Nutritive Value of Oat β-glucan and the Factors Affecting Its Accumulation

Oats, being rich in protein and linoleic acid, have long attracted the attention of nutritionists and agronomists both in China and abroad. Over the past 20 years, numerous human and animal experiments conducted in the United States, the United Kingdom, Canada, Japan, and other countries have shown that oats can prevent and treat cardiovascular and cerebrovascular diseases caused by hyperlipidemia, and help control non–insulin-dependent diabetes mellitus. Studies in the early 1990s demonstrated that β-glucan in oats possesses the ability to lower blood lipids and serum cholesterol. The U.S. Food and Drug Administration (FDA, 1995, 1996) announced twice that oat-based foods containing β-glucan and low fat can reduce the risk of heart disease. This paper summarizes the physiological functions of β-glucan and the factors affecting its content in oat grains, with the aim of providing a foundation for further research and the development and utilization of oat β-glucan.

1. β-Glucan Composition and Characteristics

Glucans are a class of polysaccharides composed of glucose as the basic structural unit and can be divided into α-type and β-type. In nature, β-type glucans are predominant, while α-type glucans are mainly synthesized artificially and have not yet been found to occur naturally in living organisms. β-Glucan is the main component of the cell walls in the endosperm and aleurone layers of cereal grains, accounting for more than 85% of the cell-wall polysaccharides in oats. Oat β-glucan is a low-molecular-weight, short-chain glucan with molecular weights ranging from 5.3 × 10⁴ to 2.57 × 10⁵ Da. Its structure contains two types of glycosidic linkages: β-(1→3) and β-(1→4). The precursor for β-glucan synthesis is uridine diphosphate glucose (UDP-glucose). The enzyme responsible for catalyzing β-glucan formation is tightly bound to the cell membrane and the Golgi apparatus membrane and may possess the catalytic ability to form both β-(1→3) and β-(1→4) linkages. Even if these two linkages are formed by different enzymes, the enzymes must act in close association. The presence of β-(1→3) linkages causes irregular arrangement of cellulose molecules, resulting in differences in the physiological and chemical properties of these substances, including water solubility. β-Glucans are generally divided into two types: water-soluble and water-insoluble, and this distinction is mainly determined by the proportion of β-(1→3) linkages and the degree of polymerization. In water-soluble β-glucans, the ratio of β-(1→3) to β-(1→4) linkages is approximately 1 : 2.5–2.6, while in water-insoluble β-glucans, the corresponding ratio is about 1 : 4.2.

2. Health Functions and Mechanisms of Oat β-Glucan

Regarding the nutritional and health-promoting functions of oats, researchers such as Ma Dequan and Huang Xiangguo have generally recognized that oat-based foods possess the ability to lower blood lipids and serum cholesterol, playing an important role in the prevention and treatment of cardiovascular and cerebrovascular diseases as well as diabetes. Among these, the active component responsible for the health-promoting effects of oats is β-glucan.

Regarding the mechanisms by which oat β-glucan lowers blood lipids, several hypotheses have been proposed. These include:

① β-glucan can reduce the intestinal absorption rate of lipids and cholesterol, thereby lowering serum cholesterol levels. In addition, β-glucan can also decrease the absorption of carbohydrates, which reduces plasma insulin concentration and weakens the stimulation of cholesterol and lipoprotein synthesis.

② β-glucan binds with bile acids in the small intestine, increases the excretion of bile acids, and accelerates the conversion of cholesterol into bile acids.

③ β-glucan is fermented by intestinal microorganisms in the colon to produce short-chain fatty acids, which inhibit cholesterol synthesis.

Although all these hypotheses have some experimental evidence, none of them can fully explain the complex mechanism of β-glucan in lowering blood lipids and serum cholesterol. Some studies have also shown that β-glucan can regulate blood glucose, hormonal responses, and the bioavailability of vitamins and minerals, and may play a positive role in the prevention of colon cancer. β-glucan is a low-calorie food ingredient that is not easily digested or absorbed by the human body. After consumption, it can slow down the increase of glucose concentration in blood plasma, helping to prevent and control obesity, diabetes, and cardiovascular diseases. Due to its strong water-holding and swelling properties, β-glucan can absorb water and expand in the stomach, producing a sense of fullness and preventing overeating. In the intestines, it can promote peristalsis, exert osmotic effects in the colon, and shorten the time of waste transit through the intestinal tract, thereby reducing the contact between carcinogenic substances and the intestinal wall and achieving anti-cancer effects. Since β-glucan cannot be decomposed by cariogenic bacteria in the mouth, it does not produce acid and thus helps prevent dental caries. Studies on some aquatic animals have shown that β-glucan acts as an immunostimulant. Feeds containing β-(1→3) glucan can significantly improve shrimp growth, increase survival rate, and enhance resistance to white spot virus infection. Long-chain β-glucans, represented by mushroom and yeast β-glucans, have already been widely applied in antitumor and antiviral research. However, whether oat β-glucan possesses similar physiological activity remains to be further studied.

3. Differences in Oat β-Glucan Content and Its Influencing Factors

3.1 Effects of Genotypic Differences on β-Glucan Content

Although β-glucan is generally present in the grains of cereal crops, numerous studies have shown that the β-glucan content varies significantly among different species and genera. Guo Xiaoqiu and others measured the β-glucan content in several cereal grains and found that oats and barley have relatively high β-glucan levels, while wheat and corn contain much lower amounts. Among them, the β-glucan content in oat grains is 37 times higher than that of corn. Longland et al. compared the β-glucan content of seven oat varieties and found that naked oats generally contained more β-glucan than hulled oats. Conciator and colleagues examined 19 naked oat cultivars and 3 self-crossing lines and also found significant genotypic differences. Among these, cultivars A89106 and HJAN72095N showed a high potential for β-glucan accumulation, with β-glucan contents reaching 5.27%–5.57%, while one genotype with a relatively low level contained only 2.52%. Other studies have also confirmed that there is extensive genetic variation in oat β-glucan content. China, as the center of origin for oats, possesses an extremely rich collection of oat germplasm resources. Nearly 3,000 hulled and naked oat accessions have been collected, and while their protein, fat, and linoleic acid contents have been measured, a comprehensive analysis of β-glucan content has not yet been conducted.

3.2 Effects of Environmental Factors on β-Glucan Content

The β-glucan content in oat grains is influenced not only by varietal genetic factors but also significantly by environmental conditions. Studies by **Brunner** and others have shown that the β-glucan content of oats grown in different years and regions varies considerably, indicating that rainfall, temperature, soil type, and other environmental factors have an important impact on the formation and accumulation of β-glucan in oats. In addition to ecological conditions, cultivation practices also have a noticeable effect on β-glucan content. Research by Bannr and Brunner demonstrated that nitrogen supply is closely related to β-glucan content in oat grains. With increasing nitrogen supply, the β-glucan content tends to rise. However, the effects of other mineral nutrients—such as phosphorus and potassium—on β-glucan synthesis and accumulation have not yet been reported.

Although it is now clear that environmental factors play an important role in determining the β-glucan content of oat grains, the specific patterns by which various environmental factors—such as light, temperature, and water supply—affect β-glucan synthesis and accumulation remain unclear.

4. Conclusion

Both domestic and international research has made valuable progress in understanding the physicochemical properties of β-glucan, the health benefits and mechanisms of action of oat β-glucan, and the factors influencing its accumulation. However, many issues—such as the physiological and biochemical mechanisms underlying the synthesis and accumulation of β-glucan in oats—remain unclear. Therefore, continued in-depth research on the physiological mechanisms, genetic patterns, and environmental factors affecting β-glucan synthesis and accumulation in oat grains is essential. Such work will provide a solid foundation for the development and effective utilization of oats as a valuable agricultural resource.