The glycemic index measures the effect a food has on your blood glucose level. The higher a food scores on the glycemic index, the faster your blood sugar rises and more insulin your pancreas will release to help stabilize blood sugar levels. Only foods that contain carbohydrates, such as rice and grains, can be scored on the GI -- protein and fat have little impact on blood sugar.
Glycemic Index Testing
The glycemic index testing facility, located in Sydney Australia, measures the GI of food using real people. A test subject group of at least 10 people are fed 50 g of the carbohydrate being tested. Their blood sugars levels are measured periodically. On a separate occasion, those same test subjects are given 50 g of pure glucose and blood sugar levels are tested at the same timed intervals. The difference between the two levels is calculated for each individual, then all the individuals results are averaged together to determine the GI score of that food. The higher a food scores, the larger the impact it has on your glucose level.
Carbohydrates and GI Scores
There are three main types of carbohydrates: fiber, starch and sugar. Fiber slows digestion and helps keep glucose levels low and stable. Foods high in fiber tend to score lower on the glycemic index. Sugar and starch are quickly digested and can have a significant effect on blood sugar. The faster a food is digested, the faster your body converts it to glucose for use as energy. Because fat and protein slow digestion, you can help lower glucose and insulin production by combining sugar and starch with fat and protein. Sweets high in fat, such as chocolate bars, tend to score low on the GI.
Rice
The higher the fiber content, the lower the score on the glycemic index. Brown rice, with the hull and bran intact has more fiber than white rice. Glycemic scores also vary based on the type of rice. According to the glycemic index database, organic basmati rice has the lowest GI at 57. Ordinary white rice scores high on the GI with 89. Brown rice scores 66. Foods that score 55 or less are considered low-GI foods and should make up the majority of your diet. Medium-GI foods score between 56 and 70 and any food that scores 71 or more is a high-GI food and should be eaten in moderation.
Grains
Grains that are high in fiber and low in sugar and starch will score low on the glycemic index. Pearled barley is such a grain and scores a very low 25 on the GI. In general, whole grains score lower than refined flours. Pumpernickel bread scores about 50 and whole wheat bread scores about 55, but white bread made from refined flour can score as much as 20 points higher on the GI than whole wheat. Cake, with its high fat content, may score lower than bread, even though bread may be the healthier choice. The GI can be a guideline for choosing carbs, but you'll still need to use your best judgment to make healthy choices.
References
- The Glycemic Index: GI Testing and Research
- Harvard School of Public Health: The Nutrition Source -- Carbohydrates: Good Carbs Guide the Way
- MayoClinic.com; Glycemic Index -- A Helpful Tool for People With Diabetes?; Maria Collazo-Clavell, M.D.; March 2010
- Harvard Medical School Health Publications: Glycemic Index and Glycemic Load for 100+ Foods
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