Oat beta-glucan

Oat β-glucan repeat structure

Oat β-glucans are water-soluble β-glucans derived from the endosperm of oat kernels known for their cholesterol lowering and hypoglycemic properties, as well as their use in various cosmetic applications. They have been used by hypercholesterolemic subjects and by diabetics for blood glucose regulation. Recent research has shown their potential application in immunomodulation and wound healing.

History

Oat 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. After joining Agriculture and Agri-Food Canada in 1969, Peter J Wood played an instrumental role in isolating and characterizing the structure and bioactive properties of oat β-glucan. A public interest in oat β-glucan arose after its cholesterol lowering effect was reported in 1984.

In 1997, after reviewing 33 clinical studies performed over the previous decades, the FDA approved the 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.

Structural Properties

Cereal β-glucans - including β-glucan from oat, barley and wheat - are linear polysaccharides joined by 1,3 and 1,4 carbon linkages. The majority of cereal β-glucan bonds consist of 3 or 4 beta-1,4 glycosidic bonds (trimers and tetramers) interconnected by 1,3 linkages. In β-glucan, these trimers and tetramers are known as cellotriosyl and cellotetraosyl. Oats and barley differ in the ratio of cellotriosyl to cellotetraosyl, and barley has more 1-4 linkages with a degree of polymerization higher than 4. In oats, β-glucan is found mainly in the endosperm of the oat kernel, especially in the outer layers of that endosperm (a marked difference from barley, which contains β-glucan uniformly throughout the endosperm).

Most oats contain 3-6% β-glucan by weight. Oats can be selectively bred based on favourable β-glucan levels. Often millers only process oat cultivars with at least 4% by weight β-glucan. Oat β-glucans are linear and linked at the 1,3 and 1,4 carbon sites.

Oat β-glucans can form into a random coil structure and flow with Newtonian behaviour until they reach a critical concentration at which point they become pseudoplastic. The gelling ability of oat β-glucan correlates to the percentage of trimers.

Extraction

β-glucan extraction from oat can be difficult due to tendency of depolymerization – which often occurs in high pH. Thus β-glucan extraction is usually performed under a more neutral pH and generally at temperatures of 60-100 degrees Celsius. Usually β-glucan is solubilized in the extraction process with residual starch, which is then removed by hydrolysis with alpha-amylase. The residual solution usually contains coextracts of hemicelluloses and proteins which can then be separated through selective precipitation. Through wet milling, sieving, and solvent-extraction, oat beta-glucans can achieve up to 95% extraction purity.

Viscosity of oat β-glucan

In oats, β-glucan makes up the majority of the soluble fibre; however, oat β-glucans do become insoluble above a certain concentration. The total viscosity is determined by the level of solubility, the molecular weight, and the trimer-to-tetramer ratio. The lower the trimer-tetramer ratio, the higher the β-glucan viscosity in solution. A more viscous internal β-glucan solution generally leads to beneficial physiological effects – including a more pronounced hypoglycemic effect and lowered cholesterol levels, and a decrease in postprandial blood glucose levels.

Physiological Effects

Cholesterol

In 1997, the FDA recognized the cholesterol lowering effect of oat β-glucan. In Europe, several health claim requests were submitted to the EFSA NDA Panel (Dietetic Products, Nutrition and Allergies), related to the role of β-glucans in maintenance of normal blood cholesterol concentrations and maintenance or achievement of a normal body weight. In July 2009, the Scientific Committee issued the following statements:

• On the basis of the data available, the Panel concludes that a cause-and-effect relationship has been established between the consumption of beta-glucans and the "reduction of blood cholesterol concentrations."
• The following wording reflects the scientific evidence: "Regular consumption of beta-glucans contributes to maintenance of normal blood cholesterol concentrations." In order to bear the claim, foods should provide at least 3 g/d of beta-glucans from oats, oat bran, barley, barley bran, or mixtures of non-processed or minimally processed beta-glucans in one or more servings. The target population is adults with normal or mildly elevated blood cholesterol concentrations.
• On the basis of the data available the Panel concludes that a cause-and-effect relationship has not been established between the consumption of beta-glucans and the maintenance or achievement of a normal body weight.

In November 2011, the EU Commission published its decision in favour of oat beta-glucans with regard to Article 14 of the EC Regulation on the labelling of foodstuffs with nutrition and health claim statements permitting oat beta-glucan to be described as beneficial to health. Following the opinion of the Panel on Dietetic Products, Nutrition and Allergies (NDA) the EFSA and the Regulation (EU) no. 1160/2011 of the Commission, foodstuffs through which 3 g/day of oat beta-glucan are consumed (1 g of oat beta-glucan per portion) are allowed to display the following health claim: "Oat beta-glucan reduces the cholesterol level in the blood. The lowering of the blood cholesterol level can reduce the risk of coronary heart disease."

β-glucan lowers cholesterol by increasing the viscosity of digesta in the small intestine, although cholesterol reduction is greater in those with higher total cholesterol and LDL cholesterol in their blood. The degree of cholesterol reduction will also depend upon the particular strain of β-glucan and its composite food matrix. To achieve an observable reduction, β-glucans must range between a molecular weight of 26.8 and 3000 kD. Although more viscous β-glucans result in a more viscous solution of intestinal digesta and thus more cholesterol uptake, after a certain molecular weight, β-glucans become less soluble and thus contribute less to solution viscosity. The intake of β-glucan in liquid form generally results in greater solubilization and oat β-glucan is more effective at lowering cholesterol in juices than in hard foods like bread and cookies. Despite the recognized impact of viscosity on serum cholesterol levels, no current data exists comparing internal solution viscosity and serum cholesterol.

Total cholesterol levels lower up to a β-glucan intake of 3g per day, after which there is no further increase. This corresponds to a maximum decrease of 16.5% LDL.

Digestion

Throughout digestion, β-glucan alters the physical properties of digesta while chemicals in the digestive tract break down β-glucan, changing its composition. Fermentation of β-glucans by microbiote results in the production of short chain fatty acids and changes to gut microbes as well as the depolymerization and structural change of the original β-glucan. In the stomach, β-glucans swell and cause gastric distension – which is associated with the signal pathway of satiation – the feeling of fullness, leading to a decreased appetite. Studies demonstrating β-glucan’s effect on delayed gastric emptying may differ due to variants in food combination, β-glucan dosage, and molecular weight, and variety of food source. In the small intestine, β-glucan may reduce starch digestibility and glucose uptake – significant in the reduction of postprandial glucose levels.

Oat β-glucans have a prebiotic effect where they selectively stimulate growth of specific strands of microbes in the colon, where the particular microbe stimulated depends on the degree of polymerization of the β-glucan. Specifically, Lactobacillus and Enterococcus are stimulated by all oat β-glucan while Bifidobacterium bacteria also stimulated by oat β-glucan oligosaccharides. Soluble β-glucan increases stool weight through the increase in microbial cells in the colon.

Postprandial Blood Glucose

Postprandial blood glucose levels lower after consumption of a meal with solubilized viscous β-glucan as a result of increased gut viscosity, which delays gastric emptying and lengthens travel through the small intestine. The net decrease in blood glucose absorption reduces postprandial insulin concentrations in the blood which improves insulin sensitivity and glucose metabolism.

Cosmetics

β-glucan is used in a variety of creams, ointments and powders to increase collagen production, and reduce undesirable skin perturbations - including wrinkles, acne, cellulite, eczema, and dermatitis.

Wound Healing and Immunomodulation

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.

Beta-glucans immunomodulate by stimulating dectin-1 to trigger monocytes, macrophages and neutrophils. In 2002, Wei et al demonstrated that glucans directly stimulate human fibroblasts. Low molecular weight oat beta glucan increased macrophage proliferation and cytotoxicity to cancer cells while leaving healthy tissue intact. This effect also increased significantly with continued application over time. The beta-glucan further significantly reduced human melanoma and human epidermoid carcinoma, without toxic effects to human tissue, keratinocytes, or macrophages. Through immunostimulation they can also serve a preventative role against tumours. But beta-glucan's immunomodulatory properties are heavily dependent on the degree of branching and molecular size. For instance, cancer-fighting properties have been demonstrated in fungi and yeast beta-glucans, and more recently in low molecular weight oat beta-glucans, but high molecular weight oat beta-glucans are hindered by their high viscosity.

Recently, the wound healing effectiveness of both yeast and oat beta-glucan were tested on carp. After 14 days, beta-glucan applied to carp in both yeast and oat form significantly reduced wound size compared to the control group, though significantly more in the yeast group. The glucan-treated group displayed faster proliferation and fibroblast migration. In this carp study, the oat beta-glucan used had a molecular weight of 6.3 kiloDaltons and might have been more effective at macrophage stimulation and overall wound healing with a lower molecular weight.