A study of 88,000 women, whose health and diets were followed for 16 years, found that those who ate a relatively small amount of fiber had the same risk of colorectal cancer as those eating more than twice as much. The theory that diets high in fiber protect against colorectal cancer was first seriously propounded 30 years ago. Fiber is the structural material of plants that can't be absorbed by the human digestive tract. Contrary to many public perceptions, however, evidence for the theory has been sketchy at best. But there are many other potentially effective ways to do that.
Exercise lowers the risk for colon cancer, as does the regular use of aspirin.
Eating red meat tends to raise it. There is some evidence that obesity, as well as consumption of fat and sugar, also raise the risk. As with most epidemiological studies, it's impossible to say with certainty these results are correct. A more definitive test of the theory is to randomly assign people to follow either a low-fiber diet or a regular diet, and then observe them for cancer. At least two such studies are under way.