The term glycemic index was coined by researchers more than 25 years ago, and it offered a new way of classifying foods containing carbohydrate according to their effect on blood glucose level following a meal. Up until that point, for the purposes of diabetes meal planning, carbohydrates were divided into complex carbohydrates (or starches) and simple carbohydrates (or sugars). It was assumed that sugars had a much more pronounced effect on blood glucose level than starches, and people with diabetes were routinely told to avoid sugar. But nutrition studies in which people ate different foods and then had their blood glucose level measured showed that this assumption was incorrect. Today it is known, for example, that a bagel can raise blood glucose more than a serving of table sugar containing the same amount of carbohydrate. In other words, the bagel has a higher glycemic index.
Like any new research that shakes up long-held beliefs, information on the glycemic index of foods sparked both excitement and controversy (as well as a whole new category of popular weight-loss books and programs). Twenty-five years later, that controversy is still alive. So how useful is the glycemic index in improving blood glucose control, helping with weight loss or maintenance, and improving long-term health? This article takes a look at what’s been learned about the glycemic index in the years since the term was introduced.
How a food’s glycemic index is determined
Determining a food’s calorie and nutrient content involves only a laboratory technician and a machine. Determining a food’s glycemic index, on the other hand, involves significantly more people and more work. Normally the way it works is that a group of 5–10 volunteers first eats an amount of either glucose or white bread containing 50 grams of carbohydrate. (White bread has a glycemic index similar to that of pure glucose.) The volunteers’ blood glucose level is then measured at intervals over the next two hours, and the numbers are plotted on a graph. The points on the graph form a curved line, and the area under the curve is the food’s glycemic index.
The glycemic index of the glucose or bread is given a value of 100, and all other foods tested are compared to it and given a relative value. For example, the volunteers might next eat a bowl of cornflakes containing 50 grams of carbohydrate and have their blood glucose level similarly measured and plotted on a graph. The area under that curve is calculated for each volunteer, and based on the average of all the volunteers’ scores, cornflakes are assigned a glycemic index of 80, indicating that cornflakes cause somewhat less of a rise in blood glucose than glucose (or bread) causes.
A glycemic index of 80 is still high, however. Generally, a glycemic index of 55 or lower is considered low, a glycemic index from 56 to 69 is considered intermediate, and a glycemic index of 70 or higher is considered high.
While determining a food’s glycemic index is somewhat labor-intensive, it would seem that if enough foods were tested, it would be easy enough to construct a meal plan incorporating mostly foods with a low glycemic index, resulting in lower blood glucose levels after eating. In practice, however, it’s not that simple.
In the laboratory, study subjects eat only one test food at a time, but in real life, people usually eat meals or snacks containing several different foods. The presence of protein or fat in the accompanying foods can alter a food’s effect on blood glucose level. Some other factors that can affect a food’s glycemic index include the following:
- The botanical variety of the food. Different types of rice or potatoes, for example, have different glycemic index values.
- How the food has been processed. The more finely ground a grain, for example, the higher its glycemic index, so instant oats have a higher glycemic index than steel-cut oats. Conversely, parboiling rice lowers its glycemic index, so parboiled, or “converted,” rice products generally have a lower glycemic index than regular rice.
- How a food is prepared. For example, pasta cooked al dente has a lower glycemic index than pasta that is cooked until fully soft.
- Acidity. Acidic foods, such as sourdough bread, tend to have a lower glycemic index than similar, less-acidic foods. In addition, adding lemon juice or vinegar to a dish, such as a salad, will lower its glycemic index.
- Presence of sucrose. Table sugar (sucrose) is half glucose, which has a high glycemic index, and half fructose, which has a low glycemic index. As a result, the glycemic index of table sugar is intermediate, and foods that are high in sugar may similarly have an intermediate glycemic index.
- Fiber. Some types of fiber lower a food’s glycemic index, but some do not. Insoluble fiber, the type found in wheat, has little effect on glycemic index, so whole wheat bread has a glycemic index similar to that of white bread. Soluble fiber, on the other hand, lowers glycemic index. Foods high in soluble fiber such as oats, barley, and legumes (dried beans and lentils) have low glycemic index values.