- The Glycemic Index (GI) is a number implemented to determine how fast a food or drink can alter the blood sugar level and cause it to rise. Protein and fat do not have much power in altering blood sugar levels, and thus the Glycemic Index looks only at carbohydrate foods, and measures them appropriately. A food with a high GI level will cause a large jump in blood sugar levels, while lower numbers will cause a smaller jump.
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There is a more specific formula as to how the Glycemic Index is measured. A glucose or white bread substance has a GI rating of 100, and these items set the benchmark for the index and by which all other foods are judged. All other foods have a GI that is relative to the pure, ultimate number represented by white bread and glucose. For example, an index of 55 or lower is considered low, while 56 to 69 is medium, and a glycemic index of at least 70 is considered high.
However, there is a more complex science underlying these measurements, as complex and simple carbohydrates have a differing effect on the numerical system and throw a bit of a wrench into the index. Complex carbohydrates, for example, are potatoes, whole grains and fruits, and simple carbohydrates include white bread and table sugar, among other things. Complex carbohydrates were once thought to have only a minimal impact on the body and on blood sugar levels due to a slower digestion cycle, but as always, science evolves, and the new emerging viewpoint is that complex carbohydrates have a more formidable role in raising blood sugar levels. Indeed, complex carbohydrates are now seen as having as large an impact as their "simple" brothers. - There are additional factors that can affect the index of a food. Ripeness, for instance, affects the Glycemic Index of a food--the more ripe, the higher the GI. Acidic foods such as lemon juice-drenched snacks and pickled items decrease the GI of a meal, because they naturally slow digestion. Salty foods make a large impact, as the rate of digestion hikes up as a result, and processed foods suffer from a dramatically higher GI thanks to their higher levels of fiber, which as a result slows digestion and creates a slower effect on blood sugar. Fiber in general, of course, slows down digestion and glucose absorption, affecting blood sugar more slowly as well. Overall, low GI foods are considered more desirable.
