As many as 40 percent of cancer cases, and half of cancer deaths, come down to things people could easily change, researchers said Thursday.
While Americans often worry about whether chemicals, pollution or other factors out of their control cause cancer, the new analysis shows otherwise: People are firmly in charge of much of their own risk of cancer.
The team at Harvard Medical School calculated that 20 to 40 percent of cancer cases, and half of cancer deaths, could be prevented if people quit smoking, avoided heavy drinking, kept a healthy weight, and got just a half hour a day of moderate exercise.
They used data from long-term studies of about 140,000 health professionals who update researchers on their health every two years for the analysis, published in the Journal of the American Medical Association's JAMA Oncology.
"Not surprisingly, these figures increased to 40 percent to 70 percent when assessed with regard to the broader U.S. population of whites, which has a much worse lifestyle pattern than our cohorts," Dr. Mingyang Song and Dr. Edward Giovannucci of Harvard Medical School wrote.
The analysis was simple. They broke the 140,000 people into two groups: those with a healthy lifestyle, and everyone else.
The healthy lifestyle definition was based on a large body of studies that have shown what personal habits are linked with higher or lower risks of cancer.
They include not smoking; drinking no more than one drink a day for women, two drinks a day for men; keeping a healthy weight, defined as body mass index of between a very slender 18.5 and a slightly overweight 27.5; and getting the equivalent of just over an hour of vigorous exercise or two and a half hours of moderate exercise a week.
Heavy drinking raises colon, breast, liver and head and neck cancer rates. Obesity raises the risk of esophageal, colon, pancreatic and other cancers. Smoking causes 80 to 90 percent of lung cancer deaths.
Only about 28,000 of the people qualified as following a healthy lifestyle. When the rates of cancer in their group were compared to rates in the rest of the volunteers, the differences were clear.
The incidence rates of cancer were 463 per 100,000 for women in the "healthy" group, versus 618 per 100,000 for those not meeting the healthy goals. For men, it was 283 per 100,000 who met the healthy lifestyle goals versus 425 among those who did not.
And these were health professionals, who should at least try to be healthier. When Song and Giovanucci compared the healthy group to the general, white, U.S. public, the differences were even bigger.
Their findings are not mean to be exact, and they did fudge a little. "We excluded from all cancers those in the skin, brain, lymphatic, and hematopoietic (blood and bone marrow) tissues because these cancers likely have other strong environmental causes than the ones considered in the current study, such as UV exposure, infections, irradiation, and exposures to carcinogenic substances," they wrote.
And they didn't add in other known factors, such as eating a healthy diet rich in fruits and vegetables - although they said those who followed the other healthy patterns did tend to eat better, also.
"These compelling data together with the findings of the current study provide strong support for the argument that a large proportion of cancers are due to environmental factors and can be prevented by lifestyle modification." By "environmental," they mean non-genetic causes. To a scientist, environment includes diet, exercise and other factors.
"Cancer is preventable," Dr. Graham Colditz and Dr. Siobhan Sutcliffe of Washington University School of Medicine in St. Louis agreed in a commentary.
"In fact, most cancer is preventable—with estimates as high as 80 percent to 90 percent for smoking-related cancers, such as lung and oropharyngeal cancer, and as high as 60 percent for other common, lifestyle-related cancers, such as colorectal and bladder cancer."
They said health insurance companies, medical societies and regulators should encourage doctors to help their patients do what they need to do to prevent cancer, the No. 2 cause of death in the United States.
"Guidelines and reimbursement structures should foster patient counseling on being more active, cutting back on screen time, eating a plant-based diet, keeping weight in check, and getting vaccinated," they wrote.
"The broader physical communities in which people live can also help support and build on health behaviors. Efforts here can focus on things like farmers' markets in underserved neighborhoods, safe and affordable places for people to exercise, and buildings that help foster activity," they added.
"Finally, from a policy perspective, it is important to consider actions such as cigarette taxes; subsidies on fruits, vegetables, and whole grains; quality physical education in schools; and more broadly, efforts to reduce income disparity, which is a major cause of cancer disparities."
They said Americans need to stop thinking that cancer is down to bad luck or a result of factors out of their control.
"Instead we must embrace the opportunity to reduce our collective cancer toll by implementing effective prevention strategies and changing the way we live."