The glycaemic index is a measure of the blood sugar rise associated with a particular food following ingestion, compared to a standard food which is usually glucose. The glycaemic index is a useful maker of postprandial blood glucose rises, but the difficulty in associating the glycaemic index with blood sugar diseases is that low glycaemic index foods also contain different micronutrient and fibre levels, which confounds the results. If the energy density of the food is considered, the glycaemic load can be calculated, and this is considered a more reliable indicator of glycaemic response. It is therefore difficult to assess the impact of low glycaemic diets on diabetes, metabolic disorder and hyperglycaemia because other factors need to be considered. In this respect, comparisons should really occur using diets that have fixed energy, fibre and micronutrient contents, while differing only in their glycaemic index.
Researchers1 have assessed the effects of glycaemic index on 29 healthy overweight women (age 35.5 years, body mass index 27.6kg/m2). Subjects consumed a higher or lower glycaemic index diet for 10 weeks that was composed of meal with glycaemic indexes of either 79 or 103. About 75% of the carbohydrates came from a pre-prepared meal while the remaining 25% of carbohydrate was consumed from a list of low or high glycaemic index foods (eaten ad libitum), depending on the group selection. Following this protocol, the a sub-group of subjects were randomly selected to receive a test meal, which consisted of consuming either a high or low glycaemic index breakfast following an overnight fast. The high and low glycaemic index rye bread test meals had been made to contain similar amounts of micronutrients, fibre and energy, and differed only in glycaemic index.
Subjects from the lower glycaemic index group experienced a lower rise in plasma glucose, plasma glucagon-like peptide and serum insulin concentrations, but higher plasma glucose-dependent insulinotropic peptide concentrations, in response to the breakfast test meal when compared to the subjects from the higher glycaemic index group. The lower glycaemic index group also experienced higher ratings of fullness and lower desires to eat something fatty following the breakfast test meal. However, there was no significant difference in plasma ghrelin, plasma leptin, plasma glucagon-like peptide-2, plasma glucagon, plasma insulin, energy expenditure or substrate utilisation between groups following a subsequent lunch test meal later in the day. This would suggest that postprandial glycaemia, insulinaemia and subjective appetite ratings are improved following a 10 week lower glycaemic index diet, when compared to a higher glycaemic index diet.
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