Glycemic index
Glycemic index (also
glycaemic index,
GI) is a ranking system for
carbohydrates based on their immediate effect on
blood glucose levels. It compares carbohydrates gram for gram in individual
foods, providing a numerical, evidence-based index of postprandial (post-meal)
glycemia. The concept was invented by Dr.
David J. Jenkins and colleagues in
1981 at the
University of Toronto.
The glycemic index of a food is defined by the area under the 2 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) 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
[Brouns et al (2005). Glycaemic index methodology. Nutrition Research Reviews 18; 145-171].
Carbohydrates that break down rapidly during
digestion have the highest glycemic indices. Such carbohydrates require less energy to be converted into glucose, which results in faster digestion and a quicker increase of blood glucose. Complex carbohydrates that break down slowly, releasing glucose gradually into the blood stream, have a low glycemic index. A lower glycemic index suggests slower rates of digestion and absorption of the sugars and starches in the foods and may also indicate greater extraction from the liver and periphery of the products of carbohydrate digestion. Additionally, a lower glycemic response equates to a lower
insulin demand, better long-term blood glucose control and a reduction in blood lipids.
Glycemic index values for different foods are calculated by comparing measurements of their effect on blood glucose with an equal carbohydrate portion of a reference 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 in that it is universal and it results in 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.
|
The effect on blood glucose of foods with different glycemic indices. |
GI values can be interpreted intuitively as percentages on an absolute scale and are commonly interpreted as follows:
| Classification | GI range!Examples | | Low GI | Below 55 | sour dough bread (54), apple juice (40), pumpernickel (41), oatmeal (48) |
| Intermediate GI | 56 to 69 | croissant (67), coca-cola (63), raisin bran (61) |
| High GI | Above 70 | white bread (70), wholemeal bread (75), corn flakes (72), jasmine rice (109) |
A low GI food will release energy slowly and steadily and is generally appropriate for everyone, especially
diabetics, dieters and endurance athletes. A high GI food will provide a rapid rise in blood sugar levels and is suitable for energy recovery after endurance exercise.
The glycemic effect of foods depends on a number of factors such as the type of
starch (
amylose vs
amylopectin), physical entrapment of the starch molecules within the food, fat content of the food and increased acidity of the meal - adding
vinegar for example, will lower the GI. The presence of fat or
dietary fiber can inhibit
carbohydrate absorption, thus lowering the GI. Unrefined breads with higher amounts of fibre generally have a lower GI value than white breads, but, while adding butter or oil will lower the GI of bread, 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. Many brown breads, however, are treated with
enzymes to soften the crust, which makes the starch more accessible. This raises the GI, with some brown breads even having GI values over 100.
The glycemic index can only be applied 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. High fat or high protein foods such as meat, eggs, nuts and cheese have a negligible GI. Furthermore, many fruits and vegetables (but not potatoes) contain very little carbohydrate per serving, and also have very low GI values. This also applies to carrots, which were originally and incorrectly reported as having a high GI
[Brand-Miller et al (2005). The Low GI Diet Revolution: The Definitive Science-based Weight Loss Plan. Marlowe & Company. New York, NY]. Alcoholic beverages have been reported to have low GI values, however 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%
[Brand-Miller, in press].
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. High blood glucose levels or repeated glycemic "spikes" following a meal may promote these diseases by increasing oxidative damage to the vasculature and also by the direct increase in
insulin levels
[Temelkova-Kurktschiev et al (2000). 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]. In the past, postmeal
hyperglycemia has been a risk factor mainly associated with diabetes, however more recent evidence shows that postprandial hyperglycemia presents an increased risk for
atherosclerosis in the non-diabetic population
[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].
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 from other potentially confounding factors such as fibre content, palatability, and compliance. In the study (Pawlak et al, 2004), male rats were split into high and low GI groups over 18 weeks while mean bodyweight 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 three-fold greater in the high GI fed rats. Furthermore,
pancreatic islet cells suffered "severely disorganised architecture and extensive
fibrosis". The evidence in this study showed that continued consumption of high glycemic index carbohydrates would likely have led to the development of severe
metabolic abnormalities.
The glycemic index has been criticised for the following reasons:
*the GI of a food varies depending on the kind of food, its ripeness, the length of time it was stored, how it was cooked, its variety (potatoes from Australia, for example, have a much higher GI than potatoes from the United States), and how it was processed or manufactured.
*the GI of a food varies from person to person and even in a single individual from day to day, depending on blood glucose levels, insulin resistance, and other factors.
*the GI of a mixed meal can be difficult to predict.
*the GI value is based on a portion that contains 50 grams of carbohydrate only.
*a limited range of data and daily fluctuations in an individual's glycemic response.
Some of these criticisms can be addressed by taking the
Glycemic load into account. This combined approach is, however, somewhat more complicated, and therefore harder to use in giving dietary advice.
*DJ Jenkins
et al (1981). Glycemic index of foods: a physiological basis for carbohydrate exchange.
Am J Clin Nutr 34; 362-366
*Pawlak
et al (2004). Effects of dietary glycaemic index on adiposity, glucose homoeostasis, and plasma lipids in animals.
Lancet 28;364(9436):778-85
*
Glycemic index website and official GI database - University of Sydney*
Glycemic Index newsletter*
International table of glycemic index and glycemic load values: 2002*
A list of foods by GI and GL*
Searchable database of GI and GL for over 1500 food items*
Tree/freetext search-style database of GI and GL*
GiListing.com - Alphabetic GI list
