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Fiber — How it Helps, How it Doesn't
For years, fish, vegetables and rice composed the bulk of the Japanese diet, and rates of colon cancer there remained low. As a "Western" diet gained a foothold in Japan, the rate of colon cancer  began rising too. This observation, coupled with other comparative studies around the world, has led scientists to try to determine what foods might reduce or increase our risk of colon cancer. On the positive side, fiber, known to reduce the risk of constipation and heart disease, was thought to protect against colon cancer. On the negative side, broiled, fried and grilled meats are high on the list of cancer-causing suspects, especially meats prepared well done.
Disappointing news on fiber
A high fiber diet has long been presumed to reduce the risk of colon cancer. Numerous studies in laboratory rats and mice have shown that various forms of dietary fiber do reduce the risk of colon cancer. Some human studies have also suggested the same thing. But new information suggests this may not be the case. A very large, recently published study of nearly 90,000 women tracked their dietary and disease histories for 16 years, and found no association between fiber intake and any reduction in colon cancer risk.
More direct interventional trials have also shown no correlation. One study of 1,300 people who had been diagnosed with colorectal adenomas (precancerous lesions in the colon) were assigned to take fiber supplements in either high or low amounts. The rate of recurrence of the adenomas was about 50% in both groups. In a similar study of 1,900 patients with adenomas, patients were randomly assigned to follow either their ordinary diets or a high fiber, low fat diet. There was no difference in the rate of recurrent adenomas between the two groups (39.7% for the fiber group and 39.5% for the control group). Whether this effect on precancerous lesions holds for actual cancers remains to be determined.
But still some hopeful news
The good news is that fiber does have some benefits. The Nurses' Health Study, conducted by Harvard Medical School, followed over 75,000 adult women of various ages, recording the incidence of several diseases. The researchers looked at the risk of heart disease relative to various life-style factors. As dietary fiber intake increased, the risk of heart disease fell by about a third (after factoring out the influence of age and smoking). The Iowa Women's Health Study, which looked at the health habits of 34,000 Midwest women, found a similar decline in the risk of heart disease as total dietary fiber increased. The Health Professionals' Follow-up Study, also out of Harvard, has followed nearly 45,000 men over years, observing health habits and disease risk, and this study too found that the risk of heart disease fell as dietary fiber increased. All of these studies, however, are observational, and their data still needs to be confirmed in randomized trials.
Recently, researchers in Finland published evidence that diets rich in whole grains--which are high in fiber--may also protect against the development of Type 2 diabetes. They reported in the American Journal of Clinical Nutrition that over a 10-year period, study participants with the highest amount of whole grains in their diet were 35% less likely to develop Type 2 diabetes than those with the lowest intake of whole grains.
Though a diet high in fiber might not reduce your risk of colon cancer, it may help reduce your risk of heart disease, and it has one more very tangible benefit for the older adult. About 26% of older men and 34% of older women suffer from constipation. Although the colon's motility is unchanged with normal aging, medications, a sedentary lifestyle, and other illnesses (an underactive thyroid gland, a mass in the colon) can make constipation a real problem. A high fiber diet is the first step toward reducing the discomfort of constipation, and is vastly preferred to the chronic use of laxatives.
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