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Vegan Nutrition with Dina Aronson, M.S. R.D.Dina Aronson, MS, RD is a vegan dietitian whose specialties include chronic disease prevention, vegetarian/vegan nutrition, and lifestyle management. She is the founder and director of VeganRD.com, a nutrition consulting company. Active in many vegetarian nutrition organizations, Dina was the recipient of the American Dietetic Association's Recognized Young Dietitian of the Year Award in 2002.
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How does a diet high in fiber help protect/fight against disease, such as cancer? -Karrie
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Dietary fiber is a non-digestible carbohydrate found only in plant foods. Whole plant foods: vegetables, fruits, whole grains, and legumes (beans, nuts, and seeds) are the best sources.
A high intake of fiber keeps our digestive tract healthy, and studies have shown that such high fiber intakes may also help to reduce the risk of cancer of the colon and rectum. You may have heard that some recent studies failed to find this connection, but in those studies, the comparisons were between low fiber intakes and moderate- not high - fiber intakes. High intakes, in the range of 50 grams per day, do appear to be protective.
Here are some theories on how fiber combats cancer in the lower digestive tract. One way that fiber affects our digestion is by helping to move digested food through the large intestine (colon) more quickly, thereby decreasing the exposure time between the colon and the digested food. It is believed that because of this, there is less opportunity for the body to be in contact with ingested carcinogens (cancer-causing agents). Another way fiber may help is by keeping the stool soft, providing less opportunity for repeated injury to the lower intestine and rectum, which may lead to cancer. These are two possible reasons why vegetarians tend to have lower rates of colorectal cancer than non-vegetarians (not to mention that vegetarians tend to consume fewer carcinogens overall).
Fiber also appears to protect against heart disease, particularly soluble fiber, which is found in oats, beans, peas, certain grains like barley, and certain fruits like apples. This works by helping to lower blood cholesterol, which reduces heart disease risk. Studies have been more consistent with this finding, and fiber is often recommended as a part of a cholesterol-lowering diet plan. Soluble fiber acts by blocking the absorption of cholesterol in the intestines. In addition, soluble fiber can bind to certain compounds in the intestine that would keep cholesterol levels up if left intact; it can also slow down the liver's manufacture of cholesterol.
The risk of developing type 2 diabetes, it appears, may also be kept at bay by consuming a high-fiber diet. The mechanism for this may have something to do with preventing impaired blood glucose tolerance, which is one of the first steps in developing diabetes. According to Brenda Davis, co-author of Defeating Diabetes, "Dietary fiber helps to slow carbohydrate absorption and improve blood sugar control. However, the impact of fiber is relatively moderate unless the amount of fiber eaten is substantially higher than usual intakes. It takes about 35-50 grams of fiber a day to provide significant benefits." Perhaps this is another reason vegetarians have a lower risk of diabetes than non-vegetarians.
People who have higher fiber intakes also seem to be healthier overall because high fiber-consumers tend to have healthier diets and higher nutrient intakes, exercise more, and avoid smoking and excessive drinking. Whatever the reasons, it certainly is beneficial to maximize our fiber intake. The recommended intakes are at least 35-40 grams per day, and this is a conservative recommendation; intakes in the range of 50 grams per day has been shown to be the most beneficial. The average American consumes about 15-20 grams of fiber a day; ovo-lacto vegetarians consume 20-35 grams; and vegans consume 25-50 grams.
For a chart showing how much fiber is in various foods, visit this page.