To start calculating the P.A.F. of cancer resulting from inactivity, the American Cancer Society scientists first pulled anonymized data from the U.S. Cancer Statistics database about cases, nationally and by state, for all Americans 20 and older between 2013 and 2016. The team focused both on total cancer cases and on seven types of cancer that in past studies had been closely tied in part to activity (or inactivity), which are stomach, kidney, esophageal, colon, bladder, breast and endometrial tumors.
Next they checked on how much American adults claim to move, based on more than half a million replies to two large federal surveys. Both ask people in what ways and how often they exercise. The researchers drew responses from adults in every state and grouped them, based on whether or not people met the American Cancer Society recommendations for physical activity. Those guidelines call for, ideally, 300 minutes, or five hours, of moderate exercise, like a brisk walk, every week to reduce cancer risk.
Finally, the researchers adjusted these statistics for body mass and other factors, gathered additional data about cancer risks and plugged all of the numbers into an equation, which then spit out the P.A.F. for cancers linked to inactivity. That number turned out to be 46,356, or about 3 percent of all cancers annually (excluding non-melanoma skin cancers).
When they then looked at individual types of cancer, stomach cancer was most tied to inactivity, with about 17 percent of all cases annually attributable to not moving, versus 4 percent of bladder cancers. Likewise, the numbers varied by state, rising to nearly 4 percent of cancers in many Southern states, where residents tend to report getting relatively little exercise, but about 2 percent in much of the Mountain West, which has relatively active populations.
The good news, however, is that these numbers are malleable. We have the ability to lower them. Exercise could “potentially prevent many cancers in the United States,” said Adair K. Minihan, an associate scientist at the American Cancer Society, who led the new study. If everyone in America who can exercise started walking for an hour on weekdays, she said, theoretically the 46,356 cases tied to inactivity should disappear.