In the past, carbohydrates were classified as simple or complex based on the number of simple sugars in the molecule. Carbohydrates composed of one or two simple sugars like fructose or sucrose (table sugar) were labeled simple, while starchy foods were labeled complex because starch is composed of long chains of the simple sugar, glucose. Advice to eat less simple and more complex carbohydrates was based on the assumption that consuming starchy foods would lead to smaller increases in blood glucose than sugary food. This assumption turned out to be too simplistic since the blood glucose (glycemic) response to “complex” carbohydrates has been found to vary considerably. A more accurate indicator of the relative glycemic response to dietary carbohydrates is the glycemic index.
Measuring the Glycemic Index of Foods
To determine the glycemic index of a food, volunteers are typically given a test food that provides 50 grams of carbohydrate and a control food (white bread or pure glucose) that provides the same amount of carbohydrate on different days. Blood samples for the determination of glucose are taken prior to eating and at regular intervals after eating over the next several hours. The changes in blood glucose over time are plotted as a curve. The glycemic index is calculated as the area under the glucose curve after the test food is eaten, divided by the corresponding area after the control food is eaten. The value is multiplied by 100 to represent a percentage of the control food. For example, a baked potato has a glycemic index of 76 relative to glucose and 108 relative to white bread, which means that the blood glucose response to the carbohydrate in a baked potato is 76% of the blood glucose response to the same amount of carbohydrate in pure glucose and 108% of the blood glucose response to the same amount of carbohydrate in white bread. In contrast, cooked brown rice has a glycemic index of 55 relative to glucose and 79 relative to white bread. In the traditional system of classifying carbohydrates, both brown rice and potato would be classified as complex carbohydrates despite the difference in their effects on blood glucose levels.
Physiological Responses to High vs. Low Glycemic Index Foods
By definition, the consumption of high-glycemic index foods results in higher and more rapid increases in blood glucose levels than the consumption of low-glycemic index foods. Rapid increases in blood glucose are potent signals to the beta-cells of the pancreas to increase insulin secretion. Over the next few hours, the high insulin levels induced by consumption of high-glycemic index foods may cause a sharp decrease in blood glucose levels (hypoglycemia). In contrast, the consumption of low-glycemic index foods results in lower but more sustained increases in blood glucose and lower insulin demands on pancreatic beta-cells.
The glycemic index compares the potential of foods containing the same amount of carbohydrate to raise blood glucose. However, the amount of carbohydrate consumed also affects blood glucose levels and insulin responses. The glycemic load of a food is calculated by multiplying the glycemic index by the amount of carbohydrate in grams provided by a food and dividing the total by 100. In essence, each unit of the glycemic load represents the equivalent blood glucose-raising effect of 1 gram of pure glucose or white bread. Dietary glycemic load is the sum of the glycemic loads for all foods consumed in the diet. The concept of glycemic load was developed by scientists to simultaneously describe the quality (glycemic index) and quantity of carbohydrate in a meal or diet.
Type 2 Diabetes Mellitus
After a high-glycemic load meal, blood glucose levels rise more rapidly and insulin demand is greater than after a low-glycemic load meal. High blood glucose levels and excessive insulin secretion are thought to contribute to the loss of the insulin-secreting function of the pancreatic beta-cells that leads to irreversible diabetes. High dietary glycemic loads have been associated with an increased risk of developing type 2 diabetes mellitus (DM) in several large prospective studies. In the Nurses’ Health Study (NHS), women with the highest dietary glycemic loads were 37% more likely to develop type 2 DM over the next 6 years than women with the lowest dietary glycemic loads. Additionally, women with high-glycemic load diets that were low in cereal fiber were more than twice as likely to develop type 2 DM than women with low-glycemic load diets that were high in cereal fiber. The results of the Health Professionals Follow-up Study (HPFS), which followed male health professionals over 6 years were similar. In the NHS II study, a prospective study of younger and middle-aged women, those who consumed foods with the highest glycemic index values and the least cereal fiber were also at significantly higher risk of developing type 2 DM over the next 8 years. The foods that were most consistently associated with increased risk of Type 2 DM in the NHS and HPFS cohorts were potatoes (cooked or French-fried), white rice, white bread and carbonated beverages.
Impaired glucose tolerance and insulin resistance are known to be risk factors for cardiovascular disease as well as type 2 DM. In addition to increased blood glucose and insulin concentrations, high dietary glycemic loads are associated with increased serum triglyceride concentrations and decreased HDL cholesterol concentrations, both cardiovascular disease risk factors. High dietary glycemic loads have also been associated with increased serum levels of C-reactive protein (CRP), a marker of systemic inflammation that is also a sensitive predictor of cardiovascular disease risk. In the NHS cohort, women with the highest dietary glycemic loads had a risk of developing coronary heart disease (CHD) over the next 10 years that was almost twice as high as those with the lowest dietary glycemic loads. The relationship between dietary glycemic load and CHD risk was more pronounced in overweight women, suggesting that people who are insulin resistant may be most susceptible to the adverse cardiovascular effects of high dietary glycemic loads.
In the first two hours after a meal, blood glucose and insulin levels rise higher after a high-glycemic load meal than they do after a low-glycemic load meal containing equal calories. However, in response to the excess insulin secretion, blood glucose levels drop lower over the next few hours after a high-glycemic load meal than they do after a low-glycemic load meal. This may explain why 15 out of 16 published studies found that the consumption of low-glycemic index foods delayed the return of hunger, decreased subsequent food intake, and increased satiety (feeling full) when compared to high-glycemic index foods. The results of several small short-term trials (1-4 months) suggest that low-glycemic load diets result in significantly more weight or fat loss than high-glycemic load diets. Although long-term randomized controlled trials of low-glycemic load diets in the treatment of obesity are lacking, the results of short-term studies on appetite regulation and weight loss suggest that low glycemic-load diets may be useful in promoting long-term weight loss and decreasing the prevalence of obesity.
Evidence that high overall dietary glycemic index or high dietary glycemic loads are related to cancer risk is somewhat inconsistent. Prospective cohort studies in the US and Denmark found no association between overall dietary glycemic index or dietary glycemic load and breast cancer risk. In contrast, a prospective cohort study in Canada found that postmenopausal but not premenopausal women with high overall dietary glycemic index values were at increased risk of breast cancer, particularly those who reported no vigorous physical activity, while a prospective study in the US found that premenopausal but not postmenopausal women with high overall dietary glycemic index values and low levels of physical activity were at increased risk of breast cancer. Higher dietary glycemic loads were associated with moderately increased risk of colorectal cancer in a prospective study of US men, but no association between dietary glycemic load and colorectal cancer risk was observed in a prospective study of US women. In contrast, another prospective cohort study of US women found that higher dietary glycemic loads were associated with increased risk of colorectal cancer. Although there is some evidence that hyperinsulinemia (elevated serum insulin levels) may promote the growth of some types of cancer, more research is needed to determine the effects of dietary glycemic load and/or glycemic index on cancer risk.
Low-glycemic index diets appear to improve the overall blood glucose control in people with type 1 and type 2 diabetes mellitus (DM). A meta-analysis of 14 randomized controlled trials that included 356 diabetic patients found that low-glycemic index diets improved short-term and long-term control of blood glucose levels, reflected by clinically significant decreases in fructosamine and hemoglobin A1C levels. Episodes of serious hypoglycemia are a significant problem in people with type 1 DM. In a study of 63 men and women with type 1 DM, those randomized to a high-fiber, low-glycemic index diet had significantly fewer episodes of hypoglycemia than those on a low-fiber, high-glycemic index diet.
Lowering Dietary Glycemic Load
Some strategies for lowering dietary glycemic load include:
• Increasing the consumption of whole grains, nuts, legumes, fruits and nonstarchy vegetables
• Decreasing the consumption of starchy high-glycemic index foods like potatoes, white rice and white bread
• Decreasing the consumption of sugary foods like cookies, cakes, candy and soft-drinks