The glycemic index, glycaemic index, or GI is a measure of the effects of carbohydrates on blood sugar levels. Carbohydrates that break down quickly during digestion and release glucose rapidly into the bloodstream have a high GI; carbohydrates that break down more slowly, releasing glucose more gradually into the bloodstream, have a low GI. The concept was developed by Dr. David J. Jenkins and colleagues[1] in 1980–1981 at the University of Toronto in their research to find out which foods were best for people with diabetes.
A lower glycemic index suggests slower rates of digestion and absorption of the foods' carbohydrates and may also indicate greater extraction from the liver and periphery of the products of carbohydrate digestion. A lower glycemic response usually equates to a lower insulin demand but not always, and may improve long-term blood glucose control[2] and blood lipids. The insulin index is also useful, as it provides a direct measure of the insulin response to a food.
The glycemic index of a food is defined as the area under the two hour blood glucose response curve (AUC) following the ingestion of a fixed portion of carbohydrate (usually 50 g). The AUC of the test food is divided by the AUC of the standard (either glucose or white bread, giving two different definitions) and multiplied by 100. The average GI value is calculated from data collected in 10 human subjects. Both the standard and test food must contain an equal amount of available carbohydrate. The result gives a relative ranking for each tested food.[3]
The current validated methods use glucose as the reference food, giving it a glycemic index value of 100 by definition. This has the advantages of being universal and producing maximum GI values of approximately 100. White bread can also be used as a reference food, giving a different set of GI values (if white bread = 100, then glucose ≈ 140). For people whose staple carbohydrate source is white bread, this has the advantage of conveying directly whether replacement of the dietary staple with a different food would result in faster or slower blood glucose response. The disadvantages with this system are that the reference food is not well-defined and the GI scale is culture dependent.
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GI values can be interpreted intuitively as percentages on an absolute scale and are commonly interpreted as follows :
| Classification | GI range | Examples |
|---|---|---|
| Low GI | 55 or less | most fruits and vegetables (except potatoes and watermelon), whole-grain breads, pasta, legumes/pulses, milk, yogurt, products extremely low in carbohydrates (some cheeses, nuts), fructose |
| Medium GI | 56–69 | whole wheat products, basmati rice, sweet potato, table sugar |
| High GI | 70 and above | corn flakes, rice krispies, baked potatoes, watermelon, croissants, white bread, extruded breakfast cereals, most white rices (e.g. jasmine), straight glucose (100) |
A low-GI food will release glucose more slowly and steadily. A high-GI food causes a more rapid rise in blood glucose levels and is suitable for energy recovery after endurance exercise or for a person experiencing hypoglycemia.
The glycemic effect of foods depends on a number of factors such as the type of starch (amylose versus amylopectin), physical entrapment of the starch molecules within the food, fat and protein content of the food and organic acids or their salts in the meal — adding vinegar, for example, will lower the GI. The presence of fat or soluble dietary fiber can slow the gastric emptying rate, thus lowering the GI. In general, unrefined breads with higher amounts of fiber have a lower GI value than white breads.[4] Many brown breads, however, are treated with enzymes to soften the crust, which makes the starch more accessible (high GI).
While adding butter or oil will lower the GI of a meal, the GI ranking does not change. That is, with or without additions, there is still a higher blood glucose curve after white bread than after a low-GI bread such as pumpernickel.
The glycemic index can be applied only to foods with a reasonable carbohydrate content, as the test relies on subjects consuming enough of the test food to yield about 50 g of available carbohydrate. Many fruits and vegetables (but not potatoes) contain very little carbohydrate per serving, and the average person is not likely to eat 50 g of carbohydrate from these foods. Fruits and vegetables tend to have a low glycemic index and a low glycemic load. This also applies to carrots, which were originally and incorrectly reported as having a high GI.[5] Alcoholic beverages have been reported to have low GI values, but it should be noted that beer has a moderate GI. Recent studies have shown that the consumption of an alcoholic drink prior to a meal reduces the GI of the meal by approximately 15%.[6] Moderate alcohol consumption more than 12 hours prior to a test does not affect the GI.[7]
Many modern diets rely on the glycemic index, including the South Beach Diet, Transitions by Market America and NutriSystem Nourish Diet.[8] However, others have pointed out that foods generally considered to be unhealthy can have a low glycemic index, for instance chocolate cake (GI 38), ice cream (37), or pure fructose (19), whereas foods like potatoes and rice, eaten in countries with low rates of diabetes, have GIs around 100.[9][10]
The GI Symbol Program is an independent worldwide GI certification program that helps consumers identify low-GI foods and drinks. The symbol is only on foods or beverages that have had their GI values tested according to standard and meet the GI Foundation's certification criteria as a healthy choice within their food group, so they are also lower in kilojoules, fat and/or salt.
Several lines of recent scientific evidence have shown that individuals who followed a low-GI diet over many years were at a significantly lower risk for developing both type 2 diabetes and coronary heart disease than others. High blood glucose levels or repeated glycemic "spikes" following a meal may promote these diseases by increasing oxidative stress to the vasculature and also by the direct increase in insulin levels.[11]
In the past, postprandial hyperglycemia has been considered a risk factor associated mainly with diabetes. However, more recent evidence shows that it also presents an increased risk for atherosclerosis in the non-diabetic population.[12]
On the converse, there are regions such as Peru and Asia, where people eat high-glycemic index foods such as potatoes and high-GI rices, but without a high level of obesity or diabetes.[9] The high consumption of legumes in South America and fresh fruit and vegetables in Asia likely lowers the glycemic effect in these individuals. The mixing of high- and low-GI carbohydrates produces moderate GI values.
A study from the University of Sydney in Australia suggests that having a breakfast of white bread and sugar-rich cereals, over time, may make a person susceptible to diabetes, heart disease, and even cancer.[13]
A study published in the American Journal of Clinical Nutrition found that Age-related Adult Macular Degeneration (AMD) is higher in 42 percent in those with a high-GI diet and concluded that eating a lower-GI diet would eliminate 20 percent of AMD cases.[14]
The glycemic index is supported by leading international health organisations including the American Diabetes Association.[15]
Recent animal research provides compelling evidence that high-GI carbohydrate is associated with increased risk of obesity. In human trials, it is difficult to separate the effects from GI and other potentially confounding factors such as fiber content, palatability, and compliance. In one study,[16] male rats were split into high- and low-GI groups over 18 weeks while mean body weight was maintained. Rats fed the high-GI diet were 71% fatter and had 8% less lean body mass than the low-GI group. Postmeal glycemia and insulin levels were significantly higher, and plasma triglycerides were threefold greater in the high-GI-fed rats. Furthermore, pancreatic islet cells suffered "severely disorganised architecture and extensive fibrosis." However, the GI of these diets was not experimentally determined. Because high-amylose cornstarch (the major component of the assumed low-GI diet) contains large amounts of resistant starch, which is not digested and absorbed as glucose, the lower glycemic response and possibly the beneficial effects can be attributed to lower energy density and fermentation products of the resistant starch, rather than the GI. Therefore, it is crucial not to confound between low-GI diets, which have been appropriately tested using the approved GI methodology, and low-glycemic diets, which elicit low glycemic response not necessarily because they have a low GI. In addition, some human studies show that lowering the glycemic load and glycemic index of weight reduction provides no added benefit to energy restriction in promoting weight loss in obese subjects. [17]
If a person consumes 50% of his or her calories from carbohydrates, the glycemic index can enable him or her to consume the same number of calories and have lower, more stable glucose and insulin levels. The use of the glycemic index, however, is limited by several factors:
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