Dietary fiber or “roughage” is that part of plant substances that are resistant to digestion and hydrolysis by the digestive enzymes as it passes through the gastrointestinal system. Dietary fiber is by no means a single substance, the term “fiber” generally includes a diverse, complex group of substances, e.g. structural polysaccharides such as cellulose and hemicellulose, nonstructural polysaccharides such as pectines, B-glucans, gums and mucilages. The vast bulk of dietary fiber is constituted by polysaccharides, the remnants of edible plant cells, of plant origin. The cellulosic wall surrounding plant cells is the most obvious dietary fiber. Lignin is structural non-polysaccharide found in dietary fiber. Dietary fibers are classified into soluble and insoluble fiber by their water solubility and the relative resistivity to being converted into fermentable and nonfermentable fiber fractions by microbial degradation. Insoluble fiber, as the name suggests, remains substantially intact as it passes through the gastrointestinal or digestive system. Soluble fiber remains undigested by undergoes metabolic processing through fermentation. In general, cellulose (including hemicellulose) and lignin are water insoluble, while gums, pectines and mucilages are water soluble. Soluble fiber is typically found in fruits and vegetables, in particular citrus fruits and apples.

It has been known that dietary fiber is essential to good health. Dietary fiber detoxifies and expediates the removal of toxins and carcinogens. It assists the digestion process since it provides an indigestible biomass which is delivered through the alimentary canal resulting in undigested food to be thrusted out before it. The fibers bind to toxic substances in the small intestine and stomach and then push them out through the gastrointestinal tract till the toxins and carcinogens are excreted. Cholesterol, bile acids, and toxic heavy metals are all attached to the dietary fibers and  excreted in the same manner. Fibers also clear out harmful bacteria and ensure the proper functioning of the peristaltic muscles. Medical and nutritional research shows that diet high in fiber can help prevent diabetes, obesity, hypertension, cardiovascular disease, osteoporosis, gastrointestinal diseases, and several types of cancer. Foods high in insoluble fiber content are known to increase bile acid secretion, modify fat absorption, decrease endogenous cholesterol production, improve regularity and bulk formation, and promote peristalsis to reduce transit time of toxins and waste out of the body. Water-soluble fiber has been linked to such beneficial health effects as blood sugar regulation in diabetic patients, reduction of high serum cholesterol and triglyceride levels (anti-hypercholesterolemia), and prevention of colon cancer, coronary heart disease and arteriosclerosis. Soluble fiber does not dissolve in water but absorb water like a sponge in the stomach and small intestine. This creates a feeling of satiety, helps to soften the stool and ease evacuation, and leads slow food absorption which makes it useful in weight loss and the treatment of diabetes.

Due to the overall nutritional value of high dietary fiber diet, there has been considerable interests in using dietary fiber supplements for weight loss, lowering rates of intestinal and colon cancer, minimizing the risk of cardiovascular diseases and reducing other problems such as ulcer formation, ileitis and colitis, which are commonly resulted from poor digestion. A high fiber diet may provide protection from several diseases of the gut because fiber dilutes colonic contents, speeds transit time, increases bulk and modifies the bacterial metabolism. Dietary fiber can change lipid metabolism by altering fatty acid metabolism or lipoprotein lipase activity, interacting with cholesterol absorption process. Fiber has been shown to influence carbohydrate metabolism and change hormonal levels. High fiber diets are typically lower in animal products such as cholesterol and saturated fat which are a big contributor to heart diseases. Epidemiological evidence suggests the reduced incidence of colon cancer and chronic heart diseases is associated with in the consumption of high fiber diets. In fact, the deficiency in dietary fiber is associated many diseases including heart disease, colon cancer, irritable bowel syndrome, appendicitis, colitis, diverticulitis, intestinal toxemia, cerebral apoplexy, cholelithiasis (gall stones), hemorrhoids, varicocele, and diabetes. The important physiological effects of fibers including restricting insulin secretion, reducing serum cholesterol and accelerating of bowel evacuation make dietary fiber a critical nutritional supplement. Because of the concrete evidence of health benefits from high fiber diets, the dietary guidelines for Americans from American Heart Association (AHA) recommend consuming fiber foods to obtain the health benefits aforementioned. The AHA suggests that a total intake of 25 to 30 grams of dietary fiber per day to maximize the cholesterol-lowering effects of a fat-modified diet. Many health care professionals encourage people to consume high fiber diets that are rich in vegetables, fruits, legumes and whole grains.

Back to the topic of weight loss, it has been found that dietary fiber may control the rate at which fats and sugars are ingested from the intestine. The absorption rate is slowed by expediating transit speed of substances through the digestive tract. By binding cholesterol and the bile salts to fiber so that these substances are excreted out of the body than being absorbed. Weight loss diets have become extremely popular and various diet regimes are available on the market. The accumulating body fat is directly related to caloric intake. Therefore, one of the most common weight control techniques is the intake of low-fat, high fiber diets. High fiber diets help in obesity treatment by reducing the meal size and giving a delay in stomach emptying. The high viscosity fibers is specially advantageous in weight loss diets because the high viscosity of fibers may produce a sensation of satiety. However, the intake of dietary fiber should be controlled at 25-30 grams/day, but not to exceed 45 grams per day. Excess fiber intake can create undesirable gastrointestinal problems such as diarrhea, flatulence, and abdominal cramps, etc. High fiber intake may result in some side effects such as increased stool volume, decreased palatability of food, reduced food digestibility, elevated defecation frequency, poor hair and skin and improper mineral balance. Moreover, some fibers are susceptible to microbial degradation or fermentation in the colon. Such degradation or fermentation leads to the production of hydrogen, methane, carbon dioxide, and short chain fatty acids.

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