Your individual glycemic response to a particular food is affected by many factors, such as its fat and protein content, the type and amount of carbohydrate it contains, the amount of fiber and the nature of any starches in it, its preparation (raw or cooked), its ripeness, and even its acidity. In addition, the exact response to a particular food can vary considerably between individuals, just as metabolism does.
While you may be able to easily understand why the higher fiber content of whole-grain bread makes it more slowly digested than a French baguette made of white flour, not all GI differences may be quite as obvious. For example, the glycemic response to diced potatoes is somewhat lower than to mashed potatoes, and thick linguine has a lower GI value than thinner spaghetti. Moreover, cooking in general–and particularly overcooking of foods–can raise their GI value, so al dente pasta is always better (although your pasta intake should always be limited due to its large amount of carbs). Values for many ripe fruits (such as bananas), though, are actually lower than their greener, unripe precursors. Highly acidic foods, such as vinegar, can lower the GI value of another food when they are eaten in combination.
Critics of the GI concept like to point out that GI rankings reflect glycemic responses only over a two-hour time span after a particular food is eaten and that individual responses to any given food may vary widely. Such debates over the GI have led some to conclude that low-GI diets are not necessarily effective for blood glucose control or diabetes prevention because the GI value of a particular food may or may not reflect its entire glycemic effect. However, what’s important to remember is that GI testing of foods is performed on people who are diabetes-free. Moreover, an excessive intake of high-GI carbohydrate foods has been shown to increase insulin resistance even in people without diabetes. Since people with diabetes generally lack the ability to immediately release enough insulin in response to food intake (their “first phase” secretion), GI tables more than likely underestimate, rather than overestimate, the glycemic spikes caused by most carbohydrate-rich foods if you have diabetes.
More on the GI next week.