The finding comes from a vast international research effort headed by a panel of experts in obesity, nutrition, cancer, public health, and epidemiology. Funding came from the nonprofit World Cancer Research Fund.
The panel's detailed, 537-page report -- Food, Nutrition, Physical Activity, and the Prevention of Cancer -- can be summed up in one sentence, says panel member W. Philip T. James, MD, DrSc, chairman of the London-based International Obesity Task Force.
"The single message is how much obesity affects cancer risk," James tells WebMD. "The message is absolutely clear as a bell: The relation of cancer to obesity is so robust, it is going to rank close to the smoking problem in America pretty soon."
High-Normal Weight Still Risky
Here's worse news: You don't have to be obese to be at increased risk of cancer.
"The risk is lowest not in the crude normal-weight range. Actually, it is better to be slim, to be lean," James says. "This is what the science shows. Even if you are fortunate in being in the upper-normal range of body weight, you still have cancer risk if you are not doing physical activity."
James says the panel went to great lengths to ensure that their findings were based solely on scientific evidence. To this end, researchers working on the project reviewed every published study on weight and cancer to come up with the 7,000 studies on which they base their conclusions and recommendations.
"You could say, 'We've heard this before.' But you never had this kind of statement backed up by mathematical analysis of 7,000 studies," James says. "This is not a report from a select group of people who have their own biases. It has been done in a three-tier system to make sure it is as rigorous and up to date and savagely analyzed as any group could do."
The panel didn't stop at analyzing cancer risk. It also came up with 10 recommendations for cutting risk.
10 Ways to Cut Cancer Risk
"Risk isn't fate. The evidence clearly shows that risk can be changed," panel member Walter J. Willett, MD, PhD, professor of medicine at Harvard University, said in a news release.
To this end, the panel came up with 10 recommendations for cutting cancer risk:
- Be as lean as possible within the normal range of body weight. Not everyone can become lean, but everyone can stop gaining weight. "Don't put an inch on your waistline or a pound on the scale," James says.
- Be physically active as part of everyday life. Get at least 30 minutes of moderate activity, such as brisk walking, every day. And cut back on couch-potato activities such as watching television.
- Eat fewer energy-dense foods. Avoid sugary drinks. Cut way back on fast food -- avoid it if possible.
- Eat mostly foods of plant origin -- at least five portions (14 ounces) of various nonstarchy vegetables and fruits every day. Eat unprocessed grains and/or legumes (beans) with every meal. Limit refined starchy foods.
- Limit intake of red meat -- beef, lamb, and pork -- to less than 18 ounces a week. Avoid smoked, cured, or salted meats.
- Limit alcoholic drinks. Zero alcohol is best for cancer prevention. But as moderate alcohol has heart benefits, limit intake to no more than two drinks a day for men and one drink a day for women.
- Limit sodium consumption to 2.4 grams per day. Avoid salty foods. And watch out for imported foods that may be made from moldy grains -- they contain cancer-causing aflatoxins.
- Dietary supplements are not recommended for cancer prevention. "We looked at this issue in great detail," James says. "The current evidence that the use of supplements can lower cancer rates is explicitly not there."
- Breastfeeding protects both the mother and the child against cancer. Aim to breastfeed infants exclusively up to age 6 months, and supplement baby food with breastfeeding thereafter.
- Cancer survivors should receive nutritional care from a qualified professional. The recommendations for diet, healthy weight, and physical activity are particularly important for cancer survivors.