A study published this week in the British Journal of Sports Medicine suggests that the average adult should aim to get around nine to 10 hours of exercise a week—far above the World Health Organization’s (WHO’s) guidance of 150 minutes a week—to see a substantial reduction in their risk of stroke or heart attack. But some cardiovascular and fitness experts say the new research needs to be approached with a healthy dose of caution.
The study’s authors are “playing down the benefit of physical activity,” says Sean Heffron, an assistant professor of medicine and a cardiologist specializing in exercise science at New York University.
The study analyzes accelerometer data collected over the course of a week from 17,000 individuals included in the U.K. Biobank, a long-term health study. It finds that for the average person to experience a 30 percent reduction in the risk of stroke or heart attack, they need to be doing around 560 to 610 minutes of moderate to vigorous physical activity a week. And people who are less fit may need to do as much as 50 minutes more exercise than people who are more fit to see the same benefits, according to the study.
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For comparison, the WHO guidelines of 150 minutes of moderate exercise a week is associated with an 8 to 9 percent reduction in risk. That’s not nothing, Heffron says. “If we came up with a new medicine that improved risk that much, we’d be thrilled,” he says.
The results don’t indicate that the WHO’s guidance on exercise is wrong, Heffron says. Nor do they suggest that short, high-intensity bursts of exercise such as high-intensity interval training aren’t also associated with large reductions in cardiovascular disease risk, says Ulrik Wisløff, a professor at the Norwegian University of Science and Technology, who heads the university’s Cardiac Exercise Research Group.
“The 150-minute recommendation was never intended to represent an ‘optimal’ target,” Wisløff says. “Rather it was designed as a realistic, achievable public health threshold associated with meaningful health benefits in the population.”
The study’s design also risks obscuring the relative intensity of physical activity for different individuals, Wisløff says. An older person, for example, might view the same task, say, a walk around the block, as vigorous exercise, while a younger and fitter person might not see that as exercise at all.
In turn, many individuals may underestimate how much exercise they are already getting, Heffron says. Moderate walking, tennis playing, gardening or anything else that makes you eventually break a sweat counts as vigorous activity. “The gym does not hold a monopoly on exercise,” he says.
Wisløff agrees, stressing that the health effect of any kind of activity depends on its intensity. He points to previous studies that found that even five minutes a day of moderate to vigorous physical activity could reduce mortality risk by around 30 percent in people who otherwise didn’t exercise. “Small amounts of relatively intense activity appear extraordinarily powerful,” he says.
Genetics also plays a role in fitness that the study just can’t account for, Heffron says. And the study only includes a week of data: it’s entirely possible participants exercised more than usual during that week, which would complicate the findings. And as for the result that less fit people need to do more to see the same benefits, Heffron says that “as a clinician, I would ignore that wholly.”
“Your individual health and longevity is not a competition,” he says. “We’re confident that exercising is beneficial for multiple reasons, and we can’t determine who is going to benefit most from exercise.”
“It’s clear that no activity puts you at markedly increased risk [of conditions such as diabetes and hypertension],” he adds. “Going from zero to anything on an individual basis can be helpful.”
