Glycemic index is a fairly well-known concept for helping people with diabetes to plan their meals.
By Shelley Diamond, BScPhm
Illustration by Martin Bregman
However, there is also the measurement of glycemic load to consider, and it is important to understand the difference between these two different food rankings.
The GI, or glycemic index, is a measure of how much a carbohydrate-containing food raises postprandial blood glucose. Foods are given a value based on how they compare to a reference food, which is usually glucose or white bread, and these both have a GI of 100.
Foods are divided into low GI (up to 55), medium GI (56-69) and high GI (over 70) scores. Examples of low GI foods include all non-starchy vegetables and some starchy vegetables, most fruit, many whole grain breads and cereals, as well as dried beans and legumes. There are several things that impact the GI of a food, such as the ripeness of a fruit (the riper, the higher the GI), processing (some processes increase the GI while other processes decrease the GI), and the cooking method (for example, al dente pasta has a lower GI than pasta that is cooked longer).
Consumption of high glycemic index foods may cause an initial period of elevated blood glucose and insulin levels. Following this, reactive hypoglycemia can occur, and eventually insulin resistance. It is thought that high glycemic index foods increase fat accumulation and reduce the body’s capacity to break down fats, and may actually contribute to obesity.
Low GI foods are digested and absorbed more slowly and result in a greater degree of satiety, and are thought to be of benefit for weight management. Lower GI foods raise blood glucose levels more slowly after meals. However, considering low GI foods is only half the equation, since the amount of food also needs to be considered, and not simply the type.
The glycemic load, or GL, which considers both the type and the amount of carbohydrate in food, has been shown to be a stronger predictor of blood glucose response than the GI. Foods with a GL under 10 are considered low-GL foods and have little impact on blood glucose. Foods with moderate impact on blood glucose have a GL of 10-20, and foods with a GL higher than 20 are considered to have a high impact on blood glucose. A food can have a high GI but a low GL. For instance, a slice of watermelon has a high GI of 72, but a low GL of 4.3, since the serving size of one slice of watermelon is mainly water. Carrots are a food that many people feel will raise blood glucose significantly because it has a high GI of 71, but an average serving of carrots has a GL of 6.
Both GI and GL can be used in conjunction with carbohydrate counting, which is still the first tool that should be considered for managing blood glucose. GI and GL can be used to further refine blood glucose management goals. It is important to remember that there is no concrete evidence that low GI/GL diets reduce weight or provide other healthy long-term outcomes. From a practical point of view, most people eat a variety of foods and not single foods at one time. While a single food may have a low or high glycemic index, a meal may combine foods of a variety of types, which limits the applicability of the glycemic index to the way most people eat.
Shelley Diamond BScPhm is the president of Pedipharm Consultants and Diabetes Care Community Inc. (www.diabetescarecommunity.ca)