Dry Beans Get High Marks for Low Glycemic Index
January 15, 2003
By Patti Bazel Geil
"Potatoes are worse than candy bars for people with diabetes!"
Over 15 years ago, this eye-catching headline drew attention to the controversial concept of the glycemic index (GI).
The idea that mashed potatoes could cause higher bloodglucose levels than a chocolate bar simply defied conventional nutrition wisdom.
For many years, the accepted dietary dogma for diabetes was to avoid
sugary foods such as candy (the so-called simple carbohydrates), because they were absorbed quickly by the body and produced a more rapid and larger rise in blood glucose. Complex carbohydrates (starches, such as potatoes) were encouraged, because it was believed they were more slowly absorbed, resulting in a smaller rise in blood-glucose levels. The research behind the glycemic index challenged many long-held nutrition beliefs.
While glycemic index continues to be of interest to diabetes and nutrition professionals, the relatively new concept of glycemic load (GL) is now being investigated as a possible contributor to the rising rates of obesity, diabetes, and cardiovascular disease in the United States.
Due to their nutrient composition and the nature of the starch in their cell walls, dry beans are low on both the glycemic index and glycemic load scales, making them an ideal food for promoting good health.
How its measured
Research into glycemic index began in the late 1970s, and it continues to be extensively tested and refined by scientists. Although the concept has been widely adopted in diabetes management programs around the world, it remains controversial in the United States. The glycemic index has been proposed as a nutrition approach for improving blood glucose control in individuals with diabetes.
Over 600 foods have been tested in individuals with and without diabetes.
Glycemic index researchers begin by assigning a glycemic index of 100 to pure glucose, because it produces the greatest rise in blood glucose levels. Volunteers responses to other foods are then compared to their response to pure glucose, and thus a rating of foods is developed. Another approach is to assign white bread a GI value of 100, essentially shifting the entire scale, but preserving the relative rankings of food.
Each serving of the food being tested contains 50 grams of carbohydrate,
which may be equal to 1 cup of spaghetti, 20 jellybeans, 3-1/2 slices of bread, or 3 tablespoons of pure glucose powder. After the research volunteers eat their test foods, their blood-glucose levels are measured every 15 to 30 minutes over the next two to three hours. The blood-sugar
levels are plotted on a graph, and calculations are made by computer program to determine the glycemic index of a food.
Carbohydrates break down faster
Glycemic index research has revealed a number of surprises, including the attention-grabbing headline touting the benefits of candy bars versus mashed potatoes. The explanation? It appears that carbohydrate foods that break down quickly during digestion, such as most flaked breakfast cereals, have the highest GI values.
Carbohydrates that break down slowly, such as most fiber-rich breakfast cereals, release glucose gradually into the bloodstream and have low GI values. In the case of mashed potatoes (GI of 70) versus a chocolate candy bar (GI of 40), several factors are at work. First, the actual starch molecules in the potato are fully gelatinized or swollen, making them easier for digestive enzymes to attack. Therefore, the carbohydrate is rapidly digested and absorbed. In addition, foods such as candy bars, which contain a large amount of table sugar (or sucrose), have a lower glycemic index because sucrose is a disaccharide, a double sugar made of both glucose and fructose.
The fructose portion of sucrose is converted very slowly to glucose in the blood stream, thus lowering the glycemic index of the sucrose-containing food.
Finally, a chocolate candy bar is a rich source of fat, which slows down the rate of stomach emptying, thereby slowing the digestion of the carbohydrate.
High-fat foods may have lower GI numbers than their lowfat versions. Does this information mean that candy bars are a better food choice than mashed potatoes for people with diabetes? Certainly not! Glycemic index information should be used side by side with lowfat, consistent carbohydrate guidelines for overall good nutrition for individuals with diabetes.
Dr. Thomas Wolever, an originator of the glycemic index concept,states, "As originally intended, the glycemic index was meant to supplement the information in food tables, not replace it." People with diabetes may want to test the effects of glycemic index individually by substituting a portion of their daily carbohydrate intake with low-GI foods (e.g., eating wholegrain/whole-seed bread rather than white bread, or oat bran cereal in place of cornflakes). The results of these dietary changes should be followed closely by checking blood glucose levels frequently.
A relatively new way to assess the impact of carbohydrate foods on blood glucose, glycemic load takes into account the glycemic index, yet provides additional information about health effects.
While the glycemic index tells you how rapidly a carbohydrate food turns into sugar, it doesnt tell you how much of that carbohydrate is in a serving of a particular food. For example, consider the glycemic load of carrots. In one study, carrots were noted to have a glycemic index of 95, which is quite high. However, in order to obtain the standard 50 grams of carbohydrate needed for the glycemic index test, volunteers had to eat about 3/4 pound of carrots, far more than the normal serving size. A normal serving of carrots will contribute only a small amount to the rise in blood sugar. So although the glycemic index of the carbohydrate in carrots is high, the amount of carbohydrate in an average serving is small.
As the glycemic load of the typical American diet has increased over the years due to increases in carbohydrate consumption and changes in food-processing technology, so have the rates of obesity and diabetes among Americans of all ages. Research shows that high-GL diets correlate with lower HDL (good) cholesterol concentrations and higher triglyceride levels. These factors increase the risk of cardiovascular disease, particularly in overweight women. Experts theorize that the sharp rise in blood glucose that occurs after eating high-GL foods results in a surge of insulin to restore a normal bloodglucose level. Insulin promotes the deposit of fat into cells, and the constant demand for insulin can exhaust the pancreatic cells responsible for its production, resulting in diabetes.
Knowing the glycemic load of a food can lead to better carbohydrate
choices. Cutting back on high-GL processed and refined-carbohydrate
snack foods is a good idea. Using more low-GL whole grains and beans in place of higher-GL foods such as white rice will also improve your nutrition profile.
High marks for dry beans
Several factors influence the glycemic index and glycemic load of a food: cooking methods, physical form of the food, type of starch, fiber, sugar, and amount of carbohydrate per serving. The nutrient profile of dry beans, particularly their high soluble-fiber content, places them in the low glycemic index/low glycemic load category. Research continues to define the many health benefits this way of eating can provide.
-- Patti Bazel Geil is a registered dietitian, cookbook author and certified diabetes educator in Lexington, Kentucky.
-- Source: Michigan Bean Commission