A new study adds more evidence to the theory that a low-fat diet helps prevent colon cancer. But yet another study says a sort-of-low-fat diet won't work, that it has to be really low-fat.

Researchers from the Harvard School of Public Health reported Monday that men with fat intakes averaging 24 percent of calories had only half the rate of adenoma colon polyps, a common precursor of colon cancer, as men with the fat intakes averaging 41 percent."This would come close to explaining most of the variation we see between countries in colon cancer," said Dr. Edward Giovannucci of Harvard. "This adds evidence to the fat and fiber hypothesis."

"A modest reduction will probably not appreciably reduce the risk," said Dr. Tim Byers of the Centers for Disease Control. "We will not see significant effects from the kinds of small dietary changes that are now under way in this country."

The studies were presented Monday at the annual convention of the Federation of American Societies for Experimental Biology.

The Atlanta-based CDC's study found no significant difference in the risks of colon cancer - or breast or prostate cancer - between people averaging 37 percent fat intake and people averaging 32 percent.

The current government recommendation for calories from fat is 30 percent or less.

Byers described a truly low-fat diet as five or six servings of fruits and vegetables and about six servings of legumes or whole grains a day. Americans currently won't eat that, he added.

An official with the American Cancer Society agreed that Americans need to drastically reduce their fat intake.