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(NaturalNews) The U.S. Food and Drug Administration (FDA) recently granted Cargill, Inc. the right to make health claims for their beta-fiber product Barliv, a β-glucan (beta-glucan) extract from whole barley flour (1, 2). This so-called interim final ruling, effective immediately, is an amendment of the FDA's health claim regulation entitled "Soluble fiber from certain foods and risk of coronary heart disease (CHD)" (2).

The ruling adds barley β-glucan to the list of eligible β-glucan sources for which the same health claim can be made. The others are oat bran, rolled oats and whole oat flour, whole grain barley and certain dry milled barley products.

Beta-fiber is produced from whole grain barley flour by enzymatic digestion of the starch and partial enzymatic breakdown of the β-glucan. The result is a product containing >70% β-glucan with a molecular weight of 120 to 400 kDa, compared to the 500-3,300 kDa range of native barley β-glucan (since each glucose residue contributes 162 Daltons, a 120 kDa polymer has about 740 glucose units). A study involving 155 hypercholesterolemic patients demonstrated that the partially digested and the native β-glucan lowered serum LDL cholesterol levels to about the same degree; HDL cholesterol levels remained unchanged (3).

The FDA acknowledged that β-glucan is the oat or barley component primarily responsible for the cholesterol-lowering effect. However, the agency also concluded that the effect of soluble fiber on coronary heart disease is less significant than a diet low in saturated fat and cholesterol. The FDA therefore mandated that the CHD health claim for β-glucan had to be accompanied by the qualifier "when consumed as part of a diet low in saturated fat and cholesterol" (2).

The FDA further stipulated that the product had to contain at least 0.75 g of β-glucan per serving. This value is based on the observation that foods providing at least 3 g of β-glucan per day are effective in lowering serum cholesterol levels. Spreading these 3 g over three daily meals and one snack, they arrived at the figure of 0.75 g per serving.

There is obviously more to fibers than β-glucan and a reduction in serum cholesterol levels. First, there are other soluble fibers that do what β-glucan does. Secondly, soluble fibers also affect the digestion and absorption of nutrients other than cholesterol. Finally, it is debatable whether LDL cholesterol levels are valid predictors of CHD risk, but I won't go into that subject here.

What are fibers?

Fibers are plant materials that are indigestible by animals, including humans (4,5). With the exception of the polyphenol lignin, all fibers are non-starch polysaccharides, i.e. complex carbohydrates. What makes some polysaccharides (e.g. starches) digestible and others (e.g. β-glucan) indigestible, is the stereochemistry at the anomeric carbons of the individual sugars (monosaccharides) making up these polymers. We do not synthesize the necessary enzymes to break down β-linkages, but intestinal bacteria are capable of fermenting soluble fibers.

Plant cell walls are the major sources of dietary fiber. Some fibers are soluble, viscous (gel-forming) and fermentable, whereas others are insoluble, non-viscous and non-fermentable. The most important dietary fibers are cellulose, β-glucans, pectins, and hemicellulose (4).

Cellulose is a linear polymer of several thousand glucose units connected by β(1->4) linkages. It is a structural fiber, insoluble and resistant to microbiol breakdown. Vegetables are the richest dietary sources of cellulose, where it accounts for about 30% of the fiber content.

β-glucans are also glucose polymers connected by β-linkages. However, unlike cellulose, β-glucans are mixed-linkage polymers. The soluble β-glucans from cereals contain β(1->3) and β(1->4) linkages. Oats and barley are the richest cereal sources. β-glucans from baker's yeast and some mushrooms, on the other hand, contain β(1->3) and β(1->6) linkages and are insoluble.

Pectins are polymers of galacturonic acid units connected by β(1->4) linkages; some of the carboxyl groups are methylated. Pectins have side chains containing simple sugars such as glucose, galactose, and rhamnose, and may have polygalacturonic acid branches at rhamnose residues. Pectins are soluble fibers and are degraded by the gut microflora. Citrus fruit and apples are good sources of pectins.

Hemicelluloses are branched heterogeneous polymers with a wide range of chemical structures. The pentose sugar xylose is usually present in the greatest amount. Other pentoses, such as arabinose, and hexoses like mannose, galactose and rhamnose are also found. Hemicelluloses are present in almost all plant cell walls, together with cellulose. Some hemicelluloses are soluble and fermentable, whereas others are not.

Soluble fibers and the upper gastrointestinal system

The most important characteristic of soluble fibers for the upper gastrointestinal tract is their viscosity, which affects nutrient intake in several ways.

Viscous soluble fibers decrease the rate of digestion. Soluble fibers slow the mixing of the stomach contents, thereby affecting the rate of enzymatic food breakdown.

Viscous soluble fibers delay gastric emptying of the stomach contents into the small intestine, which slows the rate of nutrient absorption. A slower rate of glucose uptake, for instance, is of obvious benefit to diabetics. The delay in gastric emptying also results in a sense of fullness that lasts longer and may therefore help with weight control.

Soluble fibers may alter the degree of nutrient absorption, in both positive and negative ways. For example, interference with micelle, i.e. lipoprotein, formation reduces lipid uptake. On the other hand, negatively charged fiber residues, such as the galacturonic acids of pectins, can decrease metal ion absorption.

Soluble fibers and the colon

The most important characteristic of soluble fibers for the colon is fermentability. Intestinal bacteria use soluble fibers as food sources. Byproducts of this fermentation process include acetic, propionic and butyric acids, as well as gases like hydrogen, carbon dioxide and methane.

The acetic and propionic acids are absorbed and used as fuels by non-hepatic tissue and by the liver, respectively. Therefore, soluble fibers provide us with a small amount of energy, even though we cannot digest them. Butyric acid, on the other hand, is an important fuel for the cells lining the colon (colonocytes).

Insoluble fibers

Consumption of poorly fermented dietary fiber increases fecal bulk and lessens constipation. Insoluble fiber has also been hypothesized to reduce the risk of colon cancer, but this has not been clearly established yet.

How does β-glucan lower serum cholesterol levels?

Let's get back to the cholesterol-lowering effect of β-glucan. There are several ways in which this can happen (5).

First, β-glucan may reduce or suppress the uptake of dietary cholesterol. This could happen by retaining cholesterol in the viscous, gel-like medium, or by interfering with micelle formation, i.e. preventing the formation of the lipoproteins required for lipid transport.

Secondly, soluble fibers may reduce the hepatic cholesterol pool. Cholesterol absorption requires micelles formed from bile acids. The liver synthesizes these bile acids from cholesterol and releases them into the small intestine. Interference in the reabsorption of these bile acids requires further synthesis, thus lowering cholesterol stores.

Thirdly, soluble fiber may prevent cholesterol biosynthesis in the liver by interfering with the activity of a key enzyme, 3-hydroxy,3-methylglutaryl coenzyme A (HMG CoA) reductase. This can happen in one of two ways. Elevated insulin levels stimulate HMG CoA reductase activity, but soluble fibers slow glucose absorption and therefore insulin secretion. Secondly, propionic acid, one of the products of bacterial fermentation, has been shown to inhibit HMG CoA reductase activity.

Adequate fiber intakes

The Food and Nutrition Board of the Institute of Medicine in the U.S. has not set a Recommended Dietary Allowance (RDA) for total fiber consumption, but considers the following range of values to be Adequate Intakes (AI): Males 30 - 38 g/day, females 21 - 28 g/day, and children 19 - 25 g/day (6). These estimates are based on intake levels observed to protect against coronary heart disease. It is estimated that average fiber intake in the U.S. is only about 17 g/day for men and 13 g/day for women, i.e. considerably short of AI values.

One very important health benefit of dietary fibers hasn't been mentioned here, namely the fact that a number of fibers have been found to activate the immune system. But that is a topic for another day.

References:

1. Lorraine Heller. Cargill's barley beta-fiber achieves heart health claim. NutraIngredients 2/26/2008.


2. (http://www.fda.gov/OHRMS/DOCKETS/98...)


3. Keenan JM, Goulson M, Shamliyan T et al. The effects of concentrated barley β-glucan on blood lipids in a population of hypercholesterolemic men and women. Br. J. Nutr. 2007;97:1162-1168.


4. Tom Brody. Nutritional Biochemistry. Academic Press 1999.


5. Martha H. Stipanuk. Biochemical and Physiological Aspects of Human Nutrition. Saunders 2000.

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