The oat grain and its components.
(1-3),(1-4)-beta-d-glucan is a water-soluble, plant-based dietary fibre which is also known as beta-glucan (ß-glucan). It is found in the cell walls of the grains of sweet grasses (Gramineae), particularly in oats. Oat beta-glucan is a linear, unbranched polysaccharide chain made up of ß-glucopyranosyl units which are either 1.3 (30%) or 1.4 (70%) glycosidically linked (diagram 1). Beta-glucans belong to the group of the indigestible carbohydrates which include non-starch polysaccharides, resistant starch and oligosaccharides. These indigestible carbohydrates are not absorbed in the small intestines, but are fermented in the large intestines and fully or partly changed into short-chain fatty acids.
Diagram 1Beta-glucan displays a high viscosity, even at concentrations of only 1%, which remains stable at a wide pH range. Molecular mass and concentration have a large influence on the rheological behaviour of beta-glucan in solutions and in the intestines. Beta-glucans from oat bran have a higher viscosity than those from oat sprouts (Wikstrom et al. 1994). Beta-glucan must be soluble and have a molecular mass between 1000-3000 kDa in order to be physiologically active and be able to build viscous solutions (Doublier & Wood 1995; Aman et al. 2004). The quantity, solubility, molecular mass and structure of beta-glucan in foodstuffs can be affected by the raw materials used, endogenous beta-glucanase activity, and the preparation and storage conditions (Beer et al. 1997; Wood 2007). These factors all contribute to the cholesterol-lowering capacity, this capacity is also decreased when oats are treated with enzymes that destroy the beta-glucan (Shinnick & Marlett 1993).
Two recent studies have proven the direct relationship between the molecular mass and the viscosity of beta-glucan and the influence on its cholesterol-lowering characteristics. In the first study, several oat brancereal-products, which all contained around 3-4g of beta-glucan per portion, were produced at different temperatures and with varying concentrations of oat beta-glucan. The molecular masses thus ranged from 251 to 1930 kDa. The viscosity was measured in a test tube that simulated digestion. The viscosity increased exponentially with higher molecular masses. There was a 20-fold difference in viscosity between the products with the lowest and the highest molecular masses (Tosh et al. 2010).
Subsequently, the same products were tested in a large clinical study (Multicenter: Europe/North America/Australia). 345 participants with high cholesterol levels each received two portions of ready-to-eat breakfast flakes for daily consumption over four weeks. Participants were divided into 4 groups, each group consumed 3g of oat beta-glucan per day with differing molecular masses. The cereal flakes with the highest (2210 kDa) and medium (530 kDa) molecular masses achieved similar results in the reduction of the bad LDL cholesterol. The effectiveness in reducing LDL cholesterol, however, reduced by 50% when the molecular mass was only 210 kDa (Wolever et al. 2010).
In conclusion, these studies showed that oat products in which the molecular mass of the beta-glucan is approximately 2000 kDa have the greatest cholesterol-lowering effect. These results probably explain some of the differences which were determined in the comprehensive analysis of several cholesterol-reducing effects of oat products (Ripsin et al. 1992; Brown et al. 1999; EFSA Journal 2010; 8(12):1885). The molecular mass of several commercially available beta-glucan preparations, which have been tested for their effectiveness in the reduction of bad cholesterol, lay only between 40-80 kDa.
Cholesterol and oat beta-glucan viscosity
“The proof presented shows that the cholesterol-reducing effects of oat beta-glucan may be dependent on the increased viscosity in the small intestine. Through this, the re-absorption of bile acids is reduced, the production of bile acids from cholesterol is promoted and the concentration of circulating bad cholesterol (LDL) reduced”. EFSA Journal 2010; 8(12):1885 EFSA Journal 2010; 8(12):1885
Blood sugar and oat beta-glucan viscosity
Oat beta-glucan slows down the rise in the blood sugar levels after a meal and delays the lowering of the blood sugar level to the pre-meal level. The method of function can be described as follows: Beta-glucan in the soluble dietary fibres of the oats is digested, a gel forms which makes the contents of the stomach and the small intestine more viscous. Digestion is thus slowed, the uptake of carbohydrates into the blood stream takes longer, and sudden fluctuations in the blood sugar levels are prevented.
Oat beta-glucan slows down the rise in the blood sugar levels after a meal and delays the lowering of the blood sugar level to the pre-meal level: 1 without oat beta-glucan | 2 with oat beta-glucan
The blood sugar level is defined as the amount of glucose dissolved in the blood. This form of sugar is the energy supply for the cells. Glucose is transported from the blood stream into the cells by the hormone insulin. The cells use the glucose to create valuable energy through a chain of chemical reactions. Without the hormone insulin, the glucose remains in the blood and does not enter the cells. The result is a high blood sugar level whilst the cells still urgently require glucose. An increased glucose concentration in the blood is known as a “sugar high” or hyperglycaemia.
Proven reduction of the glycemic response after eating – reduction of the sugar level.
In order to benefit from the blood sugar level lowering property of beta-glucan, a product must contain at least 4g of bioactive oat beta-glucan per 30g of carbohydrate consumed in the meal.
“The working mechanism behind the claimed effects of oat beta-glucan has been conclusively proven and depends on the increased viscosity of the consumed food and the beta-glucan contained therein. When the consumed food reaches the small intestine, a high viscosity delays the uptake of nutrients, including glucose.” (EFSA Journal 2011;9(6):2207)
Digestion and oat beta-glucan
Oat products contain high quantities of soluble and insoluble dietary fibres which are important for balanced bowel function.
Improvement of the bowel function to promote a healthy digestion.
Oats contains a unique combination of soluble and insoluble dietary fibres. Soluble fibres are responsible for increasing the viscosity of the gut and increasing the transit time of stool through the bowel while insoluble dietary fibres add bulk to the stool.. In order to achieve these advantages, a product must contain at least 6g of oat fibres per 100g, or respectively 3g per 100 kcal.
- Beer, M, Wood, P, et al. (1997). Effect of cooking and storage on the amount and molecular weight of (1-3)(1-4)-beta-D-glucan extracted from oat products by an in vitro digestion system. Cereal Chem 74: 705-709.
- Beer, M, Wood, PJ, et al. (1997). Effects of cooking and storage on the amount and molecular weight of (1-3)(1-4)-beta-D-glucan extracted from oat products by an in vitro digestion system. Cereal Chem 74: 705-709.
- Davidson, A (1999). The Oxford Companion to Food. Oxford, Oxford University Press.
- Doublier, J and Wood, P (1995). Rheological properties of aqueous solutions of (1-3)(1-4)-beta-D-glucan from oat (Avena sativa L.). Cereal Chem 72: 335-340.
- EFSA Journal 2010; 8(12):1885
- Wikstrom, K, Lihdahl, L, et al. (1994). Rheological studies of water soluble (1-3)(1-4)-beta-D-glucans from milling fractions of oat. J Food Sci 59: 1077-1080.
- Wolever, T, SM, T, et al. (2010). Physicochemical properties of oat beta-glucan influence its ability to reduce serum LDL cholesterol in humans: a randomized clinical trial. Am J Clin Nutr 92: 723-732.
- Wood, P (2007). Cereal beta-glucans in diet and health. J Cereal Sci 46: 230-238.
Document of the EFSA references: Art14 Opinion oat beta glucan (700kB)