As mentioned, the GI has much to do with carbohydrates. In the past, carbs were either simple (fructose, sucrose, table sugar) or complex (starch). This was based on the number of sugars in the chemical structure.
Now, to measure the GI of a food, people are given a test food with 50 grams of carbs and a control food (like white bread or pure sugar) that gives a similar amount of carbs. Blood samples are taken before people eat the foods, and then after eating them. Doctors look at how the blood sugar levels change.
How they change is the “glucose curve.” A food’s GI is calculated using this curve — how a food impacts your blood sugars compared to sugar itself! For instance, corn flake cereal has a GI of 83 relative to sugar itself, but whole-grain pumpernickel bread is only 46%. What this means is that the blood sugar response to carbs in the corn flake cereal is 83% of what the response is to pure sugar. The pumpernickel bread falls in at 46% of what the body’s response is to the same amount of carbs in pure sugar. Still with me?
Overall, we can use the GI to this effect: if you eat high-GI foods, you’ll have a higher, quicker rise in blood sugar and insulin. Over time, this puts you at greater risk for type 2 diabetes.
In part two of this series, I’ll assess the GI values of a variety of foods, so you can see which tend to impact blood sugar levels more severely than others.