Glycemic index

Glycemic index

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The glycemic index, glycaemic index, or GI is a measure of the effects of carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...

s on blood sugar
Blood sugar
The blood sugar concentration or blood glucose level is the amount of glucose present in the blood of a human or animal. Normally in mammals, the body maintains the blood glucose level at a reference range between about 3.6 and 5.8 mM , or 64.8 and 104.4 mg/dL...

 levels. Carbohydrates that break down quickly during digestion and release glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

 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
David J. Jenkins
David J.A. Jenkins is a British-born University Professor in the department of Nutritional Sciences at the University of Toronto, Canada....

 and colleagues in 1980–1981 at the University of Toronto
University of Toronto
The University of Toronto is a public research university in Toronto, Ontario, Canada, situated on the grounds that surround Queen's Park. It was founded by royal charter in 1827 as King's College, the first institution of higher learning in Upper Canada...

 in their research to find out which foods were best for people with diabetes
Diabetes mellitus
Diabetes mellitus, often simply referred to as diabetes, is a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin that is produced...

.
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
Blood lipids
Blood lipids are lipids in the blood, either free or bound to other molecules. They are mostly transported in a protein capsule, and the density of the lipids and type of protein determines the fate of the particle and its influence on metabolism. The concentration of blood lipids depends on...

. The insulin index
Insulin index
The Insulin Index is a measure used to quantify the typical insulin response to various foods. The index is similar to the Glycemic Index and Glycemic Load, but rather than relying on blood glucose levels, the Insulin Index is based upon blood insulin levels...

 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
Staple food
A staple food is one that is eaten regularly and in such quantities that it constitutes a dominant portion of a diet, and that supplies a high proportion of energy and nutrient needs. Most people live on a diet based on one or more staples...

 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:
Classification GI range Examples
Low GI 55 or less most fruits and vegetables, legumes/pulses, whole grains, nut
Nut (fruit)
A nut is a hard-shelled fruit of some plants having an indehiscent seed. While a wide variety of dried seeds and fruits are called nuts in English, only a certain number of them are considered by biologists to be true nuts...

s, fructose
Fructose
Fructose, or fruit sugar, is a simple monosaccharide found in many plants. It is one of the three dietary monosaccharides, along with glucose and galactose, that are absorbed directly into the bloodstream during digestion. Fructose was discovered by French chemist Augustin-Pierre Dubrunfaut in 1847...

 and products low in carbohydrates
Medium GI 56–69 whole wheat products, basmati rice, sweet potato
Sweet potato
The sweet potato is a dicotyledonous plant that belongs to the family Convolvulaceae. Its large, starchy, sweet-tasting, tuberous roots are an important root vegetable. The young leaves and shoots are sometimes eaten as greens. Of the approximately 50 genera and more than 1,000 species of...

, sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...

, baked potato
Potato
The potato is a starchy, tuberous crop from the perennial Solanum tuberosum of the Solanaceae family . The word potato may refer to the plant itself as well as the edible tuber. In the region of the Andes, there are some other closely related cultivated potato species...

es
High GI 70 and above white bread
White bread
White bread is made from wheat flour from which the bran and the germ have been removed through a process known as milling. Milling gives white flour a longer shelf life by removing the bran which contains oil, allowing products made with it, like white bread, the ability to survive storage and...

, most white rice
White rice
White rice is the name given to milled rice that has had its husk, bran, and germ removed. This alters the flavour, texture and appearance of the rice and helps prevent spoilage and extend its storage life. After milling, the rice is polished, resulting in a seed with a bright, white, shiny...

s, corn flakes
Corn flakes
Corn flakes are a popular breakfast cereal originally manufactured by Kellogg's through the treatment of maize. A patent for the product was filed on May 31, 1895, and issued on April 14, 1896.-History:...

, extruded breakfast cereals, glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...

, maltose
Maltose
Maltose , or malt sugar, is a disaccharide formed from two units of glucose joined with an αbond, formed from a condensation reaction. The isomer "isomaltose" has two glucose molecules linked through an α bond. Maltose is the second member of an important biochemical series of glucose chains....



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
Hypoglycemia
Hypoglycemia or hypoglycæmia is the medical term for a state produced by a lower than normal level of blood glucose. The term literally means "under-sweet blood"...

.

The glycemic effect of foods depends on a number of factors such as the type of starch
Starch
Starch or amylum is a carbohydrate consisting of a large number of glucose units joined together by glycosidic bonds. This polysaccharide is produced by all green plants as an energy store...

 (amylose
Amylose
Amylose is a linear polymer made up of D-glucose units.This polysaccharide is one of the two components of starch, making up approximately 2-30% of the structure...

 versus amylopectin
Amylopectin
Amylopectin is a soluble polysaccharide and highly branched polymer of glucose found in plants. It is one of the two components of starch, the other being amylose.Glucose units are linked in a linear way with α glycosidic bonds...

), 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
Vinegar
Vinegar is a liquid substance consisting mainly of acetic acid and water, the acetic acid being produced through the fermentation of ethanol by acetic acid bacteria. Commercial vinegar is produced either by fast or slow fermentation processes. Slow methods generally are used with traditional...

, for example, will lower the GI. The presence of fat or soluble dietary fiber
Dietary fiber
Dietary fiber, dietary fibre, or sometimes roughage is the indigestible portion of plant foods having two main components:* soluble fiber that is readily fermented in the colon into gases and physiologically active byproducts, and* insoluble fiber that is metabolically inert, absorbing water as it...

 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 enzyme
Enzyme
Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

s to soften the crust, which makes the starch more accessible (high GI).

While adding lipids 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
Pumpernickel
Pumpernickel is a type of very heavy, slightly sweet rye bread traditionally made with coarsely ground rye. It is often made with a combination of rye flour and whole rye berries. It has been long associated with the Westphalia region of Germany. The first written mention of the black bread of...

.

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
Glycemic load
The glycemic load is a ranking system for carbohydrate content in food portions based on their glycemic index and a standardized portion size of 100g. Glycemic load or GL combines both the quality and quantity of carbohydrate in one number. It is the best way to predict blood glucose values of...

. 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
South Beach Diet
The South Beach Diet is a diet plan designed by cardiologist Arthur Agatston and dietician Marie Almon as an alternative to low-fat approaches such as the Ornish Diet and the Pritikin Diet advocated by the American Heart Association in the 1980s. Although the original purpose of the diet was to...

, Transitions by Market America and NutriSystem Nourish Diet
Nutrisystems Diet
Nutrisystem, headquartered in Fort Washington, Pennsylvania, is a commercial provider of weight loss products and services. Initially, the company offered weight loss counseling and products in brick and mortar centers. In 1999, the company moved to a direct-to-consumer business model, selling its...

. However, others have pointed out that foods generally considered to be unhealthy can have a low glycemic index, for instance, chocolate cake
Chocolate cake
Chocolate cake is a cake flavored with melted chocolate or cocoa powder.-History:Chocolate cake is made with chocolate; it can be made with other ingredients, as well. These ingredients include fudge, vanilla creme, and other sweeteners. The history of chocolate cake goes back to 1764, when Dr...

 (GI 38), ice cream
Ice cream
Ice cream is a frozen dessert usually made from dairy products, such as milk and cream, and often combined with fruits or other ingredients and flavours. Most varieties contain sugar, although some are made with other sweeteners...

 (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
Diabetes mellitus type 2
Diabetes mellitus type 2formerly non-insulin-dependent diabetes mellitus or adult-onset diabetesis a metabolic disorder that is characterized by high blood glucose in the context of insulin resistance and relative insulin deficiency. Diabetes is often initially managed by increasing exercise and...

,coronary heart disease
Coronary disease
Coronary disease refers to the failure of coronary circulation to supply adequate circulation to cardiac muscle and surrounding tissue. It is already the most common form of disease affecting the heart and an important cause of premature death in Europe, the Baltic states, Russia, North and South...

, and age-related macular degeneration than others.
Chiu CJ "et al." Informing food choices and health outcomes by use of the dietary glycemic index.Nutr Rev. 2011 Apr;69(4):231-42.
High blood glucose levels or repeated glycemic "spikes" following a meal may promote these diseases by increasing systemic glycative stress otheroxidative stress
Oxidative stress
Oxidative stress represents an imbalance between the production and manifestation of reactive oxygen species and a biological system's ability to readily detoxify the reactive intermediates or to repair the resulting damage...

 to the vasculature and also by the direct increase in insulin levels.
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 other obsolete proteins.
Uchiki T "et al." "Glycation-altered proteolysis as a pathobiologic mechanism that links dietary glycemic index, aging, and age-related disease (in nondiabetics)." Aging Cell. 2011 Oct 3. doi: 10.1111/j.1474-9726.2011.00752.x. [Epub ahead of print]


In the past, postprandial
Postprandial
Postprandial means after eating a meal while preprandial is before a meal.-Usages of the term:This term is used in many contexts but also in relation to blood sugar levels, which are normally measured 2 hours after and before eating in a postprandial glucose test...

 hyperglycemia
Hyperglycemia
Hyperglycemia or Hyperglycæmia, or high blood sugar, is a condition in which an excessive amount of glucose circulates in the blood plasma. This is generally a glucose level higher than 13.5mmol/l , but symptoms may not start to become noticeable until even higher values such as 15-20 mmol/l...

 has been considered a risk factor associated mainly with diabetes. However, more recent evidence shows that it also presents an increased risk for atherosclerosis
Atherosclerosis
Atherosclerosis is a condition in which an artery wall thickens as a result of the accumulation of fatty materials such as cholesterol...

 in the non-diabetic population and that high GI diets and high blood-sugar levels more generally and are related to kidney disease as well.

Conversely, there are areas such as Peru
Peru
Peru , officially the Republic of Peru , is a country in western South America. It is bordered on the north by Ecuador and Colombia, on the east by Brazil, on the southeast by Bolivia, on the south by Chile, and on the west by the Pacific Ocean....

 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. 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
University of Sydney
The University of Sydney is a public university located in Sydney, New South Wales. The main campus spreads across the suburbs of Camperdown and Darlington on the southwestern outskirts of the Sydney CBD. Founded in 1850, it is the oldest university in Australia and Oceania...

 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.

A study published in the American Journal of Clinical Nutrition
American Journal of Clinical Nutrition
The American Journal of Clinical Nutrition is a monthly peer-reviewed medical journal in the field of clinical nutrition.According to the Journal Citation Reports, it has a 2009 impact factor of 6.307, ranking it third among 66 journals in the category "Nutrition & Dietetics".The journal was...

 found that age-related adult macular degeneration
Macular degeneration
Age-related macular degeneration is a medical condition which usually affects older adults and results in a loss of vision in the center of the visual field because of damage to the retina. It occurs in “dry” and “wet” forms. It is a major cause of blindness and visual impairment in older adults...

 (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.

The American Diabetes Association
American Diabetes Association
The American Diabetes Association is a United States-based association working to fight the consequences of diabetes, and to help those affected by diabetes...

 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.

Weight control


Recent animal research provides compelling evidence that high-GI carbohydrate is associated with increased risk of obesity
Obesity
Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems...

. In human trials, it is difficult to separate the effects from GI and other potentially confounding factors such as fiber content, palatability
Palatability
Palatability is the hedonic reward provided by foods or fluids that are agreeable to the "palate" in regard to the homeostatic satisfaction of nutritional, water, or energy needs. The palatability of a food or fluid, unlike its flavor or taste, varies with the state of an individual: it is lower...

, and compliance. In one study, male rats were split into high- and low-GI groups over 18 weeks while mean body weight
Body weight
The term body weight is used in daily English speech as well as in the contexts of biological and medical sciences to describe the mass of an organism's body. Body weight is measured in kilograms throughout the world, although in some countries it is still measured in pounds or stones and pounds...

 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
Glycemia
Glycemia means the presence, or the level, of glucose in one's blood. Related words include:* Hyperglycemia, an unusually high concentration of sugar in the blood* Hypoglycemia, an unusually low concentration of sugar in the blood...

 and insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....

 levels were significantly higher, and plasma triglyceride
Triglyceride
A triglyceride is an ester derived from glycerol and three fatty acids. There are many triglycerides, depending on the oil source, some are highly unsaturated, some less so....

s were threefold greater in the high-GI-fed rats. Furthermore, pancreatic islet cells
Islets of Langerhans
The islets of Langerhans are the regions of the pancreas that contain its endocrine cells. Discovered in 1869 by German pathological anatomist Paul Langerhans at the age of 22, the islets of Langerhans constitute approximately 1 to 2% of the mass of the pancreas...

 suffered "severely disorganised architecture and extensive fibrosis
Fibrosis
Fibrosis is the formation of excess fibrous connective tissue in an organ or tissue in a reparative or reactive process. This is as opposed to formation of fibrous tissue as a normal constituent of an organ or tissue...

." 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.

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
    Insulin index
    The Insulin Index is a measure used to quantify the typical insulin response to various foods. The index is similar to the Glycemic Index and Glycemic Load, but rather than relying on blood glucose levels, the Insulin Index is based upon blood insulin levels...

     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
    Insulin resistance
    Insulin resistance is a physiological condition where the natural hormone insulin becomes less effective at lowering blood sugars. The resulting increase in blood glucose may raise levels outside the normal range and cause adverse health effects, depending on dietary conditions. Certain cell types...

    , and other factors.
  • 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. Carbohydrate impacts 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 may have a high carbohydrate content or vice versa; this can be accounted for with the glycemic load
    Glycemic load
    The glycemic load is a ranking system for carbohydrate content in food portions based on their glycemic index and a standardized portion size of 100g. Glycemic load or GL combines both the quality and quantity of carbohydrate in one number. It is the best way to predict blood glucose values of...

    . 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.
  • 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
    Diabetic diet
    There is much controversy regarding what diet to recommend to sufferers of diabetes mellitus. The diet most often recommended is high in dietary fiber, especially soluble fiber, but low in fat . Recommendations of the fraction of total calories to be obtained from carbohydrate intake range from 1/6...

     
  • Glycemic load
    Glycemic load
    The glycemic load is a ranking system for carbohydrate content in food portions based on their glycemic index and a standardized portion size of 100g. Glycemic load or GL combines both the quality and quantity of carbohydrate in one number. It is the best way to predict blood glucose values of...

  • Insulin index
    Insulin index
    The Insulin Index is a measure used to quantify the typical insulin response to various foods. The index is similar to the Glycemic Index and Glycemic Load, but rather than relying on blood glucose levels, the Insulin Index is based upon blood insulin levels...


  • Low glycemic index diet
  • Montignac diet
    Montignac diet
    The Montignac diet is a weight-loss diet that was popular in the 1990s, mainly in Europe. It was invented by Frenchman Michel Montignac , an international executive for the pharmaceutical industry, who, like his father, was overweight in his youth...

  • Resistant starch
    Resistant starch
    Resistant starch is starch and starch degradation products that escape digestion in the small intestine of healthy individuals. Resistant starch is considered the third type of dietary fiber, as it can deliver some of the benefits of insoluble fiber and some of the benefits of soluble fiber.Some...



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