The glycaemic index (GI) is a measure of the blood glucose response to carbohydrate food. High GI foods are associated with a rise in blood glucose and this is usually accompanied by a rise in plasma insulin. Continual consumption of higher GI foods may be associated with type 2 diabetes, obesity, cardiovascular disease and metabolic syndrome, because this type of food can contribute to the development of insulin resistance. Soluble fibre is beneficial in the treatment of these diseases because it may reduce the GI of foods by increasing the viscosity of the chyme as it enters the small intestine. A number of cross-sectional and prospective studies have shown an association between the GI of foods and the risk of diabetes, and clinical trials have shown a reduction in glycated haemoglobin with a low GI diet in diabetic subjects.
Research1 published in the Journal of Nutrition in 2011 investigated the effects of a low GI diet on the weight loss and level of glycated haemoglobin (HbA1c) in individuals with type 2 diabetes. The diets chosen were a vegan diet and a diet that followed the guidelines of the American Diabetes Association (ADA). The vegan diet contained 75% carbohydrate (from vegetables, fruits, grains and legumes), 10% fat, 15% protein, and did not have any energy restriction. The ADA diet consisted of 60-70% carbohydrate, 15-20% protein, ≤200mg/d cholesterol, <7% saturated fats, and was energy restricted based on body weight. The ADA participants with a BMI >25kg/m2 were given a 500-1000kcal energy deficit. Both groups were instructed to limit alcohol consumption to 1 drink/d. The subjects also recorded 3-day dietary records that were kept over the 22 weeks of the study.
The results showed that the vegan group reduced their GI further than the ADA group and that the GI of the subjects were predictive of changes in weight. The authors calculated that for every point decrease in GI, the subjects lost ≈0.2kg. Those subjects that lost the most weight also had the best improvements in HbA1c levels. Some other aspects of the vegan diet may also have lead to improvements in HBA1c, such as increases in plant polyphenols. Interestingly the vegan group had increases in both soluble and insoluble fibre above that experienced by the ADA group. The total intake of fibre was 16g/d greater in the vegan group, which may explain the greater reduction in GI and the increased weight loss seen. The fact that the vegan group lost more weight, yet were not restricted on calories, provides more evidence that calorie restrictive diets are not beneficial for weight loss.
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