New food composition measure far more meaningful

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Dr John Monro Plant & Food Research

Dr John Monro, a Principal Investigator at the Riddet Institute and a scientist at Plant & Food Research, has devised a more effective way of measuring the glycaemic potency of foods. His measure, glycaemic glucose equivalents (GGE), represents the glycaemic potency of whole foods as consumed in relevant quantities, and he says this is more useful for consumers, nutritionists and food producers than measures based on food carbohydrate composition alone.  Basing glycaemic impact values on foods rather than on carbohydrates would allow them to be expressed per serving and per 100g of food.

Consumers have access to a food's nutritional information, such as nutrient composition and energy value, e.g. grams of ingredients per serving. But Dr Monro says there is no reliable measure people can use to choose foods that allow them to control glycaemic responses. "People still do not have access to simple data that accurately represent the glycaemic potency of foods by serving or weight," he said.

The glycaemic potency of foods to date has been expressed in several ways. It has been quantified as the amount of available carbohydrate consumed, the relative glycaemic potency of the carbohydrate constituents in a given food, and the product of the glycaemic potency of the food itself, times the quantity consumed. But carbohydrate alone is an inadequate indicator of the glycaemic properties of foods because it does not account for differences in the glycaemic potency of the component carbohydrates in the food.  For instance, the blood glucose response to fructose is about one-fifth that of an equal weight of glucose.  Carbohydrate analysis does not take account of differences in rates and availability during digestion due to food structure, or interaction of carbohydrates with other food constituents.

Dr Monro is suggesting a food-based, intake-dependent measure intended for use as a nutrient-like food value.  Basing food selection on relative glycaemic effects of servings or amounts of food customarily consumed per eating occasion of entire foods expressed as glycaemic glucose equivalents (GGE) has a number of practical advantages over data such as the glycaemic index (GI). GI is determined from the responses to a food, but is calculated and expressed as if available carbohydrate were responsible for the food effect, and the drawback is that it does not change according to actual food intake.

Glycaemic responses to foods reflect the balance between glucose loading into, and its clearance from, the blood. Current in vitro methods for glycaemic analysis do not take into account the key role of glucose disposal. Dr Monro's study aimed to develop a food intake-sensitive method for measuring the glycaemic impact of food quantities usually consumed, as the difference between release of glucose equivalents (GGE) from food during in vitro digestion and a corresponding estimate of clearance of them from the blood. Five foods: white bread, fruit bread, muesli bar, mashed potato and chickpeas were consumed on three occasions by 20 volunteers to provide blood glucose response (BGR) curves. GGE release during in vitro digestion of the foods was also plotted. Glucose disposal rates estimated from downward slopes of the BGR curves allowed GGE dose-dependent cumulative glucose disposal to be calculated. By subtracting cumulative glucose disposal from cumulative in vitro GGE release, accuracy in predicting the in vivo glycaemic effect from in vitro GGE values was greatly improved. Furthermore, the difference between the curves of cumulative GGE release and disposal closely mimicked in vivo incremental BGR curves. The study concluded that valid measurement of the glycaemic impact of foods may be obtained in vitro, and expressed as grams of glucose equivalents per food quantity, by taking account not only of GGE release from food during in vitro digestion, but also of blood glucose clearance in response to the food quantity.