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Several factors affect the glycemic index of a food: (1,3,5)
• Cooking method. Heat, amount of water and cooking time affect a food’s GI. During cooking, water and heat expand the starch granules to varying degrees. Foods containing starch that has swollen (or gelatinized) to the bursting point, like boiled or baked potatoes, are more easily digested, and therefore, have higher GIs, than foods containing starch granules that are less gelatinized, like oatmeal, brown rice, and al dente spaghetti. For example, the GI of a baked potato is 85; brown rice is 50. (3,5).
• Processing method. Grinding, rolling or milling starchy foods reduces particle size and makes it easier for water to be absorbed and digestive enzymes to attack the food. Processing can also remove the fibrous outer coat of the grain that slows down the access of digestive enzymes to the starch inside. Finely milled flours generally have a high GI. Coarse, stone-ground flours have larger-sized particles and lower GIs. (3,5).
• Type of starch. The two types of starch in foods, amylose and amylopectin, have different effects on GI. Amylose molecules are harder to digest than amylopectin molecules. Legumes and basmati rice have a higher ratio of amylose to amylopectin, and therefore, have a lower GI than foods with more amylopectin. For example, the GI of basmati rice is 58; instant rice is 87. (3).
• Fiber. The extent that fiber affects GI is unclear.(1). Some researchers believe that viscous, soluble fiber thickens the mixture of food in the digestive tract, which slows down enzymes from digesting the starch. This results in a lower blood sugar response, and a lower GI. Legumes and oats contain soluble fiber and have low GIs. (3).
• Sugar. GI is affected by the type of sugar in a food. Sucrose (table sugar), which is made up of glucose and fructose, has a lower GI than glucose because half of the sucrose molecule is made up of fructose, a type of sugar that elicits a very small blood sugar response. (3). For example, the GI of sucrose is 68; the GI of glucose is 100. It would seem that adding sugar to a meal should lower the GI, but, surprisingly, a few studies have not found this to be so. (1).
• Fat. Fat increases the time it takes for food to leave the stomach and enter the intestine. By slowing the rate that carbohydrates are digested in the intestine, fat-containing foods may temper the rise in blood sugar and yield a lower GI than similar foods without fat. For example, the GI of potato chips is 57, French fries is 75 and baked potato is 85. However, that does not make potato chips a better choice than the more nutritious baked potato. (3,5).
• Acidity. Acid in food slows down stomach emptying, which slows the rate that carbohydrates are digested. Increasing the acidity in a meal can lower its GI and the blood sugar response. Vinegar, lemon juice and sourdough bread provide this benefit. (1,3).
The glycemic index is highly variable. As summarized above, a number of factors influence the GI of a food, including the physical structure of the carbohydrate, the presence of other nutrients (e.g., fat, fiber), and the way the food is processed and prepared. Opponents question the practicality of instructing people to eat foods cooked or processed in one way but not another, or of eating certain types of a particular food because the GI is lower. For example, macaroni has a GI of 47, but spaghetti has a GI of 38; thin linguini is 52 on the GI scale, but thick linguini is 46. (3). Some proponents say to focus, instead, on the categories of GI values and to choose more foods from the low GI group ( 0-55 GI) than from the high GI group (70 or higher), rather than trying to track and control the GI of every food that is eaten. (6). Yet, some popular diet books advocate avoidance of high GI foods.
Zrodlo:
The American Institute for Cancer Research
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http://preventcancer.aicr.org/site/DocServer/Glycemic_Index.pdf?docID=341