Problems with the GI Methodology and Variability

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If foods do have different glycemic responses, why the controversy? Several problems with the methodology used to determine GI have been cited [1]. As noted above, 50 g of digestible carbohydrate from foods is used to determine the GI. Although this would seem logical, in reality it does not reflect the actual amounts of carbohydrate contributed by individual foods in the usual diet. As mentioned, foods that in usual portion sizes contribute minimal amounts of carbohydrate, despite having a high GI, would not elicit much of a glycemic response. And conversely, foods such as pizza may have a lower GI number, but the usual portion size consumed would contribute a considerably greater amount of total carbohydrate resulting in a higher GL.

The GI only measures glucose above the beginning fasting glucose. If it measured what occurs naturally, the fasting glucose value would decrease over time and the area under the curve (AUC) would be greater. Therefore, some researchers favor the use of the whole AUC as the real measure of glucose availability [1]. If the AUC is calculated in this manner, the differences in

GIs between foods are greatly attenuated. For example, a person with a fasting glucose of 75mg/dl (4.2mmol/l) ingests two foods, one with a GI of 100 and the other a GI of 72. If the GI were calculated by using the whole glucose AUC instead of only the area above the fasting glucose, the values would be 100 and 92, respectively. The difference changes from 29 to 8 units. Furthermore, the GI is measured in the morning, after an overnight fast. Several studies have reported that if the GI is measured after lunch, the differences in GI would be considerably less than after breakfast.

A major problem, however, is the reproducibility and variability of the glucose response. Reproducibility of the glucose response in the same subject has not been adequately studied, and the individual blood glucose response to any food or meal is highly variable, both within and between individuals -ranging from 23 to 54% [1, 8]. Values for the GI of foods are broad. For example, Australian potatoes have a GI of 87-101, placing them in the high GI group. In the United States and Canada potatoes have a GI ranging from 56 to 77, placing them in the moderate GI group [9]. GI values for boiled rice vary from 45 to 112. Bananas range from 30 to 70, partially depending on their degree of ripeness. The GI from different types of spaghetti varies even more widely. White, durum-wheat semolina spaghetti varies from 45 to 65, depending on the length of cooking time. Even prepared foods vary greatly. For example All-Bran cereal ranged from 30 in Australia to 51 in Canada, and corn chips varied from 72 in 1985 to 42 in 1998 [10].

Testing of blood glucose before and after eating a particular food by an individual may be the best way to determine an individual's glycemic response. However, this may not be accurate either as rarely do individuals eat only one food at a meal. The glucose response to a meal is determined first of all by the total amount of carbohydrate in a meal and secondly by the combination of carbohydrate foods in a meal. If a high GI food is eaten in combination with a low GI food, the GI response will be moderate. For example, a high GI cereal eaten with milk will have a moderate GI response. Adding fat to potatoes can also change their GI response. The GI response to a particular food can also be lowered by eating less of the food. Protein in the meal does not affect the glycemic response [11], whereas large amounts of fat may.

Another major problem is that the GI is not the best indicator of healthy food choices. Although many healthy foods have a low GI (whole grains, fruits, vegetables, legumes, dairy products), there are also foods of questionable value with low or moderate GI values. For example, soft drinks, candies, sugars, and high fat foods fall into this questionable category. The GI of foods can be lowered by adding or substituting sugars, especially fructose, sugar alcohols, or fat.

In addition, the insulin response to a given food is not linear and is not consistently related to either the carbohydrate content or glycemic effect of food [12]. The insulin response to a 100-gram portion of a particular food is not

Table 2. Summary of high versus low glycemic index diets in diabetes

Outcome Type-1 diabetes Type-2 diabetes

(4 studies; 36 subjects; (12 studies; 175 subjects; ~4 weeks duration) ~5 weeks duration)

low GI diet no difference low GI diet no difference significantly between significantly between better diets better diets

HbAlc

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Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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