The glycaemic index (GI) is a rating system established in 1981 for foods containing carbohydrates. A corresponding number is given to a food depending on how quickly it causes sugar levels (glucose) to rise when it is eaten in isolation, relative to a referent carbohydrate, generally pure glucose. The principle is that the slower the carbohydrate is digested and absorbed, the lower the rise in blood glucose and the lower the corresponding GI value. Conversely, high glycaemic foods are digested and absorbed into the bloodstream quickly, which causes a large, rapid change in blood glucose levels. The GI rating system is between 1 and 100, with glucose having a score of 100. A high GI value is considered as ≥70, a value between 56 and 69 is medium and a GI value ≤55 is low.
Generally, we are advised to choose low GI sources of complex carbohydrates (starchy carbs) as they are digested slowly and thus raise blood glucose in a controlled and regulated way. Examples include brown rice, oats, potatoes, couscous and quinoa. In contrast, sources that are categorised as high GI can cause a rapid increase in blood glucose, which signals β-cells of the pancreas to increase insulin secretion. Over the next few hours, insulin, which regulates blood glucose levels by moving glucose molecules from the blood into cells, will cause a sharp decrease in the concentration of blood glucose, resulting in hypoglycaemia.
See the table below for a list of foods and their corresponding GI value.
Table 1. GI and GL Values for Selected Foods
|Food||GI (Glucose=100)||Serving Size||Carbohydrate* per Serving (g)||GL per Serving|
|Potatoes, baked||111||1 medium||30||33|
|Potatoes, boiled||82||1 medium||30||25|
|Puffed rice cakes||82||3 cakes||21||17|
|Jelly beans||78||1 oz||28||22|
|Rice, white, boiled||66||1 cup||53||35|
|Table sugar (sucrose)||63||2 tsp||10||6|
|Dates, dried||62||2 oz||40||25|
|Spaghetti, white, boiled||58||1 cup||44||25|
|Honey, pure||58||1 tbsp||17||10|
|Banana, raw||55||1 cup||24||13|
|Parsnips, peeled, boiled||52||1/2 cup||10||5|
|Spaghetti, wholemeal, boiled||32||1 cup||37||14|
|Orange, raw||42||1 medium||11||5|
|Apple, raw||39||1 medium||15||6|
|Skim milk||33||235 ml||13||4|
|Lentils, dried, boiled||29||1 cup||24||7|
* Amount of available carbohydrate in a food serving that excludes indigestible carbohydrates, i.e. dietary fibre
Although glycaemic index should be considered when choosing carbohydrate sources, this rating system has several shortfalls.
Firstly, the GI scale does not consider the nutritional value of foods. Foods that are high GI are not necessarily unhealthy, and not all low GI foods are healthy. Many fruits and vegetables, for example, are high GI, such as watermelons and parsnips, which are both good sources of vital vitamins and minerals. Some low GI foods may also be high in calories and should be consumed in small portions.
The GI value of a food does not take real-life application into consideration. Generally speaking, we eat meals that are made up of numerous different foods, and the nutritional composition of one food may influence how another food is digested and absorbed. For example, foods that contain or are cooked with fat and protein slow down the absorption of carbohydrate, thus lowering their GI. For example, crisps have a lower GI than plain boiled potatoes, but we’re recommended to eat crisps in moderation due to their high fat content.
The rate at which your body processes carbohydrates can also be influenced by the total amount of food eaten, the processing of food and the ripeness of the food. A banana's GI value, for example, will increase as it ripens. When a banana is under-ripe, with a greenish colour and firm texture, the GI is relatively low (GI = ~30). Over time, as the starch breaks down into simple sugars, the banana will turn yellow and eventually to brown. The GI of a ripe banana is ~62.
Glycaemic load (GL) is the total glycaemic effect of the diet. It is the product of dietary GI and total dietary carbohydrates consumed. GL measures how much a single serving of food influences the blood glucose response and therefore takes into consideration some of the shortfalls of GI.
To calculate the GL of a single serving of food, multiply the GI number by the grams of carbohydrate in one serving, then divide the total by 100[8-10].
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