Low glycaemic index (GI) diets may confer beneficial fat loss effects. This relates to their ability to allow slower digestion of starch and slower absorption of glucose which has blood glucose and insulin lowering effects postprandially. The slower rise in blood sugar decreases nutrient overload on the cells of the peripheral and hepatic tissue and these effects in combination may improve insulin sensitivity, or at least not increase insulin resistance. Low carbohydrate diets are by their nature low glycaemic index diets because lowering the carbohydrate content of the diet in exchange for protein and fat decreases the glycaemic index of the meals as a whole due to the modifying effects of protein and fatty acids in the gut alongside starch. As well as weight loss, low glycaemic index diets may confer other health benefits which include changes to blood lipid levels, reductions in inflammation and decreases in oxidative stress. These changes suggest that low glycaemic index diets are beneficial at preventing the metabolic syndrome.
Researchers have assessed the efficacy of low glycaemic index diets on weight loss in overweight individuals. For example, in one study1 researchers randomly assigned obese individuals to receive a moderate carbohydrate (42 %) diet with a high glycaemic index (GI: 64), a moderate carbohydrate (42 %) diet with a low glycaemic index (GI: 34) or a low fat high carbohydrate (65 %) high glycaemic index (GI: 65) diet for 6 months. All diets were 500 kcals lower than the estimated maintenance energy required for the individuals. By the end of the study the low glycaemic index diet group had significantly greater reductions in body mass index compared to the low fat group. In contrast, the high glycaemic index diet group had reductions in body mass index that were intermediate between the other diet groups but did not differ significantly from either group. Therefore low glycaemic index diets may produce better weight loss effects when compared to more traditional low fat high carbohydrate diets.
In addition, the low glycaemic index group also experiences reductions in fasting insulin levels, measures of insulin resistance, as well as greater improvements in β-cell function, when compared to the low fat group. These changes suggest that the weight loss effects seen in the subjects consuming the low glycaemic index diet may have resulted from improvements to the insulin system. One likely scenario is that consumption of a typical Western diet had resulted in the subjects becoming insulin resistant and this had caused their weight gain. Consuming a moderate carbohydrate low glycaemic index diet in the study reversed this insulin resistance and improved the function of the insulin system. These results support many previous studies that show superior fat loss effects from lower carbohydrate and lower glycaemic index diets. This would tend to suggest that Western high carbohydrate high glycaemic index diets are the drivers of weight gain and obesity, characterised as they are by refined starch and sugar.
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