Glycemic Index and Diabetes
What is the glycemic index?
The glycemic index, or GI, measures how a carbohydrate-containing food raises blood glucose. Foods are ranked based on how they compare to a reference food – either glucose or white bread. A food with a high GI raises blood glucose more than a food with a medium or low GI.
Meal planning with the GI involves choosing foods that have a low or medium GI. If eating a food with a high GI, you can combine it with low GI foods to help balance the meal. Examples of carbohydrate-containing foods with a low GI include dried beans and legumes (like kidney beans and lentils), all non-starchy vegetables and some starchy vegetables, most fruit, and many whole grain breads and cereals (like barley, whole wheat bread, rye bread, and all-bran cereal). Meats and fats don’t have a GI because they do not contain carbohydrate.
What affects the GI of a food?
Fat and fiber tend to lower the GI of a food. As a general rule, the more cooked or processed a food, the higher the GI; however, this is not always true.
Below are a few specific examples of other factors that can affect the GI of a food:
- Ripeness and storage time – the more ripe a fruit or vegetable is, the higher the GI
- Processing – juice has a higher GI than whole fruit; mashed potato has a higher GI than a whole baked potato, stone ground whole wheat bread has a lower GI than whole wheat bread.
- Cooking method: how long a food is cooked (al dente pasta has a lower GI than soft-cooked pasta)
- Variety: converted long-grain white rice has a lower GI than brown rice but short-grain white rice has a higher GI than brown rice.
Other things to consider if using the GI:
- The GI value represents the type of carbohydrate in a food but says nothing about the amount of carbohydrate typically eaten. Portion sizes are still relevant for managing blood glucose and for losing or maintaining weight.
- The GI of a food is different when eaten alone than it is when combined with other foods. When eating a high GI food, you can combine it with other low GI foods to balance out the effect on blood glucose levels.
- Many nutritious foods have a higher GI than foods with little nutritional value. For example, oatmeal has a higher GI than chocolate. Use of the GI needs to be balanced with basic nutrition principles of variety for healthful foods and moderation of foods with few nutrients.
GI a better tool than carbohydrate counting?
There is no one diet or meal plan that works for everyone with diabetes. The important thing is to follow a meal plan that is tailored to personal preferences and lifestyle and helps achieve goals for blood glucose, cholesterol and triglycerides levels, blood pressure, and weight management.
Research shows that both the amount and the type of carbohydrate in food affect blood glucose levels. Studies also show that the total amount of carbohydrate in food, in general, is a stronger predictor of blood glucose response than the GI.
Based on the research, for most people with diabetes, the first tool for managing blood glucose is some type of carbohydrate counting. Balancing total carbohydrate intake with physical activity and diabetes pills or insulin is key to managing blood glucose levels.
Because the type of carbohydrate does have an affect on blood glucose, using the GI may be helpful in "fine-tuning" blood glucose management. In other words, combined with carbohydrate counting, it may provide an additional benefit for achieving blood glucose goals for individuals who can and want to put extra effort into monitoring their food choices.