Glycemic indexes measure carbs’ from the perspective of their pure sugar/starch content in order to determine how they affect glycemia (blood sugar levels) after meals.
Glycemia refers to blood sugar levels, namely, the amount of sugar (glucose) in one’s blood. In the case of a person fasting, for example, blood sugar levels are approximately 1 g of glucose per liter of blood.
Digestion transforms carbohydrates into glucose and by so doing raises blood sugar levels. Blood sugar levels are critical to losing or gaining weight. Once we’ve digested our food, the blood sugar that is produced makes our bodies secrete insulin. Insulin is a hormone which, depending on the amount secreted, might or might not cause us to gain weight.
For years people believed that equal portions of different carbohydrates generated the same blood sugar levels (glycemic responses). As of the mid 1970s, Crapo, a Californian researcher from Stanford University, discovered evidence to the fact that carbohydrates having the same pure sugar/starch content did not necessarily have the same impact on blood sugar levels.
Crapo established that what has to be taken into consideration is a carbohydrate’s potential to raise blood sugar levels and how it rates in comparison to other carbs.
Later, in 1981 and on the basis of the research carried out by Crapo since 1976, Jenkins, gave final shape to Glycemic Indexes (GI).
Rather than focusing solely on how individual carbs increased blood sugar, Jenkins sought to improve the GI notion by using estimates on the hyperglycemia triangle area. This area is given by the full blood-sugar curve of each carb tested, eaten alone and on an empty stomach.
In order to design the GI scale, Jenkins arbitrarily assigned a value of 100 to glucose (much like we arbitrarily opted for the 0° centigrade measurement). The 100 figure also corresponds to when our bodies totally (100%) assimilate the sugar consumed.
The index value of a pure carbohydrate is estimated by using standard portions and measuring them as follows: the area of the triangle of the carb being tested is divided by the area of the glucose triangle and the result is then multiplied by the 100 value.
Area of the triangle of the tested carb
Area of the glucose triangle
Glycemic Indexes measure how a carb affects our blood sugar levels, namely, its potential for releasing a certain amount of sugar into our blood after each meal. We could say that GIs actually measure a carb’s biodisposition or intestinal absorption rate.
If a carb’s sugar level index is high (as in the case of potatoes), the corresponding carb’s absorption rate will provoke high blood sugar levels.
Comparatively, carbs with low Glycemic Indexes (for example lentils) are digested at rates that generate low, or practically meaningless, glycemic response.
For example, when measured by the 100 glucose value standard, the GI for fries is 95 whereas green lentils have a 25 GI.
Nonetheless, it is important to note that carbohydrates’ GIs are not fixed. They can vary depending on a number of factors. A cereal’s GI, for instance, depends on the variety of cereal consumed and on its botanic origin. For fruit, GIs vary depending on how ripe the fruit is, on how it was processed, on its degree of hydration, and so on.
As mentioned before, GIs correspond to a carbohydrate’s potential to raise blood sugar levels. In other words, they tell us the degree to which certain carbs make our bodies secrete insulin. The more insulin our bodies secrete, the greater the chances of gaining weight.
Despite warnings by GI experts such as Professor Gérard Slama, nutritionists continue to think of carbs in terms of their absorption speed.
In fact, there a two types of nutritionists.
For them, the GI notion is only relevant in measuring a carb’s absorption speed. From this perspective, the full carb content of food would always be transformed into sugar (glucose). In this respect, the food with a low Glycemic Index would take longer to absorb and that, while the ensuing glycemia is lower, it would then tend to last longer. Seen from this angle, the sole purpose of Glycemic Indexes would be to measure the time span of glucose intestinal absorption processes.
This approach is totally mistaken since it goes against the facts of how our bodies really work.
In effect, the opposite is true. All of the tests carried out on GIs, particularly those done by Jenkins, prove that what low GIs indicate is that less quantities of glucose have been absorbed as opposed to the time span for the absorption of equal quantities of glucose.
This misguided notion regarding GIs is, unfortunately, widespread among a scientific community that awkwardly preaches it any which way.
In his book Brain Dieting (« La diététique du cerveau »), published by Odile Jacob in April 2003, Professor Jean-Marie Bourre, member of the French Medical Academy, states clearly that GIs measure the speed at which sugar is absorbed. This happens to be a totally false assumption.
In conclusion, we can only regret medical professionals’ failure to take into account Glycemic Indexes and their usefulness in regulating insulin secretion, a crucial element to gaining weight and diabetes.