Glucose is the body’s main”fuel”. It is stored in the form of glycogen in the muscles and liver. The blood glucose level (or blood sugar level, or glycaemia) is simply the level of glucose in the bloodstream. On an empty stomach, this is normally one gram per liter of blood.
When carbohydrates (bread, honey, starchy foods, cereals, sweets, etc.) are ingested on an empty stomach, the effect on the blood sugar level is found to be as follows :
The first stage is that blood glucose rises (to a greater or lesser extent, according to the nature of the carbohydrate).
The second stage is that, after insulin has been secreted by the pancreas, the blood glucose level falls and the glucose is released into the body’s tissues.
So, thirdly, the blood sugar level reverts to normal (see graph on the following page).
Traditionally, it was usual to place carbohydrates in one of two distinct categories,”quick sugars”and”slow sugars”, the terms referring to the body’s rate of absorbing them.
“Quick sugars”were simple sugars (such as glucose) and disaccharides, such as the sucrose found in refined sugars (both cane and beet), honey and fruit. The term”quick sugar”owed its existence to the belief that, because of the simple nature of the molecule, the body rapidly absorbed these sugars after ingestion.
Conversely,”slow sugars”referred to all carbohydrates whose more complex molecule had first to be chemically converted into simple sugar (glucose) in the course of digestion. This applied notably to starches, from which, it was thought, glucose was released into the body slowly and progressively.
This way of classifying carbohydrates is today completely outdated, and is based on a misconceived theory. Recent studies show that the complexity of the carbohydrate molecule does not actually determine the speed with which glucose is released and absorbed into the body.
It is now accepted that the glycaemic peak (that is, the point of maximum absorption) is reached at the same rate for any carbohydrate eaten in isolation and on an empty stomach, and occurs about half an hour after ingestion. Therefore, instead of talking about their speed of absorption, it is more to the point to consider different carbohydrates in terms of their potential to induce a greater or lesser rise in blood glucose, that is, in terms of the sheer quantity of glucose they produce.
Disaccharides (white sugar, maltose in beer, lactose in milk)
Polysaccharides (cereals, flours, potatoes, pulses)
Monosaccharides (glucose and fructose found in fruit and honey)
So scientists and others now agree in the field of nutrition that carbohydrates should be classified according to what is called their hyperglycaemic potential, as defined by the glycaemic index.