For decades, experts have routinely said that a third of all cancers were caused by smoking.
“A bunch of us got to thinking sometime about October of last year. You know in 1981 when we said a third of all cancer deaths were due to tobacco, those guys knew that 10 cancers were linked to tobacco and one additional might be,” Brawley said.
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It’s now known that 17 cancer types are linked to smoking, that obesity raises cancer risk and that fewer people smoke. And now, three-fourths of Americans are overweight or obese. So a team re-ran the numbers.
What they found surprised them. It’s still true that 45 percent of cancer deaths are due to things people can change — mostly smoking, but also, increasingly, obesity and drinking alcohol.
“Cigarette smoking was associated with far more cancer cases and deaths than any other single risk factor, accounting for nearly 20 percent of all cancer cases and 30 percent of all cancer deaths, followed by excess body weight,” the team wrote in a report released Tuesday.
And drinking too much alcohol causes 5.6 of cancer causes and 4 percent of deaths, they found.
“One of the things that came through that surprised me was alcohol,” Brawley said.
Drinking too much can cause liver cancer and is linked with breast cancer and some forms of head and neck cancer.
“The combination of excess body weight, alcohol intake, poor diet, and physical inactivity accounted for the highest proportion of all cancer cases in women and was second only to tobacco smoking in men," the researchers added.
“These four combined risk factors also accounted for the second highest proportion of cancerdeaths in both men and women.”
Diet was especially important for colon cancer, the team found. “Red and processed meat consumption accounted for 5.4 percent and 8.2 percent of colorectal cancers, respectively,” the team wrote. “Low dietary fiber accounted for 10.3 percent of colorectal cancer cases, while low dietary calcium accounted for 4.9 percent of cases.”
The team did not include air pollution as a modifiable risk factor for cancer, because the data is not terribly clear and because it’s not clear what most people can do about it if they live in an area with air pollution, Brawley said.
Cancer is caused by damaged DNA. The National Cancer Institute estimates that inherited mutations play a role in about 5 to 10 percent of all cancers. Many mutations may also arise randomly, as a result of bad luck as cells divide in the body.
Maggie Fox is a senior writer for NBC News and TODAY, covering health policy, science, medical treatments and disease.