MD Stein - The world according to fiber
Dietary fiber consists of the edible parts of plants that are resistant to absorption in the small intestine. Fiber-rich foods—whole grains, vegetable, fruits—are recommended in all dietary guidelines. But what are the health benefits of fiber intake, how much needs to be consumed, and is fiber beneficial if ingested in tablets and powder form instead of through food consumption?
An association between higher fiber intake and lower risk of cardiovascular death has been found consistently in studies across cultures over the past several decades. A diet of foods providing adequate fiber is usually less calorically dense and larger in volume than a low fiber diet. This larger mass of food takes longer to eat and its presence in the stomach may bring a feeling of satiety sooner. The benefit of a higher fiber diet therefore starts with the lower fat and simple sugar contents of such a diet, and is an effective treatment for obesity and hypertriglyceridemia associated with cardiovascular disease.
Second, dietary fiber lowers blood lipid levels. Fiber increases the amount of cholesterol-binding bile acids in the gut that promote the excretion of fats. In response, LDL cholesterol is removed from the blood and converted into bile acids by the liver to replace the bile acids lost in the stool, lowering one’s cholesterol level in many cases.
Third, dietary fiber lowers blood sugar levels (high glucose levels are another cardiovascular risk) by restricting its absorption. A fiber-rich meal is processed more slowly, and nutrient absorption occurs over a greater time period. Thus fiber slows the rate of glucose appearance in the blood, which is particularly important in persons who have diabetes. Insulin secretion is subsequently reduced. Lowering insulin levels may reduce blood pressure, another contributor to heart disease risk.
Two fibers, specifically the beta glucan in oats and psyllium husk, have been sufficiently studied for the FDA to support the health claim that foods containing 0.75g or 1.7g of these per serving, respectively, can reduce the risk of heart disease. To reach the recommended target of at least 25 grams per day, one could eat five portions of the following high fiber foods each day—1) Fruits: figs, pears, strawberries, prunes, oranges; 2) Vegetables: lentils, split peas, beans; 3) Grains: bran cereals, shredded wheat, brown rice, oatmeal—or an assortment of other fiber-containing foods.
Intriguingly, high fiber diets may protect against the development of other chronic illnesses. Fiber intake may protect against death from infectious and respiratory disease. The mechanisms remain unknown, but fiber-containing foods have anti-inflammatory properties, dampening the body’s production of inflammatory chemicals that produce and are signs of tissue damage during bacterial infections.
Assisting in the relief of both constipation and diarrhea, a high-fiber diet is standard therapy for diverticular disease of the colon. Diverticula, out-pouchings of colon that can become irritated and cause pain and internal bleeding, will not be resolved by a diet adequate in fiber, but the bulk provided by such a diet will prevent the formation of additional diverticula, lower the pressure in the lumen, and reduce the chances that one of the existing diverticula will burst or become inflamed, a common cause of hospitalization.
Finally, some studies have found a decrease in cancer deaths among those with high fiber diets. Fiber is thought to bind and promote the excretion of food-borne carcinogens. But while binding might be expected to provide greatest protection to the intestines themselves, fiber has not been consistently found to reduce the rates of colorectal cancer, making this association weaker.
A central issue in discussion of fiber-rich diets has been whether the benefit of such diets is attributable to fiber itself or to other nutritional components that are ingested along with fiber. Whole grains, for example, contain numerous minerals (selenium, zinc, magnesium) and other antioxidants (polyphenols, such as lignans and alkylresorcinols) that may be the true protective elements of a high fiber diet. Dietary fiber intake may be a marker of a healthy diet that is high in grains, fruits and vegetables that contain an assortment of vitamins and minerals. Fiber intake may also signal health consciousness beyond food, with fiber eaters those who exercise most and smoke least. Of note, while fiber is a component of whole grains, the reverse is not true. Therefore, isolated fiber products (tablets, powders), are unlikely to provide the same benefits as intact, whole grains, vegetables and fruits.
The health benefits of fiber remain observational and associative. That is, there are no large randomized trials that take a population of low-fiber eaters and feed one half a high-fiber diet and the other half their usual low-fiber over a long period of time to see what happens. Nor do we know the threshold for a dose that delivers the greatest health benefit. Some studies suggest that particular sources of fiber (whole grain) may be more beneficial than others (fruit), but this is uncertain. Still, the observational data is compelling: fiber is associated with lower mortality.
Importantly, there are no studies that demonstrate any health risk of fiber. Fiber is uniquely safe. A diet of a wide variety of fiber-containing foods is rich in micronutrients. Demonstrated reductions in the risk of premature death and chronic disease should make attention to fiber and its nutritional fellow travelers as important as awareness of fat or salt intake.