28 Apr 11 Glycaemic Index update
The glycaemic index (GI) measures how various carbohydrates affect blood sugar levels. It has been used to control diabetes and may hold promise as a method of controlling weight. But, recent research suggests it might be more complicated than once believed.
Glycemic index 101
GI measures a carbohydrate’s effect on blood glucose compared to a reference food of either glucose or white bread. Foods that are digested and absorbed rapidly, resulting in a sharp spike in blood glucose after consumption, such as potatoes and watermelon, are given a high GI (70 to 99). Foods that are digested slowly, resulting in a slow release of glucose into the bloodstream, such as vegetables, are given a low GI (55 or less). Foods with a moderate GI, including whole-wheat products and brown rice, fall within a GI range of 56 to 69 (American Journal of Clinical Nutrition, 1981; 34:362-366).
The GI has sometimes been confused with the glycemic load (GL). GL is defined as the GI of a food (divided by 100) multiplied by the grams of carbohydrate from one serving of the food. GI and GL do not go hand-in-hand; a food with a high GI will not necessarily have a high GL (American Journal of Clinical Nutrition, 2002; 76(1):5-56).
Many factors affect a food’s GI. Fat, fibre and protein slow the digestion of carbohydrates and release of sugar into the bloodstream. In addition, cooking and processing methods also affect GI by altering the structure of starch and subsequently speeding up the release of sugar into the bloodstream. For example, raw carrots have a GI of 16, whereas cooked carrots have a GI of 92. And finally, ripeness, storage time, cooking method and type of carbohydrate affect the GI (American Journal of Clinical Nutrition, 2002; 76(1):5-56).
GI can be incredibly confusing and misleading for consumers. First, published GI values for the same foods vary based on testing methods and the physical and chemical characteristics of the food. In addition, a GI given for a particular food, a slice of whole-wheat bread for instance, may vary from brand to brand of whole-wheat bread based on ingredients and processing. And, GI values for commercially prepared foods may change over time based on ingredient changes.
Plus, there may be differences in products, say a nutrition bar, depending on the SKU (an apple-cinnamon bar versus an almond-flavoured bar, for example). Additionally, GI may vary depending on where the food is grown. Rice, for instance, varies based on botanical differences. And finally, GI values may also differ because some labs use white bread as the reference, whereas others use glucose (dextrose).
Making matters more confusing, the GI of a food does not correlate to the GI of a meal that contains that food, and many people eat mixed meals versus single foods. So, for instance, white bread consumed as part of a sandwich loaded with fibre-rich vegetables, cheese and meat will elicit a different glycemic response than plain white bread.