The healthiness of high-fibre diets is generally recognised. Here Uura-Liina Pöyhönen reports on the control of blood glucose through oat soluble fibre beta-glucan.
Most of fibre intake consists of insoluble fibre, which shortens bowel transit time, increases faecal mass in the large intestine, and renders feces softer but it has no significant effect on viscosity in the small intestine.
In contrast, some viscous soluble fibres exert their effects mainly by increasing viscosity in the small intestine. They delay gut transit time and gastric emptying contributing to the feeling of satiety, and impede the absorption of certain nutrients such as glucose, cholesterol and fats. Another major difference between these two types of fibres is that bacteria in the colon can break down soluble fibres, whereas insoluble fibres are largely resistant to degradation (1, 2).
Approximately half of the total dietary fibre content of oats is soluble and the other half is insoluble. The main component of oat soluble fibre is called ß-glucan, which also causes the main physiological effects (3). In addition to insoluble and soluble fibre, oats have considerably higher concentration of protein than other cereals. The superior nutritional value of oat protein, compared to that of other cereals, has long been recognised (4). Of all cereals, oat groats have the highest lipid concentration. Majority of the fat, however, is nutritionally beneficial unsaturated fat (5). Oats contain also essential vitamins and minerals (6).
Glycemic control
Glycemic/blood glucose control by diet is essential for optimal management of diabetes in order to avoid some of the long-term complications of the disease (7). Recently, however, glycemic index (GI), which classifies foods according to their blood glucose raising potential, and its role in body weight control and provision of long-lasting energy have been intensively discussed in the scientific circles and the media. The reasons behind this increasing interest are mainly the increasing number of overweight and obese people worldwide and the related increase in type 2 diabetes.
Though the ideal composition of a diet necessary to maintain glucose values within normal limits at all times remains uncertain, current dietary recommendations emphasise sufficient intake of complex carbohydrate and fibre (both soluble and insoluble) and restricted consumption of animal fat (7).
Accordingly, the relation between dietary fibre and diabetes has received much attention. Particularly soluble fibre has generated considerable interest in diabetes management as it has repeatedly been shown to decrease postprandial glucose and insulin concentrations both in person with diabetes and without (8, 9).
The most common and readily available sources of soluble fibre such as fresh fruits and vegetables are low (<2percent) in soluble fibre. Oats, on the other hand, are rich in soluble fibre and available in convenient, palatable forms such as oatmeal and oat bran. The presumed active component of oat soluble fibre is a linear polysaccharide called ß-glucan that is present (dry weight basis) in normal rolled oats at approximately 4percent, in commercial oat bran at 710percent and can be as high as 19percent in specially processed bran fractions (10).
Several studies have shown that meals containing viscous soluble fibres reduce the rise in postprandial blood glucose and insulin concentrations more consistently than insoluble fibres. Pick et al. (7), for example, evaluated the long-term efficacy of incorporating oat bran concentrate bread products in the daily dietary pattern of subjects with non-insulin dependent diabetes mellitus (NIDDM). Their results show (table 1) that during the oat bran concentrate period the increase in postprandial blood glucose area under the response curve was less than in the white bread period, as was the case with the total insulin response and peak insulin values during the same period. These results are supported by Tappy et al (11), who have shown that by increasing the viscosity of the contents in gut, oat b-glucan slows down the uptake of sugar and has a dampening effect on both the after meal peak and the subsequent drop in blood glucose and insulin levels (table 2). Braaten et al, on the other hand, give evidence on the similar effects not only on persons with type 2 diabetes but on healthy humans as well (10).
The reduction of the glucose and insulin peak after consumption of soluble fibre such as oat ß-glucan works through an increase of the viscosity of the contents of the stomach and small intestine, which reduces the absorption rate of the digested nutrients from the small intestine. Glucose transport in the intestinal wall is inhibited in part by an increase in the resistance of the mucosal diffusion barrier brought about by the greater viscosity of the intestinal bolus (12). It has been experimentally verified that the higher the viscosity, the slower the absorption (13). However, the level of viscous ß-glucan in oat flakes used to make the porridge is too low (only 4percent) to produce a significant attenuation of the glycemic response in diabetic subjects and therefore more ß-glucan (~6-8g) is needed to adequately reduce the GI. This can easily be achieved for example by using oat bran rich in ß-glucan (11, 12).
Conclusions
Diabetic individuals should benefit from diets that are rich in oat soluble fibre ß-glucan. Most typically oats have been used in products such as hot and cold breakfast cereals, breads, biscuits, snack bars and pasta products but possibilities exist also in ready-to-eat meals, drinks or drink powders and dairy products. Finn Cereal Ltd develops, processes, and sells ß-glucan rich oat bran ingredients, ß-glucan content being 1018percentd.s. It is an excellent natural ingredient to number of healthy products such as the ones mentioned above.
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Uura-Liina Pöyhönen is with Finn Cereal, Vantaa, Finland. www.natureal.fi
