Fine Tuning Your Blood Glucose Control: Glycemic Index and Load

I’m sure nearly every patient with diabetes has experienced the following situation: you eat the exact same number of carbohydrates on two different occasions. The first go-around your sugar remains perfectly at goal, while the second time your sugar skyrockets for no discernible reason.

The most likely cause of this seemingly mystifying situation is that while the two meals had the same number of carbohydrates, they had different glycemic index levels and, therefore, different glycemic loads.

So first, some definitions:

The glycemic index refers to the degree to which 50 grams of a particular food increases the blood glucose in relation to 50 grams of white bread or glucose. It is expressed as a percent, such that foods with the exact same effect on blood sugar as white bread have a glycemic index of 100%.

The lower the number, the lower the impact of the carbohydrate on your blood sugar.

  • 55% or less = Low (good)
  • 56- 69% = Medium
  • 70% or higher = High (bad)

High index foods include cranberry juice (105%), potatoes (102%), white rice (102%), and dark bread (102%). Low glycemic index foods include non-starchy vegetables, nuts, and barley. In addition, 100% stone- ground whole wheat bread, oatmeal, converted pasta, sweet potatoes, and corn are all considered low glycemic index foods.

The glycemic load is simply the product of the total carbohydrate content of a food and its glycemic index.

Glycemic load helps patients account for both the quantity and the quality of carbs simultaneously. A glycemic load of less than 10 is considered low, and a load of more than 20 is thought to be high.

The glycemic index and, therefore, glycemic load of a food is affected by several factors. Fats, fiber, and acids (such as lemon juice and vinegar) generally lower the glycemic index.

Conversely, the glycemic index generally increases as fruits and vegetables become riper and as foods become more processed. For example, orange juice has a higher index than oranges themselves. Similarly, mashed potatoes have higher index levels than whole potatoes. On average, the longer a food is cooked, the higher the resulting glycemic index. For example, pasta served al dente has a lower glycemic index than regular cooked pasta. Finally, the variety of a type of food can modify the glycemic index; long-grain rice has a lower glycemic index than both brown and short-grain rice.

In a combined analysis (meta-analysis) of 14 randomized trials looking at patients with type 1 and type 2 diabetes following either low glycemic index diets (i.e., average index of less than 55%) or the standard American Diabetes Association diet, hemoglobin A1C levels were found to be 0.43% lower in the low glycemic index group.

So why do endocrinologists ask patients to carbohydrate count rather calculate glycemic indices for each meal?

There are several possible reasons.

First, it is an oversimplification to say that there is one glycemic index per food. As described above, the glycemic index of a banana is not actually well-defined but rather depends on the ripeness of the banana. In addition, as food is combined with fats and acids, the glycemic index is subject to change. Next, it is difficult for patients to obtain information on the glycemic index of combined foods since this information is not included on food labels. In addition, there are many factors which contribute to the overall nutritional value of foods -- an overemphasis on glycemic index can potentially backfire. For example, both ice cream and potato chips are low-glycemic foods but both should be eaten in moderation given their high levels of saturated fats. Finally, despite the studies cited, in general research shows that the total amount of carbohydrates is the primary predictor of blood glucose excursions. Therefore, we still rely on carbohydrate counting as the first line tool for managing diabetes.

However, in patients noticing discrepant blood glucose readings despite entering the meal with well-controlled sugars and eating consistent numbers of carbohydrates, glycemic index and load can potentially help improve blood sugar control. 

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