Glycemic Index

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; a disaccharide composed of one molecule of glucose and one molecule of fructose) 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 (i.e., polysaccharides) was based on the assumption that consuming starchy foods would lead to smaller increases in blood glucose than sugary foods (1). 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 should be glycemic load, which incorporates the relative quality and quantity of carbohydrates in the diet.

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 (2). 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 (3). In contrast, cooked brown rice has a glycemic index of 55 relative to glucose and 79 relative to white bread (4). 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- versus 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 β-cells of the pancreas to increase insulin secretion (2). 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 β-cells (5).

Glycemic Load

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 (1). 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.

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 β-cells that leads to irreversible diabetes (6). 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 (7). 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 (8). 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 (9). 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 (6). The Black Women's Health study, a prospective study in a cohort of 59,000 US 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 (10). 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 (11). 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 (12).

A US ecologic 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 (13). 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 (13). 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 (14, 15). 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 (16). 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 (17). 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 (1). A similar finding was reported in a cohort of middle-aged Dutch women followed for nine years (18). More recently, a prospective study in an Italian cohort of 47,749 men and women, who were followed for almost eight years, found that a high glycemic load was associated with an increased risk of CHD in women but not in men (47). Yet, studies to date have reported mixed results, and more research is needed to determine if low glycemic index diets decrease the risk for CHD (19).

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 (20). 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 (21-23). 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 (24). 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 US, Denmark, France, and Australia have found no association between overall dietary glycemic index or dietary glycemic load and breast cancer risk (25-28). 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 (29). 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 (30), 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 (31). 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]) (28). Higher dietary glycemic loads were associated with moderately increased risk of colorectal cancer in a prospective study of US men, but no clear associations between dietary glycemic load and colorectal cancer risk were observed in a prospective studies of US men (32), US women (32-35), Swedish women (36), and Dutch men and women (37). However, one prospective cohort study of US women found that higher dietary glycemic loads were associated with increased risk of colorectal cancer (38). One meta-analysis of case-control and cohort studies suggested that glycemic index and glycemic load were positively associated with colorectal cancer (39), but a more recently published meta-analysis did not find glycemic index or load to be significantly associated with colorectal cancer (40). Two separate meta-analyses reported that high dietary glycemic loads were associated with increased risk of endometrial cancer (39, 41). Although there is some evidence that hyperinsulinemia (elevated serum insulin levels) may promote the growth of some types of cancer (42), 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 (43) and in a cohort of women participating in the Nurses' Health Study (44). Likewise, higher glycemic index diets were associated with increased risks of gallstone disease in both studies (43-44). 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 (45). 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 (46).

Lowering Dietary Glycemic Load

Some strategies for lowering dietary glycemic load include:

• Increasing the consumption of whole grains, nuts, legumes, fruit, 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 Table 1 for the glycemic index and glycemic load values of selected foods (4). Foods with higher glycemic index values are at the top of the table, while foods with lower glycemic index values are at the bottom the table. To look up the glycemic index values for other foods, visit the University of Sydney’s GI website.

Table 1. Glycemic Index (Relative to Glucose) and Glycemic Load Values for Selected Foods (3, 4)
Food
Glycemic Index
(Glucose=100)
Serving Size
Carbohydrate per Serving (g)
Glycemic Load per Serving
Dates, dried
103
2 oz
40
42
Cornflakes
81
1 cup
26
21
Jelly beans
78
1 oz
28
22
Puffed rice cakes
78
3 cakes
21
17
Russet potato (baked)
76
1 medium
30
23
Doughnut
76
1 medium
23
17
Soda crackers
74
4 crackers
17
12
White bread
73
1 large slice
14
10
Table sugar (sucrose)
68
2 tsp
10
7
Pancake
67
6" diameter
58
39
White rice (boiled)
64
1 cup
36
23
Brown rice (boiled)
55
1 cup
33
18
Spaghetti, white; boiled 10-15 min
44
1 cup
48
21
Spaghetti, white; boiled 5 min
38
1 cup
48
18
Spaghetti, whole wheat; boiled
37
1 cup
42
16
Rye, pumpernickel bread
41
1 large slice
12
5
Oranges, raw
42
1 medium
11
5
Pears, raw
38
1 small
11
4
Apples, raw
38
1 small
15
6
All-Bran™ cereal
38
1 cup
23
9
Skim milk
32
8 fl oz
13
4
Lentils, dried; boiled
29
1 cup
18
5
Kidney beans, dried; boiled
28
1 cup
25
7
Pearled barley; boiled
25
1 cup
42
11
Cashew nuts
22
1 oz
13
3
Peanuts
14
1 oz
6
1

Authors and Reviewers

Originally written in 2003 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in December 2005 by:
Jane Higdon, Ph.D.
Linus Pauling Institute
Oregon State University

Updated in February 2009 by:
Victoria J. Drake, Ph.D.
Linus Pauling Institute
Oregon State University

Reviewed in February 2009 by:
Simin Liu, M.D., M.S., M.P.H., Sc.D.
Professor and Director, Program on Genomics and Nutrition
Professor of Epidemiology and Medicine
UCLA School of Public Health

Copyright 2003-2015  Linus Pauling Institute 


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