P-Glucan has been suspected of influencing glucose metabolism by modulation of glycaemia or insulin response to a meal. It works by slowing the rate of nutrient absorption leading to a smaller increase in glycaemia and insu-linaemia whenever nutrients are ingested concomitantly with P-glucans as opposed to without P-glucans.61 A study by Battilana et al.62 performed in healthy subjects was designed to observe the action of P-glucans on postprandial metabolism independently of a delayed absorption effect. The absence of any relevant effect in this case led the authors to conclude that the action of P-glucans after a single meal was mainly due to their delaying the rate of carbohydrate absorption. In addition, it is known that P-glucans taken in a meal increase the viscosity of the meal bolus, thereby reducing the rate of absorption and flattening post-prandial glycaemia. In fact, the viscosity of fibres relates positively with the degree of flattening of post-prandial glycaemia.63-66 In epidemiological studies, the precise type of fibre ingested is seldom reported but could be important to record. Few studies have been published comparing the differential effects of fermentable and non-fermentable fibres on energy intake and metabolism. Howarth et al.67 did not find any differential effects of fermentable fibres versus non-fermentable fibres (taken as a supplement in flavoured water before the meal) on energy intake or changes in body fatness in a population whose body mass index (BMI) ranged from 20 to 34 kg/m2. However, in animal models, fermentable fibres enhanced satiety to a greater extent. Therefore, a greater reduction in energy intake and body fatness over time was expected. A study in which 10 healthy men ingested either a diet with P-glucans or a diet with cellulose failed to produce significantly different effects on plasma glucose and insulin concentrations.62 In this case, the effects of the specific action of the P-glucans (colic fermentation, production of short-chain fatty acids) did not seem to interfere with carbohydrate metabolism.
The glycaemic index (GI) of the carbohydrate and fibre content can be related since viscous fibres and foods with intact natural cell walls generally have a lower GI.68 Dietary fibre was shown to account for about 40% of the variance in GI among 18 starchy foods.69 In fact, epidemiological and intervention studies showed that body weight is positively correlated with GI. Ma et al.68 carried out a study of 572 healthy men, in which BMI was positively associated with GI: a 5-unit increase in GI was significantly associated with a 0.75-unit increase in BMI (p = 0.01). Therefore, dietary fibres like P-glucans, which affect food digestion and absorption rate, also affect GI values as a consequence. Jenkins et al.66 found that addition of P-glucans reduces GI (4 units/g) while maintaining palatability.
Some intervention studies have been carried out with specific test products with high P-glucan content to evaluate the glycaemic and insulin responses in healthy and diabetic subjects. It has been shown that 8-10% of P-glucans in cereals can decrease the glycaemia peak by 50% .70 Juntunen et al.71 showed that the lowered insulin response observed after consumption of different grain products in healthy men was not dependent on the type of cereals. In another study, P-glucan-enriched products consisted of barley-enriched pasta. Healthy men fed barley pasta enriched with P-glucans did not show any difference in glycaemia response but did show a greater insulin response compared with men fed low-fibre-content pasta. This discrepancy between insulin and glycaemia schemes, also found in other studies, indicates a complicated pattern for the effect of fibres on glucose metabolism, which could be modulated by the action of hormones such as cholecystokinin65 and glucagon-like peptide 1.71 These intervention studies showed that a high P-glucan content is required to obtain a significant effect on post-prandial glycaemia and insulinaemia in healthy subjects.
The antidiabetic effects of P-glucans have been suspected in response to their actions on energy and glucose metabolism. Longer studies have been designed to observe the beneficial metabolic effects of P-glucans on type 2 diabetes. The risk of developing type 2 diabetes has been shown to be more than twice as high when consuming a diet high in glycaemic load in combination with a low fibre intake.70'72 Another study has shown that the addition of P-glucan greatly reduced hyperglycaemia but that there was a threshold level for the amount of fibre.64 In diabetics, P-glucans-enriched diets have also been shown to improve metabolic control with a decrease in haemoglobin A1c (HbA1c).7374 However, only a few studies have focused on the specific effects of P-glucans on HbA1c, and no clear and direct correlation has been found to date.61 Nevertheless, the improved glycaemic control and the reduction of cardiovascular risks (due to lowered blood lipid levels) associated with fibre intake indicate that it could be a recommended part of the diet for subjects with type 2 diabetes.
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