Beta-glucan

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Cellulose is an example of a (1→4)-β-D-glucan composed of glucose units

β-Glucans (beta-glucans) comprise a group of β-D-glucose polysaccharides naturally occurring in the cell walls of cereals, bacteria, and fungi, with significantly differing physicochemical properties dependent on source. Typically, β-glucans form a linear backbone with 1-3 β-glycosidic bonds but vary with respect to molecular mass, solubility, viscosity, branching structure, and gelation properties, causing diverse physiological effects in animals.

At dietary intake levels of at least 3 g per day, oat fiber β-glucan decreases blood levels of LDL cholesterol and so may reduce the risk of cardiovascular diseases.[1] β-glucans are used as texturing agents in various nutraceutical and cosmetic products, and as soluble fiber supplements, but can be problematic in the process of brewing.

History[edit]

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, 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]

Structure[edit]

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 typically 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]

β-Glucan Structure by Source
Source Backbone Branching Solubility in Water
Bacteria
Curdlan haworth.png
None Insoluble[8]
Fungus
Curdlan haworth.png
Short β-1,6 branching Insoluble[9]
Yeast
Curdlan haworth.png
Long β-1,6 branching Insoluble[7]
Cereal
Beta-1,3-1,4-glucan.png
None Soluble[6]

β-glucan types[edit]

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.[10] Other sources include seaweed,[11] and various mushrooms, such as reishi, shiitake, chaga, and maitake, which are under preliminary research for their potential immune effects.[12]

Fermentable fiber[edit]

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.[13] This fermentation impacts the expression of many genes within the large intestine,[14] which further affects digestive function and cholesterol and glucose metabolism, as well as the immune system and other systemic functions.[13][15]

Oatmeal is a common food source of β-glucans

Cereal[edit]

Cereal β-glucans from oat, barley, wheat, and rye have been studied for their effects on cholesterol levels in people with normal cholesterol levels and in those with hypercholesterolemia.[1] Intake of oat β-glucan at daily amounts of at least 3 grams lowers total and low-density lipoprotein cholesterol levels by 5 to 10% in people with normal or elevated blood cholesterol levels.[16]

Oats and barley differ in the ratio of trimer and tetramer 1-4 linkages. Barley has more 1-4 linkages with a degree of polymerization higher than 4. However, the majority of barley blocks remain trimers and tetramers; in oats, β-glucan is found mainly in the endosperm of the oat kernel, especially in the outer layers of that endosperm.[6]

β-glucan absorption[edit]

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.[17] 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.[18] M cells within the Peyer’s patches physically transport the insoluble whole glucan particles into the gut-associated lymphoid tissue.[19]

(1,3)-β-D-glucan medical application[edit]

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.[20][21][22] 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,[23] 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.[24] This test can aid in the detection of Aspergillus, Candida, and Pneumocystis jirovecii.[25][26][27] 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.[28]

See also[edit]

References[edit]

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  25. ^ Lahmer, Tobias; da Costa, Clarissa Prazeres; Held, Jürgen; Rasch, Sebastian; Ehmer, Ursula; Schmid, Roland M.; Huber, Wolfgang (2017-04-04). "Usefulness of 1,3 Beta-D-Glucan Detection in non-HIV Immunocompromised Mechanical Ventilated Critically Ill Patients with ARDS and Suspected Pneumocystis jirovecii Pneumonia". Mycopathologia. 182 (7–8): 701–708. doi:10.1007/s11046-017-0132-x. ISSN 1573-0832. PMID 28378239. 
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  28. ^ Ostrosky-Zeichner, Luis; Alexander, Barbara D.; Kett, Daniel H.; Vazquez, Jose; Pappas, Peter G.; Saeki, Fumihiro; Ketchum, Paul A.; Wingard, John; Schiff, Robert (2005-09-01). "Multicenter clinical evaluation of the (1→3) beta-D-glucan assay as an aid to diagnosis of fungal infections in humans". Clinical Infectious Diseases. 41 (5): 654–659. doi:10.1086/432470. ISSN 1537-6591. PMID 16080087. 

External links[edit]