Dietary And Insoluble Fibers Biology Essay

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Dietary fiber is defined as the edible parts of plants or analogous carbohydrates that resist digestion and absorption in the human small intestine, with complete or partial fermentation in the human large intestine. Dietary fiber includes polysaccharides, oligosaccharides, lignin and associated plant substances. Dietary fiber displays one or more of these characteristics, laxation, decrease in blood cholesterol, or decrease in blood glucose levels.

Fiber is categorized into soluble and insoluble fiber. Soluble fiber includes pectins, mucilages, gums, and storage polysaccharides. Soluble fiber has shown to have positive effects on glucose metabolism that are partly attributed to the increased viscosity of luminal contents. It causes delayed gastric emptying, which account for around 35% of the variance in peak glucose concentrations after ingesting glucose. It also reduces the rate of glucose diffusion through the water layer. The most important factor in regards to glycemic effect is soluble fiber's ability to increase viscosity and form gel-like stools. Soluble fiber also becomes fermented in the colon and yields short-chain fatty acids. It is thought that these short chain fatty acids may be beneficial to lipid metabolism and cardiovascular disease prevention. 1-2

Insoluble fiber includes cellulose, hemicelluloses, and lignin. Insoluble fiber does get fermented in the colon, but it attracts water which increases fecal bulk, softening, and laxation. The main effect of insoluble fiber in regards to glycemia is heightened insulin sensitivity.

As of 2007, 23.6 million people (7.8% of Americans) had diabetes. There was approximately 1.6 million new cases of diabetes diagnosed in people aged 20 years or older in 2007.3

As previously mentioned, dietary supplement sales have sky rocketed in recent years. Fiber supplements are promoted for reducing constipation, being beneficial for reducing cholesterol levels, and for lowering risk for heart disease, type 2 diabetes, colon cancer, and as being useful for diverticular diease4. However, these supplements are said to contain little fiber, and like all other supplements, they are costly. These fiber supplements contain no other nutritional value. UCLA recommends that it is better to get fiber from whole foods which will also provide vitamins, minerals, and phytochemicals. Fiber binds to minerals, so when food is eaten, it replaces the minerals that are lost, instead of just take them away (with a fiber supplement)5.

Psyllium husk is one of the most popular fiber supplements sold (source). Psyllium refers to the husk of psyllium seeds and is a very viscous mucilage in aqueous solution. The psyllium seed is small, dark, reddish-brown, odorless, and nearly tasteless. Psyllium is usually derived from the blond or Indian plantago seed. It may be classified as a functional fiber. A functional fiber is an extraction or isolation of a polysaccharide, usually through chemical, enzymatic, or aqueous means, that can either enhance its health benefit (usually because it is a more concentrated source) or diminish it. A substance is required to demonstrate a beneficial physiological effect to be classified a functional fiber.6

Psyllium has various different functions. Psyllium is the active ingredient in laxatives. Patients given psyllium treatment of idiopathic constipation have been shown to reap benefits. Psyllium increases stool water content, stool water weight, total stool output, and bowel movement frequency. Pain was also reduced.

Another function of psyllium is normalization of blood lipid concentrations. It decreases LDL cholesterol concentrations mainly by the stimulation of bile acid production. Psyllium has shown to decrease plasma concentrations in a variety of studies. It has lowered total cholesterol by 5.6% to 52%. It has lowered low density lipoprotein (LDL) cholesterol by 8.6%. HDL cholesterol was not effected in most studies.7

Psyllium has also shown a decrease of blood glucose responses. It has also shown to reduce the glycemic index of food when added to a meal. When psyllium was compared to a placebo, it lowered glucose elevation by at least 14%.6

It may be argued that fiber supplements are necessary. The average American's intake of fiber a day is 15 grams. Yet the average female requires 20-25 grams a day and the average male requires 30-35 grams a day (HARVARD 2). Americans have not been ingesting a sufficient amount of fiber and therefore, have not been benefitting from it. In some instances, fibers isolated from plants or manufactured chemically or synthetically have demonstrated more powerful beneficial physiological effects than a food source of the fiber polysaccharide (long one 83 pages).It is necessary to find out whether or not Americans must change their diet or if a supplement can reap the same benefits.4-5

Elevated levels of low-density lipoprotein cholesterol (LDL) increase the risk for coronary heart disease. Psyllium husk has been shown to lower serum cholesterol levels. A study comparing simvastatin and psyllium concluded that the mean decreases in total cholesterol and LDL levels for a group receiving 10 mg simvastatin plus psyllium were comparable to the group receiving 20 mg simvastatin plus placebo. This suggests that 10 mg simvastatin with psyllium is as effective in lowering cholesterol as 20 mg of simvastatin alone and highlights the benefit of additional fiber, even in a supplemental form.7

There have been several studies that show that a fiber supplement lowers postprandial glucose levels. However, none of the studies are comparing supplemental fiber to dietary fiber. The question if supplemental fiber has a different effect on glucose levels than dietary fiber has not been answered.

A study was conducted testing the different sources of fiber (oat powder, rye bran, beet fiber, a mixture of the three, and a control group), and compared their effects on postprandial glucose and insulin levels. The meal containing rye bran had the lowest postprandial glucose peak, even though it had the least amount of soluble fiber out of the five meals given. It also did not have the highest amount of total fiber out of the five meals.

This study also discussed the tendency that oats have to increase insulin response. Oats have a higher protein content. Food proteins give different effects depending on the combination of amino acids. For example, branched- chain amino acids are suggested to stimulate secretion of insulin which may induce insulin resistance. In a study by Nilsson et al. on whey protein of milk, a positive correlation was found between high insulin levels and the presence of certain amino acids, primarily leucine, isoleucine, valine, lysine and threonine, in the blood samples.

These findings may suggest that when comparing glucose levels and fiber sources, it may be wise to have similar protein content. The findings also suggest that postprandial glucose, insulin and triglycerides are influenced by dietary fiber-rich meals, depending on fiber source, mixture of fibers, dose of soluble and total fiber, fiber processing and possibly gender.8

An additional study that has shown a benefit of functional fiber is "The Depression of the Glycemic Index by High Levels of B-Glucan Fiber in Two Functional Foods Tested in Type 2 Diabetes."This study revealed that when subjects with type 2 diabetes consumed extruded breakfast cereals containing different levels of oat bran concentrate, reductions in postprandial glycemia ranged from 33% to 62%.9

Dietary fiber has recently received recognition for reducing the risk of developing diabetes and heart disease. The inference is that it may have a beneficial effect in preventing Type Two Diabetes. To test this hypothesis, this study investigated the effect of supplementing a high-carbohydrate diet with Konjac-Mannan (KJM), a high-viscous fiber, on metabolic control in subjects with insulin resistance syndrome, the control being Wheat Bran (WB). KJM is derived from Konjac, which is a type of sweet yam. KJM showed a fasting glycemia drop by 13 ± 2.5% (P 0.0001) compared with 9.6 ± 4.3% (P 0.05) on control. Both the supplemental fiber, KJM, and the dietary WB significantly affected postprandial glucose levels.10

The benefits of fiber and psyllium are not in question. Fiber and psyllium have been shown to lower cholesterol levels, postprandial glucose and insulin levels, as well as help with constipation and other various functions. Whether or not there is a difference between the benefits of dietary fiber and functional fiber is what our research will show.