Building the library of glycemic index values for foods has been a relatively slow, painstaking effort. That’s because each food must be tested on a number of volunteers, and each volunteer must be tested several times. The basic steps are the same. A healthy volunteer fasts overnight. The next morning he or she drinks a glass of water in which 50 grams of glucose have been dissolved (or, alternately, eats 50 grams of white bread). Over the next two hours, blood samples are taken at regular intervals to measure the rise and fall of glucose and insulin. On another day, the same volunteer eats enough of the test food—cooked potato, whole-grain bread, kiwi fruit, ice cream, and so on—to consume 50 grams of carbohydrates and sits through another two hours of blood sampling. The glycemic index for that food for that individual is calculated by dividing his or her blood sugar response to the test food by the response to pure glucose or white bread. The numbers in the tables, then, represent percentages. For example, black beans have a glycemic index of 30. This means that they boost blood sugar only 30 percent as much as pure glucose.
Because everyone processes food and responds to glucose a little differently, the glycemic index published in tables is usually the average of eight to ten volunteers.
One general trend you can see in the glycemic index table is that products made from reﬁned grains, things like white bread, bagels, and crackers, have a rapid and strong inﬂuence on blood sugar. Those that are less reﬁned, such as whole-grain breads and cereals, have relatively lower glycemic indices, as do beans, vegetables, and fruits.
Several things determine how rapidly the carbohydrates in a particular food are broken down and the resulting glucose absorbed into the bloodstream:
• How swollen (gelatinized) the starch grains are. Starch grains swollen to the bursting point with water or heat, such as those in a boiled or baked potato, are more easily digested than the relatively unswollen starch grains found in brown rice.
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• How much the food has been processed. Grinding wheat into superﬁne ﬂour dramatically increases the attack rate of digestive enzymes. Not only does ﬂour have greater surface area than coarsely ground wheat grains, but it has been stripped of the protective, hard-to-digest, ﬁbrous outer coat that temporarily fends oﬀ enzymes from digesting the starch inside. Regular oatmeal, which is made of smashed oat grains, has a higher glycemic index than oats that are intact or sliced, usually sold as steel-cut oats.
• How much ﬁber it contains. As indigestible ﬁber passes through the intestine, it carries along partly digested food, shielding it from immediate digestion. This spreads out the release of glucose into the blood.
• How much fat the snack or meal contains. Fats tend to increase the time it takes for food to leave the stomach and enter the intestine. So a food that contains fat may temper the rise in blood sugar.
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GLYCEMIC INDEX AND GLYCEMIC LOAD: BEYOND HEART DISEASE AND DIABETES
As people with diabetes come to know, chronically high levels of blood sugar and insulin aren’t good for the body. They contribute to many of the complications of diabetes, such as nerve damage, loss of vision, kidney disease, sexual dysfunction, and wounds that won’t heal. Recent research suggests that the excess blood sugar and insulin that come from eating a high-glycemic diet contribute to other chronic conditions besides heart disease and diabetes. These include breast cancer, colon cancer, and polycystic ovary syndrome.
For now, though, the main beneﬁts for managing your glycemic load are preventing heart disease and diabetes and controlling weight.