Your body runs on glucose, the molecules your cells burn for energy. (For more information on how you get energy from food, check out Chapter 3.)
Proteins, fats, and alcohol (as in beer, wine, and spirits) also provide energy in the form of calories. And protein does give you glucose, but it takes a long time, relatively speaking, for your body to get it.
When you eat carbohydrates, your pancreas secretes insulin, the hormone that enables you to digest starches and sugars. This release of insulin is sometimes called an insulin spike, which means the same thing as "insulin secretion" but sounds a whole lot more sinister.
Eating simple carbohydrates such as sucrose (table sugar) provokes higher insulin secretion than eating complex carbohydrates such as starch. If you have a metabolic disorder such as diabetes that keeps you from producing enough insulin, you must be careful not to take in more carbs than you can digest. Unmetabolized sugars circulating through your blood can make you dizzy and maybe even trip you into a diabetic coma.
What makes this interesting is that some perfectly healthful foods, such as carrots, potatoes, and white bread, have more simple carbs than others, such as apples, lentils, peanuts, and whole wheat bread. The Glycemic Index, developed at the University of Toronto in 1981, gives you a handle on this by ranking foods according to how quickly they affect blood sugar levels when compared to glucose (the form of sugar your body uses as energy), the glycemic indicator par excellence.
Most people who don't have a metabolic disorder (such as diabetes) that interferes with the ability to digest carbs can metabolize even very large amounts of carbohydrate foods easily. Their insulin secretion rises to meet the demand and then quickly settles back to normal. In other words, although some popular weight loss programs, such as the South Beach Diet, rely on the Glycemic Index as a weight loss tool, the fact remains that for most people, a carb is a carb is a carb, regardless of how quickly the sugar enters the bloodstream. Check it out in Diabetes For Dummies.
For info on why the difference between simple and complex carbs can matter for athletes, check out the section called "Who needs extra carbohydrates?"
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