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Glucans are polysaccharides made of chains of glucose molecules. Beta (β)-glucans are made of β-D-glucose, one particular form of glucose. (Starch is a polysaccharide made of α-D glucose.) Certain β-glucans are recognized as immune system modifiers, increasing resistance to infection and preventing and treating cancer.
Other β-glucans, such as those in oats and barley, lower serum cholesterol and regulate blood glucose.
There are many different β-glucans. Cellulose is a β-glucan, but it does not possess immune-modulating activity. For a β-glucan to function as an immune modulator, it must have a backbone chain of glucose molecules in 1→3 linkage plus branch chains attached to the backbone in 1→6 linkage. Such molecules are often referred to as 1,3/1,6 β-glucans. (Cellulose has the 1→3 linkages but no branches.) A 1,3/1,6 β-glucan has a shape that fits onto receptors on the surface of certain white blood cells and activates the white blood cells to engulf and destroy infectious agents—bacteria, viruses, parasites, and fungi. White blood cells with β-glucan receptors constitute the backbone of the body’s innate immune system. Some β-glucans fit the receptor sites better than others and consequently are more effective in activating them.
The human body cannot make glucans, but β-glucans occur in many foods: mushrooms and other fungi, the cell walls of yeasts (e.g., baker’s yeast, Saccharomyces cerevisiae), and the bran of cereal grains (particularly high in oats and barley). The β-glucans contained in these foods differ in structure from one another. The source providing the greatest immune modulating activity is presently thought to be baker’s yeast.
While there has been considerable research done on the effect of β-glucans on the immune system, much of it has been done in animals and with injectable forms. β-glucans are not digestible by human enzymes and hence not absorbable by typical routes; therefore, a question arises as to how orally consumed β-glucans can reach the critical sites within the body, such as cancer cells, where they can be effective. By using a fluorescently dyed β-glucan, researchers have been able to follow the passage of the glucan through the stomach, into the intestine, and into a specialized type of cell, the M cell, located in the Peyer’s patches of the ileum (the final segment of the small intestine before it enters the colon). Peyer’s patches are lymph tissue made of a variety of cells. Their M cells have the capacity to engulf β-glucans and present them to immune cells, which then carry them throughout the body via the lymphatic system.
A number of studies have shown that β-glucans in food can attenuate the rise in insulin or blood glucose following ingestion of carbohydrate. In one study, subjects with type 2 diabetes were fed breakfast cereals containing various proportions of oat bran enriched with β-glucan. Peak glucose levels resulting from breakfasts providing 4.0, 6.0 or 8.4 grams of β-glucan were 67%, 42%, and 38% as high as when a continental breakfast was eaten. Postprandial insulin was only 59% to 67% of that with the continental breakfast. Similar results were achieved with barley β-glucan when it was added to carbohydrate food. However, when the glucan was added to a high carbohydrate drink, there was no significant lowering of blood sugar or insulin response. The authors speculated that the glucan in the food increased gastrointestinal viscosity, whereas that did not happen with the drink because of dilution and rapid absorption. Another study showed that when β-glucan was combined with resistant starch (a form of starch that is not easily digested in the small intestine but is digested by colonic bacteria; found mostly in beans and grains), insulin response and blood sugar response were lower than with either β-glucan or resistant starch alone.
The FDA has approved a health claim for oat and barley β-glucan as cholesterol-lowering agents that may reduce the risk of cardiovascular disease. To make the claim, a food must provide 0.75 g of β-glucan per serving. Three grams per day is considered the amount needed for a cholesterol-lowering effect. Consumption of these glucans in oats and barley reduces total cholesterol and low-density lipoprotein cholesterol concentrations without adversely affecting high-density lipoprotein cholesterol or triglyceride concentrations.
β-glucans are macrophage activators. Topical application of yeast-derived β-glucans results in production of epidermal cell growth factor, which supports production of collagen and elastin, two structural components of skin that give it strength and elasticity. This aids the healing of wounds and improves the skin’s appearance by diminishing lines and wrinkles. β-glucans also stimulate defense against infection, making them particularly helpful against acne and herpes skin eruptions.
Most clinical studies of the effects of β-glucans on immunity and prevention of infection have used intravenous administration. In such studies, incidence of pneumonia and sepsis in post-surgical patients has been greatly reduced (e.g., from 55% to 10% for pneumonia and 35% to 10% for sepsis in one study). Studies using oral administration of β-glucans have been limited to animals. Yet, there is much anecdotal information and tradition suggesting that the use of β-glucan-containing foods, particularly mushrooms of various kinds, confer improved immunity. (Note: Any such effect may be due to compounds in the foods other than β-glucans.) Mushroom-containing products marketed for their immune-enhancing effects cite evidence of the increase in natural killer (NK) cell activity resulting from mushroom intake. Heightened NK cell activity presumably confers greater ability to resist infection. Studies to confirm that presumption are still waiting to be done.
As with immune enhancement, many of the cancer studies with β-glucans or mushroom extracts have used intravenous administration. A few studies using oral administration of mushroom extracts to advanced cancer patients have been done. In none of the studies was there a case of remission in a treated patient even though increased NK cell activity and leukocyte counts were observed to rise. There was some suggestion in one study of a slowing of deterioration and palliative effects were reported in another. The disappointing results of these studies may reflect an unfortunate choice of the particular compound, extract, or mushroom type tested. Indeed, animal studies have shown certain orally administered extracts to inhibit tumor formation when more purified forms did not. Collectively, published data supports the idea that β-glucans and certain mushrooms may have potent cancer-preventing effects. The specifics regarding the compounds and their preparation, as well as the forms of cancer that are treatable, remain to be established.
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 Poppitt SD et al. Supplementation of a high-carbohydrate breakfast with barley beta-glucan improves postprandial glycaemic response for meals but not beverages. Asia Pac J Clin Nutr 2007; 16(1):16-24.
 Behall KM et al. Consumption of both resistant starch and β-glucan improves postprandial plasma glucose and insulin in women. Diabetes Care 2006; 29:976-981.
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