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The glycemic (glycaemic) index (GI; ) is a number from 0 to 100 assigned to a food, with pure glucose arbitrarily given the value of 100, which represents the relative rise in the blood glucose level two hours after consuming that food. The GI of a specific food depends primarily on the quantity and type of
carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...
it contains, but is also affected by the amount of entrapment of the carbohydrate molecules within the food, the fat and protein content of the food, the amount of organic acids (or their salts) in the food, and whether it is cooked and, if so, how it is cooked. GI tables, which list many types of foods and their GIs, are available. A food is considered to have a ''low GI'' if it is 55 or less; ''high GI'' if 70 or more; and ''mid-range GI'' if 56 to 69. The term was introduced in 1981 by David J. Jenkins and co-workers. It is useful for quantifying the relative rapidity with which the body breaks down carbohydrates. It takes into account only the available carbohydrate (total carbohydrate minus
fiber Fiber or fibre (from la, fibra, links=no) is a natural or artificial substance that is significantly longer than it is wide. Fibers are often used in the manufacture of other materials. The strongest engineering materials often incorporate ...
) in a food. Glycemic index does not predict an individual's glycemic response to a food, but can be used as a tool to assess the insulin response burden of a food, averaged across a studied population. Individual responses vary greatly. The glycemic index is usually applied in the context of the quantity of the food and the amount of carbohydrate in the food that is actually consumed. A related measure, the
glycemic load The glycemic load (GL) of food is a number that estimates how much the food will raise a person's blood glucose level after eating it. One unit of glycemic load approximates the effect of eating one gram of glucose. Glycemic load accounts for how ...
(GL), factors this in by multiplying the glycemic index of the food in question by the carbohydrate content of the actual serving.


Measurement

The glycemic index of a food is defined as the incremental area under the two-hour blood glucose response curve ( AUC) following a 12-hour fast and ingestion of a food with a certain quantity of available 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. Foods with carbohydrates that break down quickly during digestion and release glucose rapidly into the
bloodstream The blood circulatory system is a system of organs that includes the heart, blood vessels, and blood which is circulated throughout the entire body of a human or other vertebrate. It includes the cardiovascular system, or vascular system, tha ...
tend to have a high GI; foods with carbohydrates that break down more slowly, releasing glucose more gradually into the bloodstream, tend to have a low GI. A lower glycemic index suggests slower rates of digestion and absorption of the foods' carbohydrates and can also indicate greater extraction from the liver and periphery of the products of carbohydrate digestion. 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. A disadvantage with this system is that the reference food is not well-defined, because there is no universal standard for the carbohydrate content of white bread.


Accuracy

Glycemic index charts often give only one value per food, but variations are possible due to: * Ripeness – riper fruits contain more sugars, increasing GI * Cooking methods – the more cooked, or overcooked, a food, the more its cellular structure is broken, with a tendency for it to digest quickly and raise blood glucose more * Processing – e.g., flour has a higher GI than the whole grain from which it is ground as grinding breaks the grain's protective layers and the length of storage. Potatoes are a notable example, ranging from moderate to very high GI even within the same variety. More importantly, the glycemic response is different from one person to another, and also in the same person from day to day, depending on blood glucose levels,
insulin resistance Insulin resistance (IR) is a pathological condition in which cells fail to respond normally to the hormone insulin. Insulin is a hormone that facilitates the transport of glucose from blood into cells, thereby reducing blood glucose (blood sugar ...
, and other factors. The glycemic index only indicates the impact on glucose level two hours after eating the food. People with diabetes have elevated levels for four hours or longer after eating certain foods.


Grouping

GI values can be interpreted intuitively as percentages on an absolute scale and are commonly interpreted as follows: A low-GI food will cause blood glucose levels to increase more slowly and steadily, which leads to lower postprandial (after meal) blood glucose readings. A high-GI food causes a more rapid rise in blood glucose level and is suitable for energy recovery after exercise or for a person experiencing
hypoglycemia Hypoglycemia, also called low blood sugar, is a fall in blood sugar to levels below normal, typically below 70 mg/dL (3.9 mmol/L). Whipple's triad is used to properly identify hypoglycemic episodes. It is defined as blood glucose bel ...
. The glycemic effect of foods depends on various factors, such as the type of
starch Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human die ...
(
amylose Amylose is a polysaccharide made of α-D-glucose units, bonded to each other through α(1→4) glycosidic bonds. It is one of the two components of starch, making up approximately 20–30%. Because of its tightly packed helical structure, amylose ...
versus
amylopectin Amylopectin is a water-insoluble polysaccharide and highly branched polymer of α-glucose units found in plants. It is one of the two components of starch, the other being amylose. Plants store starch within specialized organelles called amyloplas ...
), 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. The presence of fat or soluble dietary fiber can slow the gastric emptying rate, thus lowering the GI. In general, coarse, grainy breads with higher amounts of fiber have a lower GI value than white breads. 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 have GIs around 100 but are commonly eaten in some countries with low rates of diabetes.


Application


Weight control

Dietary replacement of saturated fats by carbohydrates with a low glycemic index may be beneficial for
weight control Obesity is a medical condition, sometimes considered a disease, in which excess body fat has accumulated to such an extent that it may negatively affect health. People are classified as obese when their body mass index (BMI)—a person's w ...
, whereas substitution with refined, high glycemic index carbohydrates is not. A
Cochrane review Cochrane (previously known as the Cochrane Collaboration) is a British international charitable organisation formed to organise medical research findings to facilitate evidence-based choices about health interventions involving health professi ...
found that adoption of low glycemic index (or load) diets by people who are overweight or obese leads to more weight loss (and better fat control) than use of diets involving higher glycemic index/load or other strategies. Benefits were apparent even with low glycemic index/load diets that allow people to eat as much as they like. The authors of the review concluded that "Lowering the glycaemic load of the diet appears to be an effective method of promoting weight loss and improving lipid profiles and can be simply incorporated into a person's lifestyle." In clinical management of obesity, diets based on a low glycemic index/load appear to provide better glycemic and inflammatory control than ones based on a high glycemic index/load (and therefore could potentially be more effective in preventing obesity-related diseases). In overweight and obese children, adoption of low glycemic index/load diets may not lead to weight loss but might potentially provide other benefits.


Limitations


Compared to quantity of carbohydrate

Depending on quantities, the number of grams of carbohydrate in a food can have a bigger impact on blood sugar levels than the glycemic index does. Consuming less dietary energy, losing weight, and
carbohydrate counting Carbohydrate counting or “carb” counting is a meal planning tool used in diabetes management to help optimize blood sugar control. It can be used with or without the use of insulin therapy. Carbohydrate counting involves determining whether a fo ...
can be better for lowering the blood sugar level. Carbohydrates impact glucose levels most profoundly, and two foods with the same carbohydrate content are, in general, comparable in their effects on blood sugar. A food with a low glycemic index can have a high carbohydrate content or vice versa; this can be accounted for with the
glycemic load The glycemic load (GL) of food is a number that estimates how much the food will raise a person's blood glucose level after eating it. One unit of glycemic load approximates the effect of eating one gram of glucose. Glycemic load accounts for how ...
(GL) where .


Compared to insulin index

While the glycemic index of foods is used as a guide to the rise in blood glucose that should follow meals containing those foods, actual increases in blood glucose show considerable variability from person to person, even after consumption of identical meals. This is in part because glycemic index does not take into account other factors besides glycemic response, such as insulin response, which is measured by the
insulin index The insulin index of food represents how much it elevates the concentration of insulin in the blood during the two-hour period after the food is ingested. The index is similar to the glycemic index (GI) and glycemic load (GL), but rather than relyi ...
and can be more appropriate in representing the effects from some food contents other than carbohydrates. In particular, since it is based on the area under the curve of the glucose response over time from ingesting a subject food, the shape of the curve has no bearing on the corresponding GI value. The glucose response can rise to a high level and fall quickly, or rise less high but remain there for a longer time, and have the same area under the curve.


See also

*
Diabetic diet A diabetic diet is a diet that is used by people with diabetes mellitus or high blood sugar to minimize symptoms and dangerous complications of long-term elevations in blood sugar ( i.e.: cardiovascular disease, kidney disease, obesity). Among ...
*
Disposition index Disposition index (DI) is the product of insulin sensitivity times the amount of insulin secreted in response to blood glucose levels. Insulin resistant individuals can maintain normal responses to blood glucose due to the fact that higher levels o ...
*
Glycemic efficacy Glycemic efficacy refers to the capacity of regulated glycemic levels to produce an effect in people with diabetes and heart disease. According to Zeev Vlodaver, Robert F. Wilson and Daniel J. Garry, "exenatide and liraglutide are synthetic GLP-1 ...
*
Low glycemic index diet Low-carbohydrate diets restrict carbohydrate consumption relative to the average diet. Foods high in carbohydrates (e.g., sugar, bread, pasta) are limited, and replaced with foods containing a higher percentage of fat and protein (e.g., meat, ...
*
Montignac diet The Montignac diet is a high-protein low-carbohydrate fad diet that was popular in the 1990s, mainly in Europe. It was invented by Frenchman Michel Montignac (1944–2010), an international executive for the pharmaceutical industry, who, like his ...
*
Overall nutritional quality index The overall nutritional quality index was a nutritional rating system developed at the Yale-Griffin Prevention Research Center in 2008. A proprietary algorithm assigned foods a score between 1 and 100 intended to reflect the overall nutritional va ...


References


External links

* {{cite web , author1=Human Nutrition Unit , author2=School of Molecular Bioscience , title=Glycemic Index , work=International GI database , publisher=University of Sydney , url=http://www.glycemicindex.com/foodSearch.php , ref={{harvid, Glycemic Index – Searchable database of over 2600 foods with their glycemic index and load values.
List of low GI foods

Glycemic Index and Glycemic Load calculator
Total values for any combinations of foods and any number of servings are computed according t

. Diabetes Nutrition Endocrinology