Australia's Sydney University Glycemic Index Research Service is the official laboratory that tests foods for the glycemic index. The service tests foods for their effect on blood sugar -- the less a food raises blood sugar level, the lower its score on the glycemic index, or GI. Pure glucose scores 100, and all foods are ranked in comparison to glucose.
The Glycemic Index's Importance
Originally developed as a tool to help diabetics make healthier food choices, following a low-GI diet stabilizes blood sugar and may also reduce your cholesterol levels, blood pressure and risk of heart disease. The GI can be a tool for weight loss; many popular diets, such as the Zone, South Beach and Nutrisystem, are based on eating low-GI foods. Eating foods low on the glycemic index may also help women manage symptoms of polycystic ovarian syndrome.
Glycemic Index Testing
Only foods that contain carbohydrates can be tested, because only carbohydrates raise your blood sugar. Protein, fat and fiber counteract carbohydrates and slow the digestion and absorption of sugar. To test the GI value of any food, a group of healthy adults are given 50 g of pure glucose. For a two-hour period following the ingestion of glucose, their blood sugar levels are monitored at regular intervals. A few days later, the same test subjects are given 50 g of carbohydrates of the test food, and their blood sugar levels are checked at the same intervals. The difference in each person's blood sugar levels is calculated and then an average number is calculated for the entire group -- that's the GI value of that food. Because real people are fed real foods, the GI value of specific ingredients, such as wheat flour, can't be calculated.
Potential Problems
The GI can be a useful tool for making healthier carbohydrate choices, but there are some issues with basing your food choices solely on the GI. The glycemic index doesn't take the nutrient value of food into account -- just because a food is low on the glycemic index doesn't mean it's low in calories or fat and doesn't mean it contains essential vitamins or minerals. Some high-fat foods score low on the GI. An orange is a low-GI food with a score of 44, but a Snickers bar scores an even lower 40.
The glycemic index also measures food in isolation; in real life, people tend to eat foods in combination. A baked potato may be a high-GI food, but because fat and protein slow sugar absorption, adding sour cream, butter and bacon to your baked potato will lower its GI value. But those ingredients probably don't make your potato a healthier choice.
Using the Glycemic Index
Use the glycemic index as a guideline. With a dose of common sense, the GI can be a useful tool to help you lose weight and lower your risk for Type 2 diabetes, heart disease, cancer and gallbladder disease. You don't need to memorize the GI value of every food -- just remember this simple rule: eat high-fiber foods such as whole grains, legumes and non-starchy vegetables instead of high-sugar foods, such as cakes, cookies, bread and potatoes. Remember that GI is one of many tools to help you make healthier choices.
