Some of the world's leading experts in nutrition epidemiology have cast a resounding vote in the decades-long debate between treating or preventing cancer: Prevention wins.

Their report, being released today, argues strenuously for diet and exercise as the keys to fighting cancer. It calls research and spending on the treatment of cancer "necessary but not sufficient," and contends that a far better strategy for reducing the world's annual tally of 11 million cancer cases would be to develop a public-health policy aimed at preventing people from getting the disease in the first place.

The report, issued by the World Cancer Research Fund/American Institute for Cancer Research, is based on an exhaustive review of nearly 7,000 scientific studies into whether cancer rates are influenced by diet, obesity and exercise.

Based on this review, it concludes that cancer "is mostly preventable," estimating that about one-third of all cases in advanced countries like Canada could be eliminated by diets that aren't loaded with fatty, sugary foods, by people exercising regularly and, if they are obese, by slimming down to an appropriate weight. Among the cancers with links to these factors are those of the breast, prostate, mouth and colon.

Another third of cancers are due to smoking, indicating that well over half the cases of the disease could easily be prevented.

"It's a very compelling case" that cancer incidence could be cut dramatically through prevention, said Shiriki Kumanyika, an epidemiologist at the University of Pennsylvania in Philadelphia and a member of an expert panel that oversaw the writing of the report. Also on the panel were such nutrition luminaries as the Harvard School of Public Health's Walter Willett.

Dr. Kumanyika said scientific evidence strongly supports the estimate that poor diet, obesity and lack of exercise cause one out of three cancers. "I definitely feel confident that it's at least that much," she said.

Much of the medical research into the causes of cancer has looked at factors such as genetic susceptibility. But Dr. Kumanyika said studies tracking immigrants and their children who move from areas of low cancer incidence, such as Asia, to countries with high rates, such as the United States, suggest the genetic factor may be overrated. Over time, cancer rates among migrants and their children rise toward the levels prevalent in their adopted countries, suggesting that something common to everyone in the new environment is the cause.

Although individuals can make decisions to get more exercise or eat better food, thereby reducing their chances of developing cancer, the report says that entities ranging from governments to schools need to develop public-health strategies to reduce the incidence of the disease.

Among the steps it recommends are banning advertising of sugary drinks and unhealthy foods directed at children, a step Quebec has already taken. The report says the Quebec action has led to a decrease in the amount of sugary cereals purchased, particularly among francophones.

It also calls for vending machines that dispense high-fat, sugary sweets or drinks to be moved out of schools and workplace cafeterias.

Another approach that should be considered is having processed meals, snacks and food reformulated to contain less sugar, refined starches, fat and salt, the report says.

To encourage more physical activity, it says society should place less emphasis on elite professional sports that lead people to be sedentary spectators, and instead develop entertainment that inspires everyone to be physically active.