Fiber comes from the indigestible part of plant foods such as cereals, grains, seeds, nuts, beans, vegetables and fruit. Also referred to "roughage" or "bulk," fiber may be classified as either soluble and insoluble. By passing through the digestive tract relatively intact, both types of fiber play unique and complementary roles in the maintenance and promotion of health.
Soluble Fiber
Soluble fiber dissolves in water and forms a gel-like or viscous substance in the intestine. This slows the rate of digestion and hinders the absorption of fat and cholesterol in the intestine. According to MayoClinic.com, soluble fiber can reduce cholesterol and blood glucose levels, promote insulin sensitivity, and possibly aid in weight control. A 2010 University of Illinois study also suggests that soluble fiber may have immune-boosting effects by way of its anti-inflammatory properties. Foods high in soluble fiber include oatmeal and oat bran, legumes, barley, citrus fruits, apples, carrots and flax seed.
Insoluble Fiber
In contrast to soluble fiber, insoluble fiber absorbs water rather than dissolves in it. This creates a feeling of fullness that also helps with weight management. Insoluble fiber provides fecal bulk and assists with laxation, thereby preventing constipation and protecting against bowel diseases such as diverticulitis. By decreasing bowel transit time, insoluble fiber removes toxic waste and also helps keep the large intestine at an optimal pH. Insoluble fiber-rich food sources include fruit skins, vegetables, whole grains, wheat bran and seeds.
Fiber Recommendations
The Institute of Medicine of the National Academies, IOM, established an Adequate Intake of total fiber for 38 gper day for men and 25g per day for women, both under the age of 50. For men and women over 51, the amount is lower, at 30g and 21g respectively. The IOM bases these recommendations on the levels observed to protect against heart disease and does not distinguish between soluble or insoluble fiber amounts.
Selecting High Fiber Foods
Selecting whole, plant-based foods is the easiest way to identify foods rich in soluble and insoluble fiber. Fruits and vegetables, particularly those with the skin left on, contain more fiber than fruit or vegetable juices. Legumes such as beans, lentils and chickpeas are all excellent sources of fiber. In the case of grain-based products, the American Heart Foundation recommends foods that contain one of the following ingredients first on the label's ingredient list: whole wheat, oatmeal, whole oats, brown rice, wild rice, whole-grain corn, whole-grain barley, whole-wheat bulgur and whole rye.
Increasing Fiber Intake
Despite the health benefits of a diet high in soluble and insoluble fiber, the average American consumes less than half the recommended amount. Fortunately, small dietary changes can quickly increase both types of dietary fiber. Examples include eating at least one fruit or vegetable at each meal and snack, having one or more legume-based meals a week and choosing whole grain breads and cereals instead of refined grain products. Note that when increasing dietary fiber, health professionals recommend doing so gradually, while increasing fluid intake, to prevent abdominal discomfort and gas.
Increasing Fiber Intake
References
- MayoClinic.com: Dietary Fiber---Essential for a Healthy Diet
- Sickness behavior induced by endotoxin can be mitigated by the dietary soluble fiber, pectin, through up-regulation of IL-4 and Th2 polarization
- Institute of Medicine: Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids
- American Heart Association: Whole Grains and Fiber
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