Beta-glucan

Cereal and fungal products have been used for centuries for medicinal and cosmetic purposes; however, the specific role of β-glucan was not explored until the 20th century. β-glucans were first discovered in lichens, and shortly thereafter in barley. A particular interest in oat β-glucan arose after a cholesterol lowering effect from oat bran reported in 1981.[2]

In 1997, the FDA approved of a claim that intake of at least 3.0 g of β-glucan from oats per day decreased absorption of dietary cholesterol and reduced the risk of coronary heart disease. The approved health claim was later amended to include these sources of β-glucan: rolled oats (oatmeal), oat bran, whole oat flour, oatrim (the soluble fraction of alpha-amylase hydrolyzed oat bran or whole oat flour), whole grain barley and barley beta-fiber. An example of an allowed label claim: "Soluble fiber from foods such as oatmeal, as part of a diet low in saturated fat and cholesterol, may reduce the risk of heart disease. A serving of oatmeal supplies 0.75 grams of the 3.0 g of β-glucan soluble fiber necessary per day to have this effect." The claim language is in the Federal Register 21 CFR 101.81 Health Claims: "Soluble fiber from certain foods and risk of coronary heart disease (CHD)".[3]

Glucans are arranged in six-sided D-glucose rings connected linearly at varying carbon positions depending on the source, although most commonly β-glucans include a 1-3 glycosidic link in their backbone. Although technically β-glucans are chains of D-glucose polysaccharides linked by β-type glycosidic bonds, by convention not all β-D-glucose polysaccharides are categorized as β-glucans.[4] Cellulose is not conventionally considered a β-glucan, as it is insoluble and does not exhibit the same physicochemical properties as other cereal or yeast β-glucans.[5]

Glucose molecule, showing carbon numbering notation and β orientation.

Some β-glucan molecules have branching glucose side-chains attached to other positions on the main D-glucose chain, which branch off the β-glucan backbone. In addition, these side-chains can be attached to other types of molecules, like proteins, as in polysaccharide-K.

The most common forms of β-glucans are those comprising D-glucose units with β-1,3 links. Yeast and fungal β-glucans contain 1-6 side branches, while cereal β-glucans contain both β-1,3 and β-1,4 backbone bonds. The frequency, location, and length of the side-chains may play a role in immunomodulation. Differences in molecular weight, shape, and structure of β-glucans dictate the differences in biological activity.[6][7]

In general, β-1,3 linkages are created by 1,3-Beta-glucan synthase, and β-1,4 linkages are created by cellulose synthase. The process leading to β-1,6 linkages is poorly understood: although genes important in the process have been identified, not much is known about what each of them do.[8]

β-Glucan Structure by Source

Source (Example)	Backbone	Branching	Solubility in Water
Bacteria (Curdlan)		None	Insoluble[9]
Fungus		Short β-1,6 branching	Insoluble[10]
Yeast		Long β-1,6 branching	Insoluble[7]
Cereal (Oat beta-glucan)		None	Soluble[6]

β-glucans form a natural component of the cell walls of bacteria, fungi, yeast, and cereals such as oat and barley. Each type of beta-glucan comprises a different molecular backbone, level of branching, and molecular weight which affects its solubility and physiological impact. One of the most common sources of β(1,3)D-glucan for supplement use is derived from the cell wall of baker's yeast (Saccharomyces cerevisiae). β-glucans found in the cell walls of yeast contain a 1,3 carbon backbone with elongated 1,6 carbon branches.[11] Other sources include seaweed,[12] and various mushrooms, such as lingzhi, shiitake, chaga, and maitake, which are under preliminary research for their potential immune effects.[13]

Fermentable fiber

Main article: Dietary fiber

In the diet, β-glucans are a source of soluble, fermentable fiber – also called prebiotic fiber – which provides a substrate for microbiota within the large intestine, increasing fecal bulk and producing short-chain fatty acids as byproducts with wide-ranging physiological activities.[14] This fermentation impacts the expression of many genes within the large intestine,[15] which further affects digestive function and cholesterol and glucose metabolism, as well as the immune system and other systemic functions.[14][16]

Oatmeal is a common food source of β-glucans

Enterocytes facilitate the transportation of β(1,3)-glucans and similar compounds across the intestinal cell wall into the lymph, where they begin to interact with macrophages to activate immune function.[18] Radiolabeled studies have verified that both small and large fragments of β-glucans are found in the serum, which indicates that they are absorbed from the intestinal tract.[19] M cells within the Peyer's patches physically transport the insoluble whole glucan particles into the gut-associated lymphoid tissue.[20]

An assay to detect the presence of (1,3)-β-D-glucan in blood is marketed as a means of identifying invasive or disseminated fungal infections.[21][22][23] This test should be interpreted within the broader clinical context, however, as a positive test does not render a diagnosis, and a negative test does not rule out infection. False positives may occur because of fungal contaminants in the antibiotics amoxicillin-clavulanate,[24] and piperacillin/tazobactam. False positives can also occur with contamination of clinical specimens with the bacteria Streptococcus pneumoniae, Pseudomonas aeruginosa, and Alcaligenes faecalis, which also produce (1→3)β-D-glucan.[25] This test can aid in the detection of Aspergillus, Candida, and Pneumocystis jirovecii.[26][27][28] This test cannot be used to detect Mucor or Rhizopus, the fungi responsible for mucormycosis, as they do not produce (1,3)-beta-D-glucan.[29]

• Prebiotic (nutrition)
• Resistant starch
• Xylooligosaccharides

• beta-Glucans at the US National Library of Medicine Medical Subject Headings (MeSH)