Trying to protect yourself against cancer through your diet can be a confusing proposition. Should you try to stick to a low-fat regimen? Load your plate with tomatoes and blueberries? Bulk up with fiber? The best strategy may be deceptively simple. Rather than focusing on a specific nutrient or percentage of fat, the top recommendation in the recently released set of cancer prevention guidelines is to stay slim, period.
"Weight is the critical thing," says Barry Popkin, director of the University of North Carolina's Interdisciplinary Obesity Center and a member of a panel that will translate the findings into policy recommendations. "That's a sea change from the idea that one unique little component of diet has a powerful effect."
To further confuse matters, the report, issued by the American Institute for Cancer Prevention and the World Cancer Research Fund, may appear to say just the opposite of a study out this week, which found that while obesity leads to more death from some kinds of cancer and from heart disease, 20 or so extra pounds may not pose a risk. But that study looked only at the risk of death, not of developing the diseases in the first place, cautions Meir Stampfer, who researches nutrition and epidemiology at the Harvard School of Public Health. He says research has consistently shown a steady relationship between weight and cancer risk.
The new cancer report puts it this way: "Be as lean as possible within the normal range of body weight." That means keeping your body mass index, a number that relates weight to height, between 21 and 23. For someone who is 5 foot 4, the target zone would be between 122 and 134 pounds. Some of the other recommendations found in the 500-plus-page report mention specific foods—for example, avoiding sugary drinks, alcohol, and fast food—more because they tend to lead to weight gain than because the sugar and fat themselves are harmful. "The underlying biological consequences of overweight are quite continuous down to low levels," says Stampfer. "A BMI of 25 increases the risk of postmenopausal breast cancer more than [a BMI] of 21." That's why the report recommends that people try to keep their BMI maxed out at 23, rather than 24.9, the high end of the usual healthy range. (It also recommends that people don't dip below a BMI of 21, since some cancers, like premenopausal breast cancer, may be jump-started by very low body fat.)
What about diet? Researchers have been studying the links between the components of food and cancer for years, with mixed results. This fall, a study in the Journal of the National Cancer Institute reported that postmenopausal women who stick to a low-fat diet for at least four years might cut their risk of ovarian cancer. The JNCI also published a review of research this summer revealing that lycopene—the chemical in tomatoes thought to prevent cancer—actually doesn't work as previously thought. A study of 49,000 women published in the Journal of the American Medical Association in 2006 found that those who cut fat from their diet were just as vulnerable to colon and breast cancer as those who did not. And the idea that fiber guards against colon cancer hasn't been supported by most research.
The new report didn't find convincing evidence that eating more garlic, fiber, veggies, fruits, or foods containing folate, lycopene, or selenium could reduce the chances of developing any kind of cancer. The best case could be made for fruit and its impact on the risk of mouth, esophagus, lung, and stomach cancer; researchers found a "probable" decreased risk. Compare that with body fatness. There's "convincing" evidence, the report says, that extra fat increases the risk of six types of cancer: esophageal, pancreatic, colorectal, breast (in postmenopausal women), endometrial, and kidney; and "probable" evidence that it increases the risk of gallbladder cancer. Fat is especially dangerous around the abdomen, where it's thought to affect inflammation and hormone systems, perhaps leading to cancer.