LONDON (AP) - Up to one-third of cancers of the colon, breast, kidney and digestive tract are attributable to too much weight gain and too little exercise, the World Health Organization's cancer agency said.

"Putting on weight, even if you're in the normal range, increases your risk," said Dr. Harri Vainio of the International Agency for Research on Cancer, which convened a panel to study the impact of obesity and a sedentary lifestyle on cancer worldwide. "The most important thing is not to gain weight, however much you already weigh."

Half the adults in Europe and 61 percent of American adults are overweight, and the epidemic is spreading around the globe as Western lifestyles infiltrate new areas, experts say.

Despite the link between cancer and weight gain noted by the WHO panel, the evidence is not as strong for intentional weight loss.

"There's no evidence that weight loss reduces the risk of cancer, but that doesn't mean it doesn't," Vainio said. "Nobody has been able to track it because people haven't maintained their weight loss long enough to see if it makes a difference in cancer risk."

In a preliminary report published Thursday, the WHO panel said that despite the lack of direct evidence, hormonal changes produced by weight loss seem likely to reduce risks of some cancers, and that research suggests weight loss even late in life would cut the risks of breast and uterine cancer.

Obesity is normally defined by the body mass index, or BMI, calculated by dividing weight in kilograms by height in meters squared. An index of between 18.5 and 25 is considered healthy, while those with a score between 25 and 29 are classed as overweight and those whose BMI is higher than that are considered obese.

Avoiding weight gain was found to protect against colon, kidney, uterine, digestive tract and post-menopausal breast cancers. Exercise helps prevent breast and colon cancer, the panel concluded.

Besides cancer, being too fat increases the danger of developing heart disease and diabetes and of dying early.

"Obesity cannot be prevented or managed, nor physical activity promoted, solely at the level of the individual," the group said, echoing statements it has made in the past. "Governments, the food industry, international agencies, the media, communities and individuals all need to work together to modify the environment so that it is less conducive to weight gain."

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