Beta-glucan

Did You Know?

• Enterocytes facilitate transportation of β(1,3)-glucans across the intestinal cell wall into the lymph, where they interact with macrophages.
• M cells within Peyer's patches transport insoluble whole glucan particles into gut-associated lymphoid tissue.
• β-glucans are estimated to account for about 25% of worldwide organic carbon production, mainly from marine algae.

History

Although people have turned to cereal and fungal products for medicinal and cosmetic uses over many centuries, scientists did not investigate the specific role of beta-glucan until the 1900s. The substance was first identified in lichens, and soon after in barley. Interest in oat beta-glucan grew following a 1981 report that oat bran could lower cholesterol. In 1997, the United States Food and Drug Administration permitted a health claim stating that consuming at least 3.0 grams of beta-glucan from oats each day reduces dietary cholesterol absorption and lowers the risk of coronary heart disease. This claim later expanded to include sources like rolled oats, oat bran, whole oat flour, oatrim, whole grain barley, and barley beta-fiber. A typical label statement notes that soluble fiber from oatmeal, within a diet low in saturated fat and cholesterol, may help reduce heart disease risk.

Structure

Glucans are arranged in six-sided D-glucose rings connected linearly at different carbon positions, though most β-glucans include a 1-3 glycosidic link in their backbone and generally contain 20 to 30 D-glucose rings. By convention, not all β-D-glucose polysaccharides are categorized as β-glucans; cellulose is excluded because it is insoluble and lacks the same physicochemical properties. Some β-glucan molecules have branching glucose side-chains attached to other positions on the main chain, and these side-chains can attach to other molecules like proteins, as in polysaccharide-K. The most common forms have β-1,3 links; yeast and fungal β-glucans contain 1-6 side branches, while cereal β-glucans have both β-1,3 and β-1,4 backbone bonds but no β-1,3 branching. Seaweed glucans have a backbone primarily β-1,3-glucan with some β-1,6-glucan in the backbone and side chains. Glucans play a role in carbon storage, cell wall structure, and communication between fungi, plants, and animals. The frequency, location, and length of side-chains may influence immunomodulation. β-1,3 linkages are created by 1,3-beta-glucan synthase, β-1,4 linkages by cellulose synthase, and the process for β-1,6 linkages is poorly understood. Bacteria produce linear β-1,3 glycans and cyclic β-1,3-1,6 glycans, the latter found in the periplasm with an osmotic adaptation role.

β-glucan types used in the human diet

One common source of β-glucan for supplements is the cell wall of baker's yeast (Saccharomyces cerevisiae), which contains a β-1,3 glucose backbone with elongated β-1,6 glucose branches. Other sources include seaweed and various mushrooms such as lingzhi, shiitake, chaga, and maitake, which are under preliminary research for potential immune effects. In the diet, β-glucans serve as soluble, fermentable fiber that provides a substrate for microbiota in the large intestine, increasing fecal bulk and producing short-chain fatty acids. Cereal β-glucans from oat, barley, wheat, and rye have been studied for their effects on cholesterol levels; intake of at least 3 grams daily lowers total and LDL cholesterol by 5 to 10% in people with normal or elevated cholesterol. Oats and barley differ in the ratio of trimer and tetramer 1-4 linkages, with barley having more linkages with a degree of polymerization higher than 4.

Presence in blood

An assay to detect (1,3)-β-D-glucan in blood is used to identify invasive or disseminated fungal infections, aiding in the detection of Aspergillus, Candida, and Pneumocystis jirovecii. It cannot detect Mucor or Rhizopus, which do not produce (1,3)-beta-D-glucan. The test must be interpreted in the broader clinical context, as false positives can occur from fungal contaminants in antibiotics like amoxicillin-clavulanate and piperacillin/tazobactam, or from bacteria such as Streptococcus pneumoniae, Pseudomonas aeruginosa, and Agrobacterium.