The glycemic index (GI) is a numerical system of measuring how much of a rise in circulating blood sugar a carbohydrate triggers–the higher the number, the greater the blood sugar response. So a low GI food will cause a small rise, while a high GI food will trigger a dramatic spike. A list of carbohydrates with their glycemic values is shown below. A GI is 70 or more is high, a GI of 56 to 69 inclusive is medium, and a GI of 55 or less is low.

The glycemic load (GL) is a relatively new way to assess the impact of carbohydrate consumption that takes the glycemic index into account, but gives a fuller picture than does glycemic index alone. A GI value tells you only how rapidly a particular carbohydrate turns into sugar. It doesn't tell you how much of that carbohydrate is in a serving of a particular food. You need to know both things to understand a food's effect on blood sugar. That is where glycemic load comes in. The carbohydrate in watermelon, for example, has a high GI. But there isn't a lot of it, so watermelon's glycemic load is relatively low. A GL of 20 or more is high, a GL of 11 to 19 inclusive is medium, and a GL of 10 or less is low.

Foods that have a low GL almost always have a low GI. Foods with an intermediate or high GL range from very low to very high GI.

Both GI and GL are listed here. The GI is of foods based on the glucose index–where glucose is set to equal 100. The other is the glycemic load, which is the glycemic index divided by 100 multiplied by its available carbohydrate content (i.e. carbohydrates minus fiber) in grams. (The "Serve size (g)" column is the serving size in grams for calculating the glycemic load; for simplicity of presentation an intermediate column that shows the available carbohydrates in the stated serving sizes has been left out.) Take, watermelon as an example of calculating glycemic load. Its glycemic index is pretty high, about 72. According to the calculations by the people at the University of Sydney's Human Nutrition Unit, in a serving of 120 grams it has 6 grams of available carbohydrate per serving, so its glycemic load is pretty low, 72/100*6=4.32, rounded to 4.

Disease Prevention

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 a 6-year period 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 six 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 eight 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.The Black Women's Health study, a prospective study in a cohort of 59,000 U.S. black women, found that women who consumed foods with the highest glycemic index values had a 23% greater risk of developing type 2 DM over eight years of follow-up compared to those who consumed foods with the lowest glycemic index values. In the American Cancer Society Cancer Prevention Study II, which followed 124,907 men and women for nine years, high glycemic load was associated with a 15% increased risk of type 2 DM. Further, in a cohort of over 64,000 Chinese women participating in the Shanghai Women's Health Study, high glycemic load was associated with a 34% increase in risk of type 2 DM; this positive association was much stronger among overweight women.

A U.S. ecological study of national data from 1909 to 1997 found that increased consumption of refined carbohydrates in the form of corn syrup, coupled with declining intake of dietary fiber, has paralleled the increase in prevalence of type 2 DM. Today, high-fructose corn syrup (HFCS) is used as a sweetener and preservative in many commercial products sold in the United States, including soft drinks and other processed foods. To make HFCS, the fructose content of corn syrup (100% glucose) has been artificially increased; common formulations of HFCS now include 42%, 55%, or 90% fructose. When consumed in large quantities on a long-term basis, HFCS is unhealthful and may contribute to other chronic diseases besides type 2 DM, including obesity and cardiovascular disease.

Cardiovascular Disease

Impaired glucose tolerance and insulin resistance are known to be risk factors for cardiovascular disease and 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 are risk factors for cardiovascular disease. 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 ten 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. A similar finding was reported in a cohort of middle-aged Dutch women followed for nine years. Yet, studies to date have reported mixed results, and there is little evidence to indicate low glycemic index diets decrease the risk for CHD.

Obesity

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. A recent review of six randomized controlled trials concluded that overweight or obese individuals who followed a low-glycemic index/load diet experienced greater weight loss than individuals on a comparison diet that was either a high-glycemic index diet or an energy-restricted, low-fat diet. The length of the dietary interventions in these trials ranged from five weeks to six months.

Cancer

Evidence that high overall dietary glycemic index or high dietary glycemic loads are related to cancer risk is inconsistent. Prospective cohort studies in the U.S., Denmark, France, and Australia have found no association between overall dietary glycemic index or dietary glycemic load and breast cancer risk. In contrast, a prospective cohort study in Italy reported a positive association between breast cancer risk and high-glycemic index diets as well as high dietary glycemic loads. A prospective 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 U.S. 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. In a French study of postmenopausal women, both glycemic index and glycemic load were positively associated with risk of breast cancer but only in a subgroup of women who had the highest waist circumference (median of 84 cm [33 inches]). Higher dietary glycemic loads were associated with moderately increased risk of colorectal cancer in a prospective study of U.S. men, but no clear associations between dietary glycemic load and colorectal cancer risk were observed in a prospective studies of U.S. men, U.S. women, Swedish women, and Dutch men and women. However, one prospective cohort study of U.S. women found that higher dietary glycemic loads were associated with increased risk of colorectal cancer. One meta-analysis of case-control and cohort studies suggested that glycemic index and glycemic load were positively associated with colorectal cancer, but a more recently published meta-analysis did not find glycemic index or load to be significantly associated with colorectal cancer. Two separate meta-analyses reported that high dietary glycemic loads were associated with increased risk of endometrial 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.

Gallbladder Disease

Results of two studies indicate that dietary glycemic index and glycemic load may be positively related to risk of gallbladder disease. Higher dietary glycemic loads were associated with significantly increased risks of developing gallstones in a cohort of men participating in the Health Professionals Follow-up Study and in a cohort of women participating in the Nurses' Health Study. Likewise, higher glycemic index diets were associated with increased risks of gallstone disease in both studies. However, more epidemiological and clinical research is needed to determine an association between dietary glycemic index/load and gallbladder disease.

Disease Treatment

Diabetes Mellitus

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

See the table below for the glycemic index and glycemic load values of selected foods. Foods with higher glycemic index values are at the top of the table, while foods with lower glycemic index values are at the bottom of the table. To look up the glycemic index values for other foods, visit the University of Sydney's GI Web site.

Understanding Glycemic Index and Glycemic Load

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