What is the Glycaemic Index?

Glycaemic Index

Different carbohydrate-rich foods affect blood sugar levels differently. The Glycaemic Index runs from 0 to 100 using glucose (which has a GI value of 100) as the reference. The effect other foods have on blood sugar levels are then compared with this giving an indication of whether food raises blood sugar levels dramatically, moderately or a little. Foods that have a slow, small effect on blood sugar have a low GI value, with those causing a rapid and massive rise having a high GI value.

Low GI value foods slowly release sugar into the blood, providing a steady supply of energy, leaving a feeling of fullness or satiety. Foods with a high GI value cause a rapid, but short-lived rise in blood sugar resulting in a large surge in insulin levels that then cause blood glucose to fall rapidly again. Hence high GI foods do not provide a sustained source of energy and leave a feeling of hunger within a short time. The surge in insulin levels has the added disadvantage of promoting the conversion of excess blood glucose into fat stores.

Foods in the low category usually have a GI value of 55 or less; in the medium category, a GI value of 56 to 69; and in the high category, a GI of 70 or more.

Diets based on GI index simply encourage the consumption of foods with a low GI value over those with a high GI value. This helps to prevent swings in blood sugar, increases satiety and reduces fat deposition.

The overall nutrient content of a food will affect its GI and both fat and protein affect the absorption of carbohydrate. Other factors that effect GI are the structure of the type of carbohydrate, how food is cooked, the ripeness of foods and the degree of processing involved as these factors all contribute to how the carbohydrate starch molecules are exposed to digestive enzymes. GI is only relevant to carbohydrate and so it only relates to bloods sugar levels when carbohydrates are eaten on their own. When combinations of foods are eaten together as in a meal, the GI value of the whole meal can change, for example by the fat content delaying the transit of carbohydrate from the stomach into the small intestine. So, the GI of a baked potato is higher if eaten on its own compared to with cheese. However, in general the more low GI foods that are included in a meal the lower the overall GI value will be.

Glycaemic Load

Glycaemic Load (GL) provides a more accurate indication of the likely rise in blood sugar levels on eating certain foods as it takes into account the actual amount of carbohydrate per portion. It is calculated by multiplying the GI value with the number of grams of carbohydrate in the portion of food being consumed and dividing this figure by 100. Eating foods with a high GL correlates with an increased risk of developing type 2 diabetes and coronary heart disease.

Table of GI and GL values for common foods