DIETARY FIBER IN THE TREATMENT OF HYPERGLYCEMIA IN TYPE 2 DIABETES

11.7 DIETARY FIBER IN THE TREATMENT OF HYPERGLYCEMIA IN TYPE 2 DIABETES

Soluble dietary fiber, such as oat β-glucan, psyllium, and guar gum, has been recom- mended in type 2 diabetic patients for improvement of especially postprandial insulin

and glucose metabolism, in addition to antihyperlipidemic effects. It is well documented that viscous polysaccharides in the diet slow down the rate of carbohydrate digestion and absorption. A main reason for the slower responses is the hindered mixing of luminal contents, causing retarded diffusion and contact between gastrointestinal enzymes and

Functional Food Carbohydrates

their substrates and retarded transport through the gut. 74,75 An inverse highly significant linear relationship has been shown between the postprandial blood glucose and insulin responses and the viscosity of the liquid mixtures consumed. 76,77 The importance of

viscosity for glycemic response was already emphasized by Jenkins et al. 78 Both soluble 79 and insoluble 80 dietary fibers have also been reported to increase mucin production, which increases viscosity beyond fiber alone. Insoluble dietary fiber does not have effects on glucose and insulin metabolism, at least in the short term, 81 despite epidemiological evidence that insoluble fiber may prevent diabetes independently of glycemic load or body mass index. We will not focus on the effects of dietary fiber on weight loss and weight control in type 2 diabetes, but the results have been conflicting. 82–84

11.7.1 P OSTPRANDIAL S TUDIES

Oat β-glucan is well known for its ability to reduce postprandial glucose and insulin responses after an oral glucose load in diabetic patients. 85–87 Both isolated oat gum 88 and an oat bran containing β-glucan 85,87 have been shown to be effective. In a study

by Tappy et al., 87 progressive decreases in blood glucose of 33 to 63% and in insulin of 33 to 41% relative to the control breakfast were achieved with the addition of 4 to 8.4 g of β-glucan to the oat bran breakfast. The efficacy is dependent on viscosity, however, and may be lost if viscosity is reduced in food processing due to, e.g., enzymatic breakdown. The viscosity of β-glucan, as measured in an assay simulating the physiological conditions, has also been emphasized in the case of oat bran-

containing breakfast cereals. 87 It has also been pointed out that predicting the action of a polysaccharide on the basis of preingestion viscosity can be misleading, and that viscosity should be high at the site of absorption in the gut. 89

The viscous polysaccharide psyllium 30,90,91 and guar gum 92–95 decrease postpran- dial glucose and insulin levels in patients with type 2 diabetes. In some studies, however, no effect of guar gum administration on postprandial glucose levels has been found, even at a dose of 5 g with meals. 96

11.7.2 M EDIUM - TO L ONG -T ERM S TUDIES

Medium- to long-term (i.e., lasting from weeks to months) studies of dietary soluble fiber other than guar gum on improving glycemic control in diabetes are not very

common. A study comparing the effects over 6 months of barley bread, high in β- glucan, to white bread found that barley bread improved glycemic control compared with white wheat bread in 11 men with type 2 diabetes. 97 Insulin responses were increased, which hypothetically could reflect recovered β-cell function. In men with diabetes and hypercholesterolemia participating in a crossover trial,

8 weeks of psyllium (15 g/day) decreased hemoglobin A 1c 6.1% (absolute change, 0.8%), with similar 6% decreases in fasting postprandial glucose. 98 A second-meal effect was noted, with the most marked decreases in postprandial glucose after lunch, 31% relative to the control. No changes in insulin sensitivity measured during a euglycemic hyperinsulinemic clamp were noted, however.

Improved fasting and postprandial glycemic control was found in 11 type

2 diabetic patients taking 21 g/day of guar gum or placebo in a randomized

The Role of Carbohydrates

double-blinded crossover trial. 82 Small improvements in overall glycemic con- trol and sizable improvements in postprandial glycemia after 4 weeks of treat- ment in a randomized controlled crossover trial were reported by Fuessl et al. 94 Guar gum decreased fasting blood glucose from 11.4 to 9.5 mmol/l in 19 obese patients with type 2 diabetes who were enrolled in a randomized double-blind

crossover trial. 99 Guar gum (15 g/day) has also improved long-term glycemic control and postprandial glucose tolerance in 15 type 2 diabetic patients treated with guar gum over an 8-week period. 84 On the other hand, guar gum had no effect on fasting blood glucose or hemoglobin A 1c levels in a 3-month trial in type 2 diabetic patients on oral hypoglycemic medication, 100 or in an 8-week randomized crossover trial in type 2 diabetic patients treated by diet, oral agents,

or lente insulin. 96 Guar gum has also decreased fasting glucose levels and hemoglobin A 1c in patients with type 1 diabetes participating in a randomized, double-blinded, placebo-controlled trial. In contrast, Ebeling et al. 101 found only improvements in postprandial glucose responses and a decrease in insulin doses without an improvement in overall glycemic control or insulin sensitivity. Bruttomesso et al. 102 found no benefits of adding 15 g/day of guar gum to meals.

In summary, dietary soluble fiber such as β-glucan, psyllium, and guar gum decreases glucose and insulin responses to carbohydrates if taken in sufficient amounts. A second-meal effect has also been described. Improved glycemic control with long-term use, over periods of weeks to months, has been suggested in isolated studies for β-glucan and psyllium, but more studies are needed. For the best studied dietary soluble fiber, guar gum, findings are conflicting.