References to high or low ‘GI’ foods are often banded about with little, or no explication. Indeed, I was guilty of this myself in my previous post. But how much do we really know about the term ‘GI’, and what does the GI number of a food actually indicate?

The glycaemic index (GI) ranks foods based on the rate at which the body is able to brake down the carbohydrates within them into glucose, and by the amount that this glucose then raises the glucose level of the blood.

It was originally developed for people with diabetes, whose bodies either do not produce insulin (type 1) or who have become insulin resistant (type 2), as a means of helping them to help keep their blood sugar levels under control.

Why does this matter?

When glucose levels start to rise, the pancreas releases the hormone insulin, which promotes the uptake of glucose by the cells for use as energy, bringing blood sugar levels back into a more manageable range.

Insulin removes the surplus glucose from the blood and this can then be used as energy within the cells.

Insulin acts on the cells in the liver, muscle and fat tissue in particular and stimulates them to:

Essentially insulin encourages the cells to make fatty acids into fat molecules and to store them, while also stopping the breakdown of extant fat in the cells for energy, instead using the ready supply of glucose for a quick energy fix.

Eating low-GI carbohydrate foods causes a steady rise in the level of glucose in the blood, which in turn leads to a small and gentle rise in insulin. Small increases in insulin keep you feeling full and energised for hours after eating and also encourage the body to burn fat.

Erratic rises and falls in blood glucose levels from low-GI foods however, can leave you feeling hungry and lethargic soon after eating.

While our bodies are well-designed to cope with changes in blood glucose levels, excessive amounts of glucose over an extended period can, in some cases lead to insulin resistance and later, the development of type 2 diabetes.

GI numbers

The ranking system is based by comparing the rate at which glucose is released from an item of food against glucose. For this reason glucose has a GI number of 100.

Foods with a GI of 70 or more are classified as ‘high GI‘ as they trigger a rapid increase in blood sugar levels. Foods with a GI of 55-69 are ‘medium GI‘ as they trigger a moderate increase. Foods with a GI below 55 are ‘low GI‘ because they have a minor impact on blood sugar.

What makes a food low rather than high GI depends on the proportion of amylose to amylopectin within them. Foods with a greater proportion of amylose such as lentils have lower GIs than those with more amylopectin, like potatoes, which have a high GI.

Why do GI scores vary?

A food’s GI is not fixed however; it will vary depending on a number of things including how the food has been prepared, whether it has been cooked, how hydrated it is and, in the case of fresh produce like fruit, how ripe it is.

An average serving of raw carrot, for example, has a GI of 16 but once peeled, diced and boiled this rises to 49.

Moreover, your glycaemic response to a food also depends on the other foods you eat with it. When a meal includes proteins and fat the impact of the carbohydrate foods is minimised. This is because, by combining foods in a single meal, the overall impact is to slow down the rate at which your body releases sugar from any single ingredient.

Are there any health benefits of eating low GI?

A positive side effect of choosing low GI foods is that you may lose weight since these sorts of foods tend to keep you feeling fuller for longer. However, it’s worth remembering that low GI doesn’t necessarily mean healthy or low fat.

A low GI eating plan can also be helpful if you’re worried about your risk of type 2 diabetes and heart disease since a low GI diet improves blood sugar and insulin control and helps manage cholesterol levels.

The effect of stabilising blood sugar levels should also mean you feel improvements in energy, mood and concentration levels.