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 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 and
blood lipids. The
insulin index is also useful for providing 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.
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.
Glycemic index of foods
GI values can be interpreted intuitively as percentages on an absolute scale and are commonly interpreted as follows:
A low-GI food will release glucose more slowly and steadily, which leads to more suitable postprandial (after meal) blood glucose readings.
A high-GI food causes a more rapid rise in blood glucose levels and is suitable for energy recovery after 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. 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. 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%. Moderate alcohol consumption more than 12 hours prior to a test does not affect the GI.
Many modern diets rely on the glycemic index, including the South Beach Diet, Transitions by Market America and NutriSystem Nourish Diet. 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.
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.
Disease prevention
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.
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.
Conversely, there are areas 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.
A study published in the American Journal of Clinical Nutrition found that age-related adult macular degeneration (AMD), which leads to blindness, 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.
The American Diabetes Association supports glycemic index but warns that the total amount of carbohydrate in the food is still the strongest and most important indicator, and that everyone should make their own custom method that works better for them.
Weight control
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, 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.
Limitations and criticisms
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:
The glycemic index does not take into account other factors besides glycemic response, such as insulin response, which is measured by the insulin index and can be more appropriate in representing the effects from some food contents other than carbohydrates.
The glycemic index is significantly altered by the type of food, its ripeness, processing, the length of storage, cooking methods, and its variety (white potatoes are a notable example, ranging from moderate to very high GI even within the same variety).
The glycemic response is different from one person to another, and even in the same person from day to day, depending on blood glucose levels, insulin resistance, and other factors. and two foods with the same carbohydrate content are, in general, comparable in their effects on blood sugar.
Most of the values on the glycemic index do not show the impact on glucose levels after two hours. Some diabetics may still have elevated levels after four hours.
The GI of foods is determined under experimental conditions after an overnight fast, and might not apply to foods consumed later during the day because glycemic response is strongly influenced by the composition of the previous meal, particularly when meals are consumed within an interval of few hours. Indeed, it has been shown that a high-GI breakfast cereal (GI = 124) elicited a lower increase in blood glucose concentrations at lunch than at breakfast. Also, the difference in glycemic responses induced by the low- and the high-GI breakfast cereals at lunch were lower than that predicted by the large difference in their GI, which was determined at breakfast.
See also
Diabetic diet
Glycemic load
Insulin index
Low glycemic index diet
Resistant starch
Notes
External links
GI Symbol Program The GI Foundation
Gl News University of Sydney
"International table of glycemic index and glycemic load values: 2002" American Journal of Clinical Nutrition
Michel Montignac: Glycemic Index
Glycemic Index Thefoodfarce.com
Category:Diabetes
Category:Nutrition
Category:Endocrinology
