Once upon a time there were two groups of mice, all genetically engineered to develop prostate cancer. Each group was fed the same amount of calories. One group of mice lived in cages warmed to 80.6 degrees. The other group lived in cages kept at 71.6 degrees. The mice in the cooler quarters burned more calories to keep warm. And after three weeks, they weighed less than the toasty warm mice. They were also less likely to develop prostate cancer.

Then there were two other groups of mice, also genetically engineered to develop prostate cancer. Both groups were kept in cages with temperatures 80.6 degrees and 71.6 degrees -- like above. But these mice got to eat whatever their little hearts desired. The mice in cooler cages ate 30 percent more than the mice in warmer cages. They got just as fat as the warm mice. And they all got prostate cancer at the same rate, despite the extra calories.

The moral of the story is this -- being lean rather than obese has a greater protective effect against cancer. Excess calorie retention, rather than consumption, raises cancer risk. This moral stands in contrast to what most researchers believe -- that a restricted diet cuts the risk of and slows the growth of cancer and this is directly related to calorie intake.

Tim Nagy, Ph.D. and professor of nutrition sciences at the University of Alabama, Birmingham and lead researcher on the mice story says when you eat more calories than you burn, you store the extra calories as fat. It's the fat cells -- not the extra calories themselves -- that affect cancer risk.

It's not a good idea for humans to chill themselves to avoid cancer, Nagy says. But perhaps people could get the same effect by exercising more since that, too, burns calories.

Nagy's study appears in the January 1 issue of the journal Cancer Research.