WHO: Many Colon Cancers Attributable to Weight Gain

Up to one-third of colon, breast, kidney and digestive tract cancers are attributable to weight gain and inactivity, according to the World Health Organization's Cancer Agency.

"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. "The most important thing is not to gain weight, however much you already weigh."

"There's no evidence that weight loss reduces the risk of cancer, but that doesn't mean it doesn't," said Vainio. "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."

The WHO report 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 cannot be prevented or managed, nor physical activity promoted, solely at the level of the individual," said the report. "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."

"Control of the obesity epidemic will require the participation of all segments of society and substantial investments, particularly in public education, community environments that promote walking and other physical activities, work-site and school programs that include at least one hour of physical activity on most days, and transportation systems that encourage walking and use of bicycles," said the report.

Other sources: International Agency for Research on Cancer, AP