Dietary fiber is the indigestible portion of plants that changes the way in which the gastrointestinal tract absorbs other nutrients. By itself fiber has no calories, yet it provides a feeling of fullness after meals. Whole grains, beans, nuts, wheat bran, fruits and vegetables are all major sources of fiber. Experts have studied dietary fiber for its beneficial impact upon colon and digestive health, but a diet high in fiber also has the negative consequences of slowing or stopping the absorption of certain medications.
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Absorption describes the movement of a substance from the intestines into the bloodstream before it becomes "bioavailable" to the rest of your body. The rate of absorption depends upon several factors, including food intake and the state of your gastrointestinal health, but it is typically less than 100 percent. Absorption only applies to medication that is administered orally. An injection directly into the blood bypasses the digestive system altogether.
Effects of Fiber
Various types of fiber have different roles in the body. Soluble fiber appears to bind with bile acids and form a gel-like substance in the digestive tract. This slows down the absorption of certain nutrients and medication into the bloodstream. For example, it is known to have an impact upon cholesterol and glucose, and some experts have also studied the way in which fiber affects the absorption of minerals, although no impact has been found. Rather, the presence of phytate within cereal fiber appears to slow down mineral absorption, according to the Linus Pauling Institute.
Dietary fiber can significantly interfere with the absorption and effectiveness of the following prescription medications: tricyclic antidepressants such as doxepin and imipramine; anti-diabetic medication such as glyburide and metformin; the seizure medication known as carbamazepine; digoxin, which is used to regulate heart function; lithium, which helps with various psychological disorders; penicillin; and cholesterol medication such as cholestyramine and colestipol. In addition, oat bran and fruit pectin can interfere with certain statins.
The average American diet only contains 14 g of fiber a day. A more adequate amount, according to the 2005 Dietary Guidelines for Americans, is at least 14 g of fiber for every 1,000 calories consumed. This kind of high fiber diet entails eating a significant amount of fiber each meal — some foods contain more than 5 g in a single serving — but the best way to avoid interference is to refrain from taking medication up to two or three hours after the consumption of fiber-rich foods. If you are on medication, then you may have to work with your doctor in order to establish a meal plan that includes enough fiber throughout the day.