s sufficient is unclear.

Giovannucci’s team wanted to look more closely at the exercise habits associated with optimal reduction in the risk for digestive cancers. They turned to data from three large prospective cohorts: the Health Professionals Follow-Up Study, 1988-2020; the Nurses’ Health Study, 1988-2021; and the Nurses’ Health Study II, 1991-2021.

In total, 231,067 men and women who were free of cancer at baseline were followed for up to 32 years. Every 2-4 years, participants reported how much time they devoted per week to various aerobic activities. Exercise consistency was based on how many years a participant met or exceeded CDC physical activity recommendations.

During follow-up, there were 6538 new diagnoses of digestive system cancers, including those of the oral cavity, throat, esophagus, stomach, small intestine, colon, rectum, pancreas, gallbladder, and liver. 

Overall, the researchers found that exercise consistency, rather than high intensity or heavy amounts, appeared key in reducing the risk for digestive system malignancies.

Compared with participants who maintained minimal levels of exercise (less than 3 MET hours per week), the risk of those cancers dropped by 17% among people who consistently achieved a median of 17 MET hours per week of exercise (hazard ratio [HR], 0.83; 95% CI, 0.75-0.90).

Going above the 17 MET-hour threshold brought no extra benefit. Even at double that level of exercise (median, 38.5 MET hours per week), the cancer risk reduction was roughly the same (HR, 0.87; 95% CI, 0.81-0.93). And such high levels of exercise, if done inconsistently over time, showed no protective association (HR, 0.91; 95% CI, 0.69-1.20).

Giovannucci acknowledged that the findings are subject to the usual limitations of observational studies, including a reliance on self-reporting. Perhaps most importantly, he and his colleagues pointed out, people who exercise regularly tend to be generally health-conscious — eating healthfully, getting recommended cancer screenings, and avoiding smoking or excessive drinking.

The researchers adjusted for those variables, but the possibility of confounding cannot be ruled out, Giovannucci said.

To Julie Gralow, MD, chief medical officer of the American Society of Clinical Oncology, the finding on consistency makes sense.

Gralow, who was not involved in the study, pointed to some of the proposed mechanisms by which physical activity affects the risk for cancer: body weight control, reductions in systemic inflammation, enhanced immune function, and improved insulin metabolism.

“It would just make sense that having this impact consistently, over long periods of time, would have the most benefit,” Gralow told Medscape Medical News.

An important message for patients, according to Gralow, is that exercise need not be grueling to potentially affect the risk for cancer. It can be broken down into manageable chunks, she noted, like taking a brisk walk on most days of the week.

Anasuya Gunturi, MD, chief of oncology at Lowell General Hospital in Lowell, Massachusetts, agreed. Gunturi, who was not involved in the study, noted that even exercise levels below 17 MET hours per week, done consistently, were in the direction of a protective association with the risk for digestive cancer vs minimal exercise.

“I think based on this and other data, physicians — particularly primary care physicians — should certainly recommend exercise for patients to reduce their risk for these types of cancers,” Gunturi said.

The Health Professionals Follow-Up Study and Nurses’ Health Studies were funded by the National Cancer Institute. The researchers reported no conflicts of interest.

Ernie Mundell is a freelance medical journalist based in Los Angeles. He has more than 30 years of experience, including editorial positions at Reuters Health and HealthDay.