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Diabetes and  Low Glycemic Index Diets
What is the Glycemic Index?

 

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The Glycemic Index (GI) is a ranking system for carbohydrates based on their immediate effect on blood glucose levels. It compares carbohydrates (cho) 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.

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How a Glycemic Index Value is Calculated

The glycemic index of a food is determined by the impact on a 2-hour blood glucose response curve (AUC) following the ingestion of a fixed amount of carbohydrate (usually 50 grams of cho). The AUC of the test food is divided by the AUC of the standard (either glucose or sometimes white bread with an actual GI index of 100) and multiplied by 100.

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 was culture dependent.

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).

  • 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 (think "slow" rhymes with "low").  

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.  All of which are of benefit to to persons with insulin resistant metabolic disorders including polycystic ovarian syndrome (PCOS), thyroid disorders, and type 2 diabetes.

GI 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.

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.

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, because many fruits and vegetables (but not potatoes) contain very little carbohydrate per serving, they have very low GI values and are regarded as "free" foods. This also applies to carrots, which were originally and incorrectly reported as having a high GI (Brand-Miller et al, 2005). 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).

 The Low GI Diet Cookbook: 100 Simple, Delicious Smart-Carb Recipes
The Low GI Diet Cookbook: 100 Simple, Delicious Smart-Carb Recipes

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.  

 Good Carb Cookbook: Secrets of Eating Low on the Glycemic Index
Good Carb Cookbook: Secrets of Eating Low on the Glycemic Index

The Glycemic Index and Type 2 Diabetes

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). 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).

 The Low GI Diet Revolution: The Definitive Science-Bases Weight Loss Plan (New Glucose Revolution Series)
The Low GI Diet Revolution: The Definitive Science-Bases Weight Loss Plan (New Glucose Revolution Series)

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 from 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 carbohydrates would likely have led to the development of severe metabolic abnormalities.  

Criticism

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.  Another example is aldente cooked pasta has a lower glycemic index than that of overcooked pasta.
  • 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 is difficult to predict (i.e., the carbohyrdates glycemic action in an apple is slowed down when you add peanut butter to the meal).
  • 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 (GL) into account. This combined approach is, however, somewhat more complicated, and therefore harder to use in giving dietary advice.  

Glycemic load

The Glycemic Load (GL) is a ranking system for carbohydrate content in foods based on their GI.

Glycemic load for a single serving of a food can be calculated as :

grams of carbohydrate content  x   its GI  divided by 100

For example, a 100g slice serving of watermelon with a GI of 72 and a carbohydrate content of 5g (it contains a lot of water) makes the calculation 5*0.72=3.6, so the GL is 3.6. So a food with a GI of 100 and a carbohydrate content of 10g has a GL of 10 (10*1=10), while a food with 100g carbohydrate and a GI of just 10 also has a GL of 10 (100*0.1=10).


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References, from Wikipedia, edited for style and content

  • 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
  • Brand-Miller et al (2005). The Low GI Diet Revolution: The Definitive Science-based Weight Loss Plan. Marlowe & Company. New York, NY.
  • Brouns et al (2005). Glycaemic index methodology. Nutrition Research Reviews 18; 145-171
  • 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
  • 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

 

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