Glycemic index GI / Index glycémique IG

Glycemic index

The glycemic indexglycaemic index, or GI is a measure of the effects of carbohydrates on blood sugar levels.[citation needed] 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 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.[2]

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.

Contents

Glycemic index of foods

GI values can be interpreted intuitively as percentages on an absolute scale and are commonly interpreted as follows:

ClassificationGI rangeExamples
Low GI55 or lessmost fruits and vegetables (except potatoes and watermelon), grainy breadspastalegumes/pulsesmilkyogurt, products extremely low in carbohydrates (some cheesesnuts), fructose
Medium GI56–69whole wheat products, basmati ricesweet potatotable sugar
High GI70 and abovecorn flakesrice krispies, baked potatoeswatermeloncroissantswhite breadextruded 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 — addingvinegar, for example, will lower the GI. The presence of fat or soluble dietary fiber can slow the gastric emptying rate, thus lowering the GI. Unrefined breads with higher amounts of fiber generally have a lower GI value than white breads.[3] 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.[4] 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%.[5] Moderate alcohol consumption more than 12 hours prior to a test does not affect the GI.[6]

Many modern diets rely on the glycemic index, including the South Beach Diet, Transitions by Market America and NutriSystem Nourish Diet.[7] However, others have pointed out that foods generally considered to be unhealthy can have a low glycemic index, for instancechocolate 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.[8][9]

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.[10]

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.[11]

Conversely, 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.[8] 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.[12]

The glycemic index is supported by leading international health organisations including the American Diabetes Association.[13]

Weight control

Recent animal research provides compelling evidence that high GI carbohydrate is associated with increased risk of obesity. In human trials, it is typically difficult to separate the effects from GI and other potentially confounding factors such as fiber content, palatability, and compliance. In one study,[14] 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. It is therefore 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.

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.[15]
  • 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[16]).[17]
  • 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.[17]
  • The number of grams of carbohydrate impacts blood sugar levels more than the glycemic index. Lowering glycemic index leads to small improvements in the blood sugar level, but consuming fewer calories, losing weight, and carbohydrate counting would benefit the blood sugar level more.[17] Carbohydrate impacts glucose levels most profoundly,[18] and two foods with the same carbohydrate content are generally comparable in their effects on blood sugar.[18] A food with a low glycemic index may have a high carbohydrate content or vice versa; this can be accounted for with the glycemic load. Consuming carbohydrates with a low glycemic index and calculating carbohydrate intake would produce the most stable blood sugar levels.
  • 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.[17]
  • 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.
http://en.wikipedia.org/wiki/Glycemic_index

Index glycémique

L'index glycémique est un critère de classement des aliments contenant des glucides, basé sur leurs effets sur la glycémie (taux de glucose dans le sang) durant les deux heures suivant leur ingestion. Il permet de comparer le pouvoir glycémiant de chaque aliment, mesuré directement lors de la digestion. L'index glycémique d'un aliment est donné par rapport à un aliment de référence, auquel on attribue l'indice 100 (généralement glucose pur ou "pain blanc"). L'index glycémique a été inventé par l'équipe de David J. Jenkins à l'Université de Toronto en 1981.

Sommaire

Calcul de l'index glycémique d'un aliment

S : la surface sous la courbe, ou surface du triangle

On donne à plusieurs volontaires une masse calibrée d'un certain aliment, de telle sorte que la quantité totale de glucides ingérés soit fixe (50g de glucide par exemple). Ceci permet de comparer plusieurs aliments qui ne possèdent pas la même proportion de glucides dans leur composition. On établit ensuite la courbe du taux de glucose dans le sang au cours des deux heures qui suivent l'ingestion.

Par ailleurs, on dispose de courbes pré-établies pour des aliments de référence:

Afin de calculer l'index glycémique d'un aliment, on rapporte la surface sous la courbe correspondant à l'aliment étudié, avec celle de l'aliment de référence :

IG = \frac{S_\mathrm{alim}}{S_\mathrm{ref}} \times 100

  • IG : index glycémique de l'aliment considéré;
  • Salim : surface sous la courbe glycémie/temps de l'aliment considéré;
  • Sref : surface sous la courbe glycémie/temps de la référence.

Par le terme surface sous la courbe on désigne souvent la surface du triangle formé par l'axe de la valeur normale de la glycémie (1 g⋅L-1) et par la courbe. C'est pourquoi on appelle cette technique de calcul méthode du triangle.

Ainsi, l'aliment de référence (le glucose ou le pain blanc) a toujours un index glycémique de 100.

Pour passer d'une mesure prenant pour référence le pain blanc dans le système ayant pour référence le glucose, il faut multiplier celle-ci par 0,7.

Valeurs d'index glycémique et interprétation

Voici quelques exemples d'index glycémiques d'aliments courants. La classification des index glycémiques en faibleélevé et moyen n'est pas la même selon les auteurs, de même que l'index glycémique d'un aliment n'a pas une valeur strictement définie. Cependant, les valeurs indiquées donne une bonne indication du pouvoir glycémiant de chaque aliment.

Exemple de classificationIntervalle de valeurs d'IGAliments
Index glycémique faibleinférieur à 55La plupart des fruits frais et légumes verts, légumes secscéréales en grains, chocolat noir riche en cacao, lait et produits laitiersviandesoléagineux...
Index glycémique moyenEntre 56 et 69Produits à base de céréales complètes, bananesabricots secs, pommes-de-terre à l'eau ou la vapeur
Index glycémique élevéPlus de 70Pain blanc, riz blanc, pommes-de-terre frites ou au four ou chips, confiseries, barres chocolatées, carottes, pastèque

Plus l'index glycémique d'un aliment est élevé, plus le taux de glucose s'élève rapidement dans le sang après sa digestion. Le système de l'index glycémique permet donc, par exemple, aux personnes diabétiques de surveiller leur alimentation, de prévoir celle-ci en fonction de leurs besoins.

Les glucides étaient autrefois classés de façon théorique en sucres lents et sucres rapides selon leur complexité moléculaire. Cette classification est cependant fausse et ne doit plus être utilisée aujourd'hui.

Les limites de ce système

Facteurs de variation de pouvoir glycémiant

Il existe de nombreux facteurs faisant varier le pouvoir glycémiant d'un aliment.

  • L'état physique de l'aliment (solide ou liquide) : un même aliment sous ces deux formes n'a pas le même index glycémique (ex. pomme fruit/jus de pomme).
  • Le mode de cuisson : la façon dont les aliments ont été cuits (la température, la durée, les éventuelles matières grasses) peut faire varier leur pouvoir glycémiant. (La purée de pommes de terre par exemple a un IG d'environ 70 alors que les pommes de terre cuites au four peuvent avoir un IG allant jusqu'à 90)
  • Le mode de consommation : la façon selon laquelle sont consommés les aliments fait aussi varier leur pouvoir glycémiant. De façon générale, des sucres consommés au cours d'un repas, notamment en présence de légumes verts, présentent une action sur la glycémie plus faible que les valeurs tabulaires.

Il faut donc toujours chercher des tables qui donnent l'index glycémique de la façon la plus précise possible et pour un aliment qui a la forme qui se rapproche le plus de celui qui se trouve dans l'assiette.

Indication de la charge glycémique

L'index glycémique reflète la quantité de glucide absorbée par le corps et est calculée pour une quantité de glucide standardisée. Mais il ne représente pas du tout la quantité de glucide ingéré quand on mange une portion habituelle d'une certain aliment. Par exemple, lapastèque a un index glycémique de 75, mais elle contient en fait très peu de sucre, et donc une portion habituelle raisonnable de pastèque ne provoquera en fait pas de forte augmentation de la glycémie.

Pour remédier en partie à ce problème, on a introduit la notion de charge glycémique, qui multiplie l'index glycémique par la teneur totale en glucide de l'aliment:

 CG = IG \times \frac{m_\mathrm{glucides}}{m_\mathrm{aliment}} .

D'autres index présentés comme plus pertinents ont été établis, notamment l'index insulinique et l'index de fructose. L'index glycémique étant basé sur le taux de glucose sanguin, il ne tient pas compte des autres monosaccharides circulants et métabolisés. Le fructose présente un cas intéressant. En effet, certains scientifiques suggèrent l'essai d'un index de fructose, puisque ce dernier, en doses élevées, entraîne une dysfonction hépatique (qui peut mener à une acidose métabolique) mais également pourrait contribuer à la résistance à l'insuline (études statistiques chez l'homme et plusieurs études chez les rongeurs). Cette nouvelle tournure pourrait discréditer la méthode de Montignac, réductrice quant aux effets du fructose qu'elle élève à un stade de sucre davantage bénéfique.

http://fr.wikipedia.org/wiki/Index_glycemique

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