Research progress on physiological functions of oat beta-glucan

Oat (Avena sativa), as an ancient cereal crop, has been cultivated by humans for nearly two thousand years and has long been a staple food in many countries and regions. Oats are classified into hulled oats and naked oats. In China, oats have a long cultivation history and were referred to as “que mai” (sparrow wheat) or “wild wheat” in Compendium of Materia Medica. Oats have a sweet and mild nature and are believed to strengthen the spleen and nourish the heart, as well as stop sweating. They have high nutritional value.

The ancient medical text Herbal for Famine Relief mentions that oats can be used to treat spontaneous or night sweating and pulmonary tuberculosis. They can be boiled into a decoction or processed into flour for steamed or baked foods. Oats can promote gastrointestinal motility, aid bowel movements, have low calorie content and a low glycemic index, and help lower blood lipids and blood sugar levels. Oat bran is a by-product of oat processing, and β-glucan is the main dietary fiber component in oat bran, possessing important physiological functions. In 1997, the U.S. Food and Drug Administration (FDA) recommended a daily intake of at least 3 grams of foods containing oat β-glucan and allowed this health claim to be clearly labeled on food packaging.

Early research on oats was relatively limited, and people mainly believed that oats could increase satiety and reduce the absorption of nutrients. In recent years, however, the physiological functions of oats that are beneficial to health have attracted growing attention, making oats a new focus of research on diet and health. Studies have shown that oats possess a wide range of physiological functions, including the ability to lower cholesterol, reduce blood glucose levels, regulate the immune system, and decrease the risk of intestinal cancer. Meanwhile, the molecular mechanisms underlying these health benefits have been gradually elucidated. Further research has revealed that these nutritional effects are primarily derived from a nutrient component found in the oat bran—oat β-glucan. Oat β-glucan exists in the cell walls of the aleurone and subaleurone layers of oat endosperm and flour layers, where it accumulates in large quantities. It is a type of indigestible β-D-glucan formed by linking numerous monosaccharides through β-(1,3) and β-(1,4) glycosidic bonds. This unique structural feature endows oat β-glucan with distinctive physiological functions.

1. Reduction of Cholesterol Levels and Prevention of Cardiovascular Diseases

Cardiovascular disease has become the second leading cause of death after malignant tumors and is one of the major causes of human mortality. Apart from genetic factors, excessive intake of fat and cholesterol is one of the primary causes of the high incidence of cardiovascular diseases. Recent studies have shown that oat β-glucan can effectively and steadily reduce serum cholesterol and triglyceride levels, thereby lowering the risk of cardiovascular diseases. In 1997, the U.S. Food and Drug Administration (FDA) recognized oats as a dietary fiber-rich food with the function of reducing serum cholesterol. Low-density lipoprotein cholesterol (LDL-C) is a key indicator of cardiovascular disease, and epidemiological and clinical studies have demonstrated that for every 0.026 mmol/L increase in LDL-C, the risk of developing cardiovascular disease increases by 1%. The occurrence of cardiovascular diseases is closely related to the concentration of serum LDL-C. In a four-week clinical trial, patients who consumed oat bran showed significant decreases in serum LDL-C, total cholesterol, and apolipoprotein B (apoB) compared with those who did not consume oats.

Rabey et al. found that in hypercholesterolemic rats, levels of alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase, and lipid peroxidation were all elevated, accompanied by pathological changes in organs such as the liver, kidneys, and heart. After being fed oat β-glucan, these rats exhibited significant improvements in various physiological indicators. Davidson et al. analyzed the lipid-lowering effects of oat meals and oat bran on patients with hypercholesterolemia and found that after six weeks of oat consumption, patients’ serum cholesterol levels were inversely correlated with the amount of oat β-glucan intake. Ripsin et al., through meta-analysis, discovered that consuming more than 3 g of oat β-glucan per day produces different effects, with more pronounced cholesterol-lowering effects observed in patients whose serum cholesterol levels exceed the clinical threshold, showing a clear dose–response relationship. A daily intake of 6 g of β-glucan was found to lower total cholesterol and LDL levels to normal ranges.

Oat β-glucan is a polysaccharide complex, and its cholesterol-lowering effect varies depending on processing methods and modes of consumption. When oats are added separately to milk, juice, or other beverages, the hypolipidemic effect is significant. However, inconsistent results have been observed when oat β-glucan is incorporated into solid foods. Reyna et al. found that adding 6 g of oat β-glucan to high-fat diets for mice resulted in reduced serum cholesterol levels. In contrast, earlier studies indicated that incorporating oat β-glucan into solid foods such as cookies or bread did not produce lipid-lowering effects. Researchers believe this may be due to the degradation of the linear structure of β-glucan during the production of solid foods. Some studies suggest that the lipid-lowering activity of oat β-glucan is also related to its molecular weight—higher molecular weight results in greater viscosity and thus stronger cholesterol-lowering effects, whereas lower molecular weight leads to weaker effects. Wilson et al. found that both small and large molecular weight oat β-glucans (175 and 1,000 kDa) effectively reduced serum cholesterol levels in hypercholesterolemic mice. When oat β-glucan was hydrolyzed to an average molecular weight of 750 kDa, it still demonstrated a strong ability to bind bile acids and lipids. Bae et al. used two types of oat β-glucan with different molecular weights (1,450 and 730 kDa) in rat studies and found that LDL cholesterol decreased by 25% and 31%, respectively, while HDL cholesterol increased by 42% and 63%. However, other studies suggest that the cholesterol-lowering effect of oat β-glucan is not necessarily correlated with its molecular weight.

The cholesterol-lowering effect of oat β-glucan may involve multiple mechanisms, the most widely accepted of which is based on its physical property—viscosity. After entering the intestinal tract, oat β-glucan, due to its high viscosity, can encapsulate bile acids, lipids, and cholesterol in the intestinal lumen, preventing their absorption by the intestinal mucosa and facilitating their excretion through feces. As the reabsorption of bile acids decreases, the balance of cholesterol metabolism in the liver is disrupted. Cholesterol in the liver is then converted into bile acids, which lowers hepatic cholesterol levels. To restore balance, endogenous cholesterol synthesis increases through the activation of 7α-hydroxylase and acetyl-CoA acetyltransferase A, thereby promoting bile acid synthesis and reducing cholesterol storage in the liver. In addition, low-density lipoprotein cholesterol (LDL-C) in the blood is converted and stored in the liver, resulting in decreased serum LDL-C levels. The hallmark of this mechanism is the enhanced excretion of bile acids via feces. Studies have also shown that as intestinal bile acids decrease, fat emulsification in the intestine also diminishes. The high viscosity of oat β-glucan helps reduce fat absorption, prevent fat accumulation, and promote fat excretion. Furthermore, oat β-glucan may reduce and delay nutrient absorption by modulating hormonal pathways. Research indicates that oat β-glucan can upregulate two key genes in cholesterol metabolism—3-hydroxy-3-methylglutaryl coenzyme A reductase (HMGR) and cholesterol 7α-hydroxylase (CYP7A1)—to accelerate the conversion of serum LDL-C into hepatic cholesterol, thereby lowering serum cholesterol levels. In mammals, a key hormone peptide, peptide YY₃₋₃₆ (PYY₃₋₃₆), can cross the blood-brain barrier and transmit satiety and energy balance signals to the hypothalamic arcuate nucleus. Huang et al. found that oat β-glucan significantly reduced body weight in obese mice and increased serum PYY₃₋₃₆ levels, contributing to enhanced satiety and improved energy homeostasis.

2. Hypoglycemic and Antidiabetic Effects

With the improvement of living standards, diabetes—a type of metabolic disease—has seen a steady increase in incidence in recent years. Oat β-glucan, as a soluble dietary fiber, has demonstrated significant hypoglycemic effects. Studies have shown that due to its high viscosity, oat β-glucan can alter the rheological properties of chyme in the upper gastrointestinal tract, delay gastric emptying, increase intestinal motility, and influence nutrient absorption and postprandial blood glucose and insulin release. Therefore, oat β-glucan plays a beneficial role in maintaining health and is particularly advantageous for diabetic patients. Diabetes is mainly assessed by measuring postprandial blood glucose and insulin levels or fasting blood glucose, as well as glycated hemoglobin (HbA1c). Metabolic and epidemiological evidence suggests that replacing high-glycemic-index (GI) carbohydrates with low-GI oat β-glucan can help reduce the risk of type 2 diabetes. Long-term consumption of oat β-glucan has been shown to lower serum cholesterol and postprandial glucose peaks; however, it does not significantly affect fasting glucose, insulin, or HbA1c levels. Studies comparing oat flakes and oatmeal found that both products have glycemic indices similar to white bread, but cooked oatmeal led to lower postprandial blood glucose levels and reduced insulin secretion. Tappy et al. added 4.0, 6.0, and 8.4 g of oat β-glucan to diabetic patients’ breakfasts and found that postprandial glucose levels were reduced by 67%, 42%, and 38%, respectively. Tapola et al. further found that oat bran has a more pronounced hypoglycemic effect than oat flakes, which was attributed to its higher β-glucan content. Consequently, oat β-glucan is believed to have a preventive effect against type 2 diabetes. Comparative tests using wheat and oats also showed that oats were more effective in stabilizing blood glucose levels than wheat.

The hypoglycemic effect of oat β-glucan is closely related to its viscosity and insulin sensitivity. Panahi et al. found that consuming oat β-glucan with varying viscosities reduced postprandial blood glucose levels in diabetic patients, with higher-viscosity β-glucan showing a greater effect. Wood similarly suggested that the hypoglycemic property of oat β-glucan mainly depends on its physicochemical characteristic—viscosity. Experiments using polysaccharides of different viscosities showed a positive correlation between blood glucose reduction and food viscosity. Tapola et al. observed that in the test group consuming oat bran, blood glucose levels were significantly lower than those of the control group one hour after eating, but increased after two hours, indicating that oat β-glucan delays the appearance of the postprandial glucose peak. Wang et al. reported that many factors influence the hypoglycemic effect of oat β-glucan, including the oat variety, extraction temperature, pH, and storage conditions, all of which can affect β-glucan’s blood glucose-lowering capacity. They suggested that these factors might alter the viscosity and molecular weight of oat β-glucan, thereby impacting its biological activity. However, recent research indicates that the hypoglycemic effect of oat β-glucan is not solely due to delayed glucose absorption caused by high viscosity. Zheng et al. found that oat β-glucan significantly reduced insulin resistance in rats and enhanced the activity of hepatic glucokinase, suggesting that β-glucan also plays a role in metabolic regulation. Hooda et al. demonstrated that oat β-glucan not only reduces postprandial blood glucose and insulin levels but also lowers the concentrations of glucagon-like peptide and glucose-dependent insulinotropic polypeptide, effectively reducing the likelihood of type 2 diabetes. Zhang et al. discovered that oat β-glucan increases insulin sensitivity, enhances Na⁺K⁺-ATPase and Ca²⁺Mg²⁺-ATPase activity in the small intestine, and thereby promotes energy metabolism.

3. Antitumor Effects

Cancer is one of the most severe malignant diseases affecting humans, and numerous studies have shown that oat β-glucan possesses antitumor and tumor-preventive properties. Its effects vary depending on multiple factors, such as the type of tumor, the animal’s genetic background, dosage, administration route, and timing. Research indicates that the antitumor mechanism of oats is not limited to directly activating macrophages to attack tumor cells but also involves regulating immune cells such as lymphocytes, neutrophils, natural killer (NK) cells, and other components of the innate immune system. In fact, polysaccharides derived from fungi and bacteria have already been widely investigated for their antitumor potential.

Murphy et al. conducted experiments examining the combined effects of exercise and dietary β-glucan on tumors and found that both exercise and oat β-glucan supplementation significantly slowed the spread of B16 melanoma and reduced the number of lung tumor nodules. Although there was no significant difference between the two groups, the macrophage antitumor activity in the experimental animals increased notably. Mechanistically, the body’s antitumor ability is closely associated with specific immune cells within the innate immune system—especially neutrophils. Hong et al. found that oral administration of oat β-glucan along with exogenous antibody therapy enhanced the antitumor activity of the antibody and improved its ability to target tumor cells. Injected polysaccharides were also shown to boost both antifungal and antitumor immunity. Demir et al. further discovered that oral administration of oat β-glucan in breast cancer patients significantly increased the distribution and activity of monoclonal antibodies in peripheral blood, thereby enhancing the overall antitumor response.

Oat β-glucan also exhibits direct antitumor effects. Choromanska et al. found that when low-molecular-weight oat β-glucan was added to cultured melanoma cells in vitro, it significantly reduced the viability of tumor cells while markedly increasing the expression of Caspase-12 within those cells. Caspase-12, a member of the cysteine-aspartic acid protease family, can activate Caspase-9 expression, which subsequently induces Caspase-3 activation—ultimately leading to tumor cell apoptosis. Similarly, Parzonko et al. discovered that after oat β-glucan was added to melanoma cells for 24 hours, it induced tumor cell death by regulating the cell cycle and activating Caspase-3 and Caspase-7 expression.

4. Immunoenhancing and Anti-inflammatory Effects

Oat β-glucan possesses immunoenhancing properties by stimulating the body’s immune system, thereby improving resistance against external pathogens such as parasites, bacteria, fungi, and viruses. Studies have shown that whether administered intravenously, intramuscularly, or orally, oat β-glucan can enhance the body’s bacterial clearance ability, increase antimicrobial activity, regulate the expression of immune-related genes, and elevate the number of monocytes and neutrophils—ultimately strengthening immune function. In addition, researchers have found that oat β-glucan can enhance the phagocytic ability of macrophages in mice, as well as the transcriptional activity of genes in intestinal leukocytes and epithelial cells.

Rodriguez et al. reported that intraperitoneal injection of oat β-glucan significantly improved the antibacterial capacity of tilapia, providing effective protection against infections caused by Aeromonas hydrophila. Chanput et al. added oat β-glucan to macrophage culture media and subsequently stimulated the cells with lipopolysaccharide (LPS). The results showed that macrophages treated with oat β-glucan released lower levels of inflammatory cytokines such as IL-1β, and reduced production of reactive oxygen species and nitric oxide. This indicates that oat β-glucan can effectively suppress macrophage inflammation in vitro.

The immunoenhancing and anti-inflammatory effects of oat β-glucan have also been confirmed in mice. Studies have shown that in mice infected with Streptococcus, administration of oat β-glucan significantly enhanced immune function and reduced the severity of inflammatory responses. Liu et al. reported that when mice were pretreated with oat β-glucan for one week before being induced with colitis using dextran sulfate sodium (DSS), the severity of intestinal lesions in the treated group was much lower than that of the control group. Additionally, the levels of oxidative stress markers such as malondialdehyde (MDA) and myeloperoxidase (MPO), as well as inflammatory cytokines including tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β), were significantly reduced. Yun et al. found that after administering oat β-glucan to mice infected with Trichuris muris, the number of worm eggs in feces decreased significantly, clinical symptoms improved, and immune cell counts in serum increased. The expression of inflammatory cytokines interferon-γ (IFN-γ) and interleukin-4 (IL-4) in intestinal mucosal tissues was also downregulated. These findings suggest that oat β-glucan enhances the ability of mice to resist Trichuris muris infection. Furthermore, Yun et al. also demonstrated that oat β-glucan improved the host’s resistance to Staphylococcus aureus and Streptococcus infections. Additional research showed that oat β-glucan similarly enhanced immune defense against pathogens such as Escherichia coli and Aspergillus niger.

5. Conclusion

As a health food rich in β-glucan, oats have attracted widespread attention in recent years. Studies suggest that the key reason for the lipid-lowering and hypoglycemic effects of oat β-glucan lies in its high viscosity, which inhibits the absorption of nutrients—the greater the viscosity, the stronger its ability to reduce lipids and blood glucose. Although oat β-glucan cannot be digested by human gastrointestinal enzymes, it can be metabolized and utilized by intestinal microorganisms. Moreover, oat β-glucan can bind to cell surface receptors, promote hormone secretion, and enhance satiety. In inflammation models, oat β-glucan has been shown to suppress the expression of inflammatory cytokines and prevent excessive inflammation. While many physiological functions of oat β-glucan have been identified, further research is still needed to elucidate the underlying molecular mechanisms behind these health benefits.