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The 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.
Carbohydrates that break down rapidly during digestion have the highest glycemic indices. An increased blood glucose response occurs very quickly. 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 scientific validated methods use glucose as the reference food. Glucose has a glycemic index value of 100. This has the advantages in that it is universal and it results in maximum GI values of approximately 100.
Glycemic Index of Foods
GI values can be interpreted intuitively as percentages on an absolute scale and are commonly interpreted as follows:
- Low GI - less than 55
- Intermediate GI - between 56 and 69
- High GI - higher than 70
A low GI food will release energy slowly and steadily and is appropriate for diabetics, dieters and endurance athletes. A high GI food will provide a rapid rise in blood sugar levels and is suitable for post-endurance exercise energy recovery. Previously, white bread was sometimes used as a reference food (if white bread = 100, then glucose = 140). For people whose staple carbohydrate source is white bread, this had 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 were that the reference food was not well-defined, and the GI scale culture dependent.
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 fibre 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.
The glycemic index applies to foods that contain carbohydrate only. High fat and high protein foods such as meat, eggs, nuts and cheese have a negligible GI. Furthermore, because many fruits and vegetables (but not potatoes) contain very little carbohydrate, they also have low GI values. Alcoholic beverages are also low GI.
Several lines of recent scientific evidence have shown that individuals that 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). 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).
Recent animal research provides compelling evidence that high GI carbohydrate are associated with increased risk of obesity. In human trials it is typically difficult to separate the effect from GI and other potentially confounding factors such as fibre content, palatability and compliance. In the study (Pawlak et al, 2004), male rats were split into a high and low GI group over 18 weeks while mean bodyweight was maintained. Rats fed the high GI diet were 71% fatter and 8% less lean 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 carbohydrate would likely have led to the development of severe metabolic abnormalities.
The glycemic index has been criticised for the following reasons: a limited range of data, some variation in GI measurements, GI values affected by cooking or preparation method, difficulty in predicting GI values within mixed meals and daily fluctuations in an individual’s glycemic response.
For extensive data on GI values of foods (based on glucose reference), see the links below.
- Pawlak et al (2004). Effects of dietary glycaemic index on adiposity, glucose homoeostasis, and plasma lipids in animals. Lancet 28;364(9436):778-85
- 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
- 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
- Glycemic index website - University of Sydney
- GI database
- International table of glycemic index and glycemic load values: 2002
- Simplified version of the International table
- Low GI food range
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