It is a well-known fact nowadays that carbohydrate-rich foods are one of the main reasons for the disease of modern man, as far as diet is concerned. The main measure we were (and still are) concerned with was the accuteness and the level of peaking of the hormone insulin after consumption of a carbohydrate-rich meal.
Up until a few decades ago we used to measure the effect of carbs on us by dividing them into different groups, based on their chemical make up: simple (monosacgcharides and disaccharides), and complex (polysaccharides).
Then we found out that the type of carbohyrdate doesn’t predict well what the insulin response is going to be. We discovered the Glycemic Index (GI) of carbohydrate foods. GI tells us how fast a particular food will raise the levels of insulin in the blood, or in other words, how fast any carbohydrate will be converted into glucose.
GI is usually recorded by having a person ingest 50g (or 25g) carbohydrates in the food studied and comparing the insulin spike with the one given by a reference carbohydrate – usually glucose.
Although we now know that a more accurate predictor of the threat of obesity and metabolic disease is not the GI but rather the Glycemic Load (GL) of foods. GL is the grams of available carbohydrate in the food x the food’s GI / 100 (1). But, since GL is a function of GI we still need a more accurate measure of the GI.
So, why some foods sometimes show different glycemic indexes (GI values)? And, why are the GI numbers of the same foods so markedly different in different GI tables? For example, why carrots have a GI of 92 (significant and sudden insulin spike) in one table and GI of 32 (slow release of glucose in the blood stream) in another?
There are several reasons for this, grouped in two categories: differences in the methodology of GI measuring and differences in the chemical characteristics of carbohydrate-rich foods (2).
Methodological factors:
- Different testing methods – different types of blood samples (capillary or venous), different experimental time periods, and different portions of foods (50g of total rather than of available carbohydrate, which is influenced by the presence of resistant starches, sugar alcohols, novelty carbohydrates)
- Preparation method – raw vs. cooked and chilled. Raw starches are often indigestible (resistant starches, or RS). When thermally prepared their starches become available for digestion. When chilled, their starches slowly begin to convert to resistant starches to a certain degree. This is called starch retrogradation. For example, potatoes: raw vs. boiled vs. boiled and chilled vs. boiled, chilled and left to sit (retrograde) for a while
- In vivo or in vitro testing – both methods often show very different GI values because it is not the same to test GI in the human digestive tract and in a test tube.
Chemical characteristics:
- Different ingredients in packaged foods, or different processing methods – difference in the rate of carbohydrate digestion
- Botanical differences of the same food – for example, different types of rice have different GI values, usually due to the different amylose content (amylose is more difficult to digest and digestion is incomplete in comparison with amylopectin)
- Localization – the same carohydrate-rich food may have different glycemic index values when grown in different parts of the world
So, which table should you look into for more accurate GI and GL values? Possibly, that would be the International Table of Glycemic Index and Glycemic Load Values – 2002