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Glycemic index

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The glycemic index, glycaemic index, or GI is a measure of the effects of carbohydrates in food on blood sugar levels. It estimates how much each gram of available carbohydrate (total carbohydrate minus fiber) in a food raises a person's blood glucose level following consumption of the food, relative to consumption of glucose.[1] Glucose has a glycemic index of 100, by definition, and other foods have a lower glycemic index.

Glycemic index is defined for each type of food, independent of the amount of food consumed. Glycemic load accounts for the amount of food consumed and is calculated in terms of glycemic index.

Determining the GI of a food

Foods with carbohydrates that break down quickly during digestion and release glucose rapidly into the bloodstream 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. The concept was developed by Dr. David J. Jenkins and colleagues[2] 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[3] 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 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.[4][5]

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.[citation needed]

Glycemic index of foods

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, legumes/pulses, some whole, intact grains, nuts, fructose, kidney beans, beets, chick peas
Medium GI 56–69 whole wheat products, basmati rice, sweet potato, sucrose, baked potatoes
High GI 70 and above white bread, most white rices, corn flakes, extruded breakfast cereals, glucose, maltose, maltodextrins

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, coarse, grainy breads with higher amounts of fiber have a lower GI value than white breads.[6] However, most breads made with 100% wholewheat or wholemeal flour have a GI similar to white bread.[7] Many brown breads are treated with enzymes to soften the crust, which makes the starch more accessible (high GI).

While adding fat or protein will lower the glycemic response to a meal, the relative differences remain. That is, with or without additions, there is still a higher blood glucose curve after a high GI bread than after a low-GI bread such as pumpernickel.

The glycemic index can be applied only to foods with a substantial carbohydrate content, as the test relies on subjects consuming an amount of the test food containing 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. This also applies to carrots, which were originally and incorrectly reported as having a high GI.[8] 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%.[9] Moderate alcohol consumption more than 12 hours prior to a test does not affect the GI.[10]

Many modern diets rely on the glycemic index, including the South Beach Diet, Transitions by Market America and NutriSystem Nourish Diet.[11] 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.[12][13]

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[when?] 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, coronary heart disease, and age-related macular degeneration than others.[14] High blood glucose levels or repeated glycemic "spikes" following a meal may promote these diseases by increasing systemic glycative stress other oxidative stress to the vasculature and also by the direct increase in insulin levels.[15] The glycative stress sets up a vicious cycle of systemic protein glycation, compromised protein editing capacity involving the ubiquitin proteolytic pathway and autophagic pathways,leading to enhanced accumulation of glycated and other obsolete proteins.[16]

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[17] and that high GI diets[18] and high blood-sugar levels more generally[19] and[20] are related to kidney disease as well.

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.[12] 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.[21]

A study published in the American Journal of Clinical Nutrition found that age-related adult macular degeneration (AMD), which leads to blindness, is 42 percent higher among people with a high-GI diet, and concluded that eating a lower-GI diet would eliminate 20 percent of AMD cases.[22]

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 best for them.[23][24]

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,[25] 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 low-GI diets — which have been appropriately tested using the approved GI methodology — with 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.[26][unreliable source?]

Accuracy

Glycemic index charts often give only one value per food, but variations are possible due to 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[27]).[28]

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.[28]

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.[28]

Other factors

The number of grams of carbohydrate can have a bigger impact than glycemic index on blood sugar levels, depending on quantities. Consuming fewer calories, losing weight, and carbohydrate counting can be better for lowering the blood sugar level.[28] Carbohydrates impact glucose levels most profoundly,[29] and two foods with the same carbohydrate content are, in general, comparable in their effects on blood sugar.[29] 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.

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 a 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.[citation needed]

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.[30]

See also

Notes

  1. ^ Glycemic Research Institute: Glycemic Index Defined
  2. ^ DJ Jenkins et al. (1981). "Glycemic index of foods: a physiological basis for carbohydrate exchange." Am J Clin Nutr 34; 362–366
  3. ^ David J. A. Jenkins et al. "Effect of a Low–Glycemic Index or a High–Cereal Fiber Diet on Type 2 Diabetes". JAMA. 2008;300(23):2742-2753.
  4. ^ Brouns et al. (2005). "Glycaemic index methodology." Nutrition Research Reviews 18; 145–171
  5. ^ Glycemic Research Institute: Glycemic Index Defined
  6. ^ http://www.norden.org/en/publications/publications/2005-589 Nordic Council of Ministers: Glycemic Index, TemaNord2005:589, Copenhagen 2005.
  7. ^ name='Atkinson F, Foster-Powell K, Brand-Miller J. International tables of glycemic index and glycemic load values, 2008. Diabetes Care. 2008;31:2281-3.
  8. ^ Brand-Miller et al. (2005). The Low GI Diet Revolution: The Definitive Science-based Weight Loss Plan. Marlowe & Company. New York, NY
  9. ^ Brand-Miller, in press
  10. ^ Godley R, et al. (2008). Eur J Clin Nutr
  11. ^ Nutrisystem
  12. ^ a b John A. McDougall, "The McDougall Newsletter", June 2006.
  13. ^ "International table of glycemic index and glycemic load values: 2002", American Journal of Clinical Nutrition
  14. ^ Chiu CJ, Liu S, Willett WC, Wolever TM, Brand-Miller JC, Barclay AW, Taylor A., Informing Food Choices and Health Outcomes by Use of The Dietary Glycemic Index., Nutr Rev., 2011 ;69(4): 231-42
  15. ^ Temelkova-Kurktschiev TS, Koehler C, Henkel E, Leonhardt W, Fuecker K, Hanefeld M., Postchallenge plasma glucose and glycemic spikes are more strongly associated with atherosclerosis than fasting glucose or HbA1c level., Diabetes Care. 2000 Dec;23(12):1830-4.
  16. ^ Uchiki T, Weikel KA, Jiao W, Shang F, Caceres A, Pawlak D, Handa JT, Brownlee M, Nagaraj R, Taylor A., Glycation-altered proteolysis as a pathobiologic mechanism that links dietary glycemic index, aging, and age-related disease (in nondiabetics)., Aging Cell. 2012 Feb;11(1):1-13. doi: 10.1111/j.1474-9726.2011.00752.x. Epub 2011 Nov 15.
  17. ^ Balkau et al. (1998) "High blood glucose concentration is a risk factor for mortality in middle-aged nondiabetic men. 20-year follow-up in the Whitehall Study, the Paris Prospective Study, and the Helsinki Policemen Study." Diabetes Care 1998 Mar;21(3):360-7
  18. ^ Allan L said at October 23, 2006 6:20 AM: (2006-10-21). "High Glycemic Index Diet Kidney Cancer Risk?". FuturePundit. Retrieved 2012-02-21.{{cite web}}: CS1 maint: extra punctuation (link) CS1 maint: numeric names: authors list (link)
  19. ^ http://www.diabetes.org%2Fliving-with-diabetes%2Fcomplications%2Fkidney-disease-nephropathy.html
  20. ^ "Diabetes and kidney failure | Better Health Channel". Betterhealth.vic.gov.au. Retrieved 2012-02-21.
  21. ^ White bread breakfast unhealthy? The Times of India, 10 Mar 2008.
  22. ^ Chiu C-J, et al. Association between dietary glycemic index and age-related macular degeneration in nondiabetic participants in the Age-Related Eye Disease Study. Am J Clin Nutr 2007 86: 180-188.
  23. ^ Sheard et al. (2004). "Dietary carbohydrate (amount and type) in the prevention and management of diabetes: a statement by the american diabetes association." Diabetes Care;27(9):2266-71
  24. ^ Glycemic Index and Diabetes, American Diabetes Association, retrieved 8 June 2011
  25. ^ Pawlak et al. (2004). "Effects of dietary glycaemic index on adiposity, glucose homoeostasis, and plasma lipids in animals." Lancet;28364(9436):778-85
  26. ^ The Glycemic Index and Weight Loss
  27. ^ GI Database.
  28. ^ a b c d Freeman, Janine (2005). "The Glycemic Index debate: Does the type of carbohydrate really matter?". Diabetes Forecast. Archived from the original on February 14, 2007. {{cite journal}}: Unknown parameter |month= ignored (help)
  29. ^ a b The Glycemic Index and Diabetes. Joslin Diabetes Center.
  30. ^ David Mendosa. Insulin Index. July 13, 2003.
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