Oct. 31, 2007 -- After five years of reviewing 7,000 medical studies, a team of top scientists from around the world has concluded that diet and weight directly affect whether you'll get cancer.
The researchers cite "convincing" evidence that excess body fat can cause six different types of common cancers, including those affecting the breast, bowel and pancreas.
The research, which also links consumption of alcohol, red meat and processed meat to an increased risk of cancer, has spurred debate among nutrition experts over whether the conclusions of the report are particularly earth-shattering.
"It should come as no real surprise that excess body fat is associated with cancer, as one additional chronic disease associated with obesity, since many dietary factors that protect against cancer are probably absent in the diet of an overweight or obese person," said Daniel Hoffman, assistant professor of nutritional sciences at Rutgers University in Newark, N.J.
In fact, Colleen Doyle, director of Nutrition and Physical Activity for the American Cancer Society (ACS), said the recommendations are similar to the ACS guidelines from 2006.
But new or not, the guidelines may drive the point home that an individual's cancer risk has much to do with their dietary habits.
"In a survey we did last year, over 80 percent of respondents connected excess weight to heart disease risk, over 57 percent connected it to diabetes, but less than 10 percent connected being overweight to cancer risk," Doyle said. "With the science so strong on this issue, the trends on weight so bad, and the awareness that what you weigh impacts cancer risk so low.
"This is an important message -- that people do indeed have some control over developing many types of cancer."
Dr. David Katz, director of the Prevention Research Center at Yale University in New Haven, Conn., echoed the view that the report was not new information, but adds to a wealth of existing data.
"While this report is mostly an aggregation of observational studies, the numbers involved are huge," he said. "That matters because observations at the level of the whole population become very reliable. The link between obesity and cancer is strong, consistent and biologically plausible."
Findings Hard to Swallow?
But not all researchers believe the issue is so clear-cut.
"Diet's relationship to cancer risk is complex and not well understood," said Michael Cummings of the Division of Cancer Prevention and Population Sciences at Roswell Park Cancer Institute in Buffalo, N.Y.
"It should be noted that while rates of obesity have increased steadily in the U.S. and other high income countries in the last two decades rates of cancer and cancer deaths have declined."
Tobacco, Cummings said, may be a stronger driving force of cancer death than diet.
On the other side of the coin, some say the recommendations for a healthier diet do not go far enough. Dr. Neal Barnard is president of the Physicians Committee for Responsible Medicine (PCRM), a group that promotes vegetarianism among other health-conscious behaviors.
"They place a limit on red meat, rather than recommending against it altogether," Barnard said. "They suggest that white meat is a better choice, despite evidence that it, too, is linked to cancer in some studies.
"I would argue that these recommendations should have been made -- and were easily justified -- more than a decade ago."
Digesting the New Diet Recommendations
ABC News medical editor Dr. Tim Johnson broke down the new recommendations.
You can also find more information at www.dietandcancerreport.org.
Limit body fat. Your body mass index should be between 18.5 to 24.9. Weight around the waist is most dangerous.
Limit red meat to 2.5 ounces a day. An additional 1.7 ounces a day increases cancer risk by 15 percent. That means an 8 ounce steak or a quarter-pound hamburger would be the limit for a week.
Limit alcohol to two drinks a day for men, one drink a day for women.
Limit salt intake to 2 grams a day. That's about a teaspoon a day, instead of the teaspoon and a half that most people eat. To give you an idea of how much salt is in food — just one cup of cottage cheese has half the salt you should eat in a day: half a teaspoon. Processed foods are almost always higher in salt.
Get nutrients from whole foods, not vitamin supplements.
Mothers should breast-feed children.