Beta-glucan

Examples of various β-glucan glycosidic linkages.

β-Glucans (beta-glucans) comprise a group of β-D-glucose polysaccharides naturally occurring in the cell walls of cereals, yeast, 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.

The positive health effects of β-glucan have been demonstrated through extensive research, but differ by type: yeast and medicinal mushroom derived β-glucans are notable for their ability to modulate the immune system, while cereal β-glucan is known for cholesterol-lowering, glucose modulation, and cosmetic applications. More recent studies have shown cereal β-glucan may also exhibit immunomodulatory properties. β-glucans are further used in various nutraceutical and cosmetic products, as texturing agents, and as soluble fiber supplements, but can be problematic in the process of brewing. An increasing body of research has demonstrated the potential of β-glucans in pharmaceutical applications.

History

Cereal, yeast, 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 their cholesterol lowering effect was reported in 1984.

In 1997, after reviewing 33 clinical studies performed over the previous decades, the FDA approved of a claim that intaking at least 3 g of β-glucan from oats per day decreased saturated fats and reduced the risk of heart disease. This marked the first time a public health agency claimed dietary intervention can actually help prevent disease. This health claim mobilized a dietary movement as physicians and dietitians for the first time could recommend intake of a specific food to directly combat disease. Since then, oat consumption has continued to gain traction in disease prevention with noted effects on ischemic heart disease and stroke prevention, but also in other areas like BMI reduction, blood pressure lowering and highly corroborated evidence for reduced blood serum cholesterol.

Structure

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. 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.

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.

β-Glucan Structure by Source

Source	Backbone	Branching	Solubility in Water
Bacteria		None	Insoluble
Fungus		Short β-1,6 branching	Insoluble
Yeast		Long β-1,6 branching	Insoluble
Cereal		None	Soluble

β-glucan types

The shiitake mushroom contains beta-glucans.

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 effects 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). The β(1,3)D-glucans from yeast are often insoluble. However, β(1,3)(1,4)-glucans are also extracted from the bran of some grains, such as oats and barley, and to a much lesser degree in rye and wheat. Other sources include some types of seaweed, and various species of mushrooms, such as reishi, shiitake, Chaga and maitake.

Cereal β-Glucans

Main article: Oat beta-glucan

Cereal β-glucans from oat, barley, wheat, and rye, induce a variety of physiological effects that positively impact health. Barley and oat β-glucans have shown superior effects on blood glucose regulation for diabetics and have been used by hypercholesterolemic subjects. Oat β-glucan has also shown immunomodulatory effects, antitumour properties, and stimulation of collagen deposition, tissue granulation, reepithelization, and macrophage infiltration in the wound healing process.

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.

Mushroom β-Glucans

β-D-Glucan forms part of the cell wall of certain medically important fungi, especially Aspergillus and Agaricus species. Mushroom beta-glucans are linked by 1,3 glycosidic bonds with 1,6 branches. An assay to detect its presence in blood is marketed as a means of diagnosing invasive fungal infection in patients. False positives may occur because of fungal contaminants in the antibiotics amoxicillin-clavulanate, 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.

Mushroom β-glucans have demonstrated anticarcinogenic, antiviral, and immunomodulatory effects. Because of their ability to activate the immune system they are known as "biological response modifiers". Immunologists have discovered that receptors on the surface of innate immune cells called dectin-1 and complement receptor 3 (CR3 or CD11b/CD18) are responsible for binding to β-glucans, allowing the immune cells to recognize them as "non-self". This immune response regulation also effects antitumour properties. Several studies note an impact on epithelial cell cytokine generation.

In a study of human patients with advanced gastric or colorectal cancer, the administration of β-1,3 glucans derived from shiitake mushrooms, in conjunction with chemotherapy, was reported to prolong survival times.

Yeast β-Glucans

β-Glucans found in the cell walls of yeast contain a 1,3 carbon backbone with elongated 1,6 carbon branches

Infection

A series of human clinical trials in the 1990s evaluated the impact of PGG-glucan therapy for controlling infections in high-risk surgical patients. In these studies, the PGG-glucan significantly reduced complications.

Allergic rhinitis

Allergic rhinitis is caused by an IgE-mediated allergic inflammation of the nasal mucosa. Orally administered yeast-glucan decreased levels of IL-4 and IL-5 cytokines responsible for the clinical manifestation of allergic rhinitis, while increasing the levels of IL-12.

β-Glucan absorption

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

Additional applications

Cereals, mushroom, and yeast β-Glucan facilitate bowel motility and can be used in amelioration of intestinal problems, particularly obstipation. Indigestible β-glucans, forming a remarkable portion of these materials, are also able to modulate mucosal immunity of the intestinal tract.

See also

• Medicinal mushrooms

External links

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