Glycemic Index or GI is a measure of the affect of the carbohydrates in individual foods on blood-glucose levels. Carbohydrates that break down quickly in the process of digestion have high GI values, while carbohydrates that break down slowly have low GI values.
Examples of high GI foods include candy bars, sodas, many breakfast cereals such as corn flakes and Cheerios, sweet treats made with white flour, etc. Examples of low GI foods include less refined foods such as old fashioned oatmeal, All Bran cereal, beans, lentils, pumpernickel bread, Ezekiel bread, stone ground grains, apples, and plain yogurt.
To determine the glycemic index, individuals were given 25-50 grams of carbohydrate food. Over the next 2-3 hours, the individual’s blood glucose levels were taken and recorded. The foods were tested in 2-3 separate occasions and averaged across the test group.
The results of the tests were rated using the effect of pure glucose as the reference point. High GI foods are those with a GI of 70 or greater. Intermediate GI foods are those with GI’s of 59 to 69 and low GI foods have a GI of less than 55.
Glycemic index is important because it helps to predict the blood glucose response to expect from eating particular foods. Being able to predictably lower blood glucose levels with food helps those with diabetes or glucose intolerance. It can also be a valuable tool in weight loss as seen by the popularity of diets such as Sugar Busters.
There are several defining qualities of particular carbohydrates that seem to influence the GI. One of these is called physical entrapment. This refers to a fibrous coating typically around a grain or legume that acts as a physical barrier and slows access to the starch inside. Examples of these are grains such as barley, grainy breads and legumes.
Particle size is another factor influencing GI. The smaller the particle makes the food easier to digest and therefore, the GI higher. An example is finely milled flour.
Soluble fiber also influences GI. The soluble fiber slows down digestion, making the GI value low. Good examples of foods with soluble fiber include rolled oats, beans, lentils, and apples.
Acids in foods are another factor in GI. Acid in foods slows down the rate starch will digest. Things like vinegar, lemon juice and pickled vegetables will lower GI by slowing down starch digestion.
Another big factor seems to be the amount of water the starch contains. Raw starch is hard and granular and difficult to digest. When starch is cooked, the combination of water and heat expand and soften the starch molecules making them easier to digest. An example would be a raw potato versus mashed potatoes. The GI index of the raw potato is lower than that of the high GI mashed potatoes.
Combinations of grains with sugar actually make the starch more difficult to access. This makes cookies tend to have intermediate GI values especially those made with low GI grains such as oatmeal.
There are several good books on glycemic index. These include The New Glucose Revolution, and Sugar Busters.
Margaret Durst is a naturopathic doctor who owns The Green House, a vitamin, herb and health food store in Mason, Texas