When food is eaten, it passes through a series of chemical and mechanical processes. Saliva begins to convert starches into small molecules (malto dextrins). The stomach is like a food processor. If we munch an apple, the processor would have to work harder than if we ate the same apple stewed or drink it in the form of apple juice. The time it takes for the body to absorb nutrients also varies.
The speed at which we digest carbohydrates differs according to whether they are consumed according to the solid or liquid on their own or as part of a meal. The speed of assimilation also depends on the overall composition of the meal.
During digestion, carbohydrates in food are converted into glucose, which in turn passes into the bloodstream. The body can use it immediately as “fuel” , or else it can store it in the form of glycogen in the liver, kidneys and muscles.
Glycaemia is the scientific term for the concentration of glucose in the blood. Fasting glycaemic levels normally range between 0.6 g and 1 g per litre. For an hour or two after the meal, blood glucose levels rise to no more than 1.6 g per litre before returning to basal levels in the following two hours.
Glucose obtained from consuming carbohydrates is either used immediately to keep the body functioning or stored as glycogen. If a meal is delayed or we are required to spend energy over a prolonged period, glycogen is converted back to glucose to fuel the body.
For over half a century, scientists believed that glycaemia (the concentration of glucose in the blood) varied according to the complexity of the carbohydrates absorbed. This was based on the simple idea that the levels of glucose in the blood rise more slowly after a meal with bread and potatoes, which contain high levels of starch, than after a meal rich in simple sugars.
But a carbohydrate does not have a high glycaemic index because it is simple, or a low glycaemic index because it is complex. The rate at which our bodies absorb carbohydrates depends on several factors, which have been the subject of numerous studies in recent years.
These studies show that glycaemia varies according to the food we eat, the form (liquid or solid) and texture of the food and how it is cooked, the presence of other nutrients (proteins, fats) and/or dietary fibre, how the food is eaten (consumed on its own or eaten as part of a meal consisting of a combination of foods) have an influence.
Studies also reveal that the glycaemic index of the same food absorbed under identical conditions varies noticeably from person to person. This research helps us to more accurately predict a person’s glycaemic response and to offer suitable advice to diabetics and serious athletes, for example.