Obesity is a complex multifactorial disorder, most probably caused by a serious metabolic dysfunction resulting from major diet induced hormonal and physiological changes. In spite of the mainstream belief that exercise causes long term weight loss, evidence for the successes of exercise programmes in weight control is conspicuous by its absence from the literature. Research suggests that the metabolic dysfunction associated with obesity and long-term weight gain results from macro- and micronutrient imbalances and that correction of these imbalances is pivotal in any long-term strategy to re-establish proper control over weight. One of the macronutrient imbalances that may lead to obesity is an over abundance of carbohydrate, particularly high glycaemic or high insulin index carbohydrate. Protein and fibre can both lower glycaemic response to foods and both have shown promise in weight control.
Research1 has investigated the effects of glycaemic index and exercise on insulin and glucose secretion in prediabetic obese individuals with a mean body mass index of 34.4 kg/m2. Twenty-two individuals were randomly assigned to either a low or high glycaemic diet while participating in a 12 week exercise programme (60 min of aerobic exercise for 5 d/w). The diets were matched for macronutrient content including fibre, but the high glycaemic diet contained 80 arbitrary units (au) on the glycaemic index, whereas the low glycaemic diet contained 40 au. After 12 weeks both groups had lost the same amount of body weight (8.8%), showed the same improvement in insulin sensitivity (27.1%) and lost the same amount of fat free mass (1.4%) However, when tested for oral-glucose induced insulin secretion, only the low glycaemic index group had a reduced secretion, which was related to suppression of glucose-dependent insulinotropic polypeptide.
These results are interesting because they suggest that reductions in metabolic abnormalities involving insulin homeostasis are only improved when low glycaemic index foods are consumed. The fact that the high glycaemic index subjects lost weight, but did not show improvements in insulin secretion suggests that the underlying insulin resistance that may have caused their weight gain, remained despite a 12 week aerobic exercise programme. These observations suggests that when high glycaemic index diets are combined with exercise, the compensatory hyperinulinaemia associated with insulin resistance is not corrected. Therefore although exercise does have an effect on body composition and body weight, it appears that diet is the defining variable in improvements in insulin secretion. Further, this improvement from low glycaemic index foods appears to be related to reductions in incretin (glucose-dependent insulinotropic polypeptide) secretion and their subsequent effect on the β-cell function. .
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