Diseases as complicated as cancer are often caused by genetic factors that are out of your control and risk factors that you can change. Now, researchers have calculated just how much of that risk is within a person’s control.

In a study published in CA: A Cancer Journal for Clinicians, researchers led by Dr. Farhad Islami at the American Cancer Society analyzed national cancer data and calculated how much of cancer cases and deaths can be attributed to factors that people can change. These included smoking, exposure to second-hand smoke, being overweight or obese, drinking too much alcohol, eating red and processed meats, eating too few fruits and vegetables, not exercising, exposure to ultraviolet radiation through activities like tanning and six cancer-related infections (including HPV).

Among more than 1.5 million cancers in 2014, 42% were traced to these factors, as well as 45% of deaths in that year.

Cigarette smoking accounted for 20% of the cancer cases: the largest contribution by a modifiable risk factor. Overweight or obesity was the second-greatest contributor, accounting for nearly 8% of cases and 6.5% of deaths. Alcohol accounted for 5.6% of cases and 4% of deaths; red and processed meat consumption contributed to 1.3% of cases and deaths, and not exercising enough accounted for 2.9% of cancer cases and 2.2% of deaths. Eating too few fruits and vegetables — which applies to around 90% of Americans — was responsible for nearly 2% of cancers and 3% of cancer deaths.

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The data are sobering, especially because deaths from cancer have dropped by 25% since 1991. While that’s good news, the latest data suggest that the decline might be even steeper if people address some of their risky behaviors that contribute to cancer. About 600,000 people are expected to die of cancer in 2017.



Islami says that the study should be seen as encouraging overall, since it supports the idea that a good proportion of cancer cases and deaths might be avoided. While factors such as alcohol and lack of exercise accounted for a much smaller percentage of cases and deaths than smoking, for example, together they have a sizable effect on cancer risk. The combination of excess weight, alcohol, poor diet and lack of exercise contributed to nearly 14% of cancer cases in men and 22% in women. For women, that contribution was greater than the effect of smoking.

“If [people] avoid some of these risk factors, they could substantially reduce their risk of many types of cancer,” says Islami.

He hopes that the findings encourage local, state and federal lawmakers to support more policies that reduce these modifiable risk factors, such as creating smoke-free areas, constructing walkable communities and encouraging physical activity. He says that doctors can also learn lessons from the data, since studies show that people are more likely to change their behaviors if their doctors inform them of how risky they might be.

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