Mushroom nutrition logo

BETA GLUCANS

When researchers in Japan started looking at the benefits of mushroom nutrition in the 1960's they did so on the basis of hot-water extracts from different mushrooms.

Having selected a specific mushroom as being worthy of further study they then separated the hot-water extract into a number of fractions and tested the activity of each fraction in one of a number of animal tumour models.

Based on the results from these trials they found that the fractions with the greatest benefit were rich in polysaccharides and this led to interest in fungal polysaccharides as an important class of immune modulators in their own right.

Polysaccharides are large molecules formed of long chains of sugar units and play an important role in the structure of fungal cell walls. The most common sugar present in fungal polysaccharides is glucose but many other sugars are also present in polysaccharides from mushrooms, including galactose, xylose, arabinose, rhamnose, mannose etc.

These sugars are joined with beta linkages rather than the alpha linkages found in starch and as such are not broken down by digestive enzymes. They differ from cellulose, another beta linked polysaccharide, in that the linkage betweek the glucose units in the main chain is between the first and third carbon atoms of the glucose molecules (a 1-3 linkage), rather than the first and fourth as in cellulose.

The main chain typically has side chains with the first sugar in the side chain joined via its first carbon atom to the sixth carbon atom of one of the sugars in the main chain (a 1-6 linkage). The prototypical molecule is thus referred to as a 1-3, 1-6 beta glucan.glucose

The cheapest source of beta glucans is yeast, which also lend themselves to research because they are almost pure beta glucans, unlike the more structurally diverse polysaccharides from mushrooms, and for these reasons most fungal polysaccharide research has tended to focus on yeast beta glucans, leading to the term 'beta-glucan' becoming synonymous with fungal polysaccharides.

However, when different polysaccharide extracts from any mushroom have been compared it has tended to be the heteropolysaccharides (containing a number of different sugar molecules) that have tended to exhibit the greatest activity.

In a study of 16 polysaccharide fractions from Pleurotus pulmonarius which showed anti-tumour activity the most active was found to be a xyloglucan also containing mannose and galactose, a protein containing mannogalactan with xylose and a protein containing xyloglucan. PSK and PSP, the top selling mushroom derived nutraceuticals are now understood to be heteroglucans with α(1-4)- and β-(1-3) glycosidic linkages and a protein component, which accounts for around 25% of their molecular weight. They are differentiated by the prescence of fucose in PSK and rhamnose and arabinose in PSP.

 

UPTAKE OF BETA GLUCANS AND OTHER FUNGAL POLYSACCHARIDES

Animal studies have confirmed that after oral administration the specific 1→3 linear β-glycosidic chain backbone of β-glucans cannot be digested by our digestive enzymes and so most β-glucans enter the proximal small intestine intact. Here some are captured by macrophages present in the small areas of immune tissue known as Peyer’s Patches present at a number of points along the wall of the small intestine. They are then internalized and fragmented within the cells before being transported by the macrophages to the marrow and endothelial reticular system where the small β-glucans fragments are released by the macrophages and taken up by other immune cells leading to a cascade of immune responses.

β-glucans have also been shown to be directly bound and internalized by intestinal epithelial cells as opposed to macrophages and β-glucans absorbed in this way have also been shown to be immunologically active, increasing resistance of mice to bacterial infection.

 

ACTIVITY OF FUNGAL POLYSACCHARIDES

Mushroom polysaccharides have been shown to have a broad effect on the immune system with at least 7 receptors involved in a range of responses including:

• Increased NK Cell activity
• Enhanced phagocytosis
• Increased cytokine production
• Promoting a shift from a Th2 to a Th1 immune state

With specific reference to cancer they have been shown to:

• Prevent oncogenesis
• Show anti-tumor activity through immune activation
• Prevent tumour metastasis

Other sources of beta glucans include oats and barley, whose beta glucans have been shown to have beneficial effects in controlling cholesterol levels and in blood sugar control and it is probable that beta glucans may be responsible for at least some of the benefits of mushrooms in these areas.

 

ARE POLYSACCHARIDES THE ONLY IMMUNOLOGICALLY ACTIVE COMPONENT IN MUSHROOMS?

No. Mushrooms derived immunomodulating preparations include a range of other compounds such as proteins (Flammulin from Flammulina velutipes and LZ-8 from Ganoderma lucidum), triterpenes (Ganoderma lucidum) and lignans (Lentinula edodes). However, the polysaccharide fraction has been shown to be the most effective for increasing the efficacy of immune response and where mushrooms are being used for immune support a high level of polysaccharides/beta glucans is undoubtedly desirable.

 

References:

Medicinal Importance of fungal β- (1→3), (1→6)-glucans. Jiezhong Chen, Robert Seviour. Mycological Research III (2007). 635-652

The effects of β-glucan on human immune and cancer cells. Godfrey Chi-Fung Chan, Wing Keung Chan and Daniel Man-Yuen Sze. Journal of Hematology & Oncology 2009, 2:25 Rice PJ, Adams EL, Ozment-Skelton T, Gonzalez AJ, Goldman MP, Lockhart BE, Barker LA, Breuel KF, Deponti WK, Kalbfleisch JH, et al.:

Oral delivery and gastrointestinal absorption of soluble glucans stimulate increased resistance to infectious challenge. The Journal of pharmacology and experimental therapeutics 2005, 314(3):1079-1086. Wilson TA, et al.

Reduced and high molecular weight barley beta-glucans decrease plasma total and non-HDL-cholesterol in hypercholesterolemic Syrian golden hamsters. J Nutr. 2004 Oct;134(10):2617-22. Casiraghi MC, et al.

Post-prandial responses to cereal products enriched with barley beta-glucan. J Am Coll Nutr. 2006 Aug;25(4):313-20. ll Nutr. 2006 Aug;25(4):313-20.