GI stands for Glycaemic Index and GL, Glycaemic Load. These two terms can be confusing. GI seems to have been around for ages and people are comfortable checking the GI of the foods they eat, but GL? Here's what these two terms mean - important if you have diabetes.

What is Glycaemic Index?

GI measures carbohydrate quality – i.e. how much a carbohydrate will affect blood sugar. 

It is a ranking of foods from 0 to 100 that tells us whether a carbohydrate food will raise blood sugar (glucose) levels dramatically, moderately or just a little.

• A low GI food has a GI of 55 or less.
• A medium GI food of between 56 and 69.
• A high GI food has a GI of 70 or more.

Carbohydrates with a low GI value (55 or less) are more slowly digested, absorbed and metabolised and cause a lower and slower rise in blood glucose and generally call for less insulin.

Note: the US spelling has no "a", as in Glycemic.

Complex carbs now out of date

Originally nutritionists believed that starchy carbohydrates like bread and potatoes were “complex” and because of this took longer to be digested and absorbed than “simple” sugars like sugar. It was reasoned that these “complex” carbohydrates would then produce smaller rises in blood sugar levels than “simple” carbohydrates, which were thought to cause a surge of blood sugar. That’s why, at that time, sugar was banned from diabetic diets and sugar-free recipes for desserts, cakes and biscuits became a necessary part of diabetes therapy (even though they may have been high in fat).

However, research from Canada, Australia and the UK has turned this notion on its head and has shown that many starchy foods like white bread, potato and many types of rice are digested and absorbed very quickly (and therefore have a high GI ranking).

It also showed that sugar has only a moderate effect on blood sugar, often lower in fact than bread or rice or refined starches.

How can I use the GI?

The GI concept has applications for:

Diabetes

Someone with diabetes should aim to avoid peaks in their blood sugar levels. One way to achieve this is by eating more low GI foods that are slowly absorbed and which produce only a slow or gradual rise in their blood sugar. It is important to still have a healthy balanced diet, which has plenty of vegetables and favours healthy fats, and lose weight, if needed. AIM: one low GI carbohydrate food at each meal and snack.

Weight control

Those who are trying to shed a few kilos want to manage their appetite and still feel satisfied on a reduced kilojoule (Calorie) intake. Low GI foods help keep you full for longer and so stave off hunger pangs, meaning eating less is easier.

Endurance sports

An endurance athlete needs sustained energy during an event. Low GI foods before exercise help to deliver a steady supply of energy to working muscles and can help muscles recover after an event. Athletes also need to replenish carbohydrate stores quickly afterwards and so should choose foods high in GI. A glucose-containing sports drink, wedge of watermelon or some rice can help here.

High or low?

Carbohydrates that break down quickly during digestion have the highest GI values i.e. they cause a sharp rise in blood sugar. Examples include:

• pure glucose (GI of 100)
• baked potato
• puffed crispbread
• many refined breakfast cereals
• most of our modern starchy foods (white bread, snack foods and biscuits).

At the opposite end, foods that are digested slowly (i.e. with a low GI) help delay hunger pangs, satisfy for longer and make weight loss easier. For people with diabetes, they also mean better blood sugar control.

For slow digestion, we should look to low GI foods such as:

• kidney beans, chick peas and other legumes
• pasta
• muesli
• porridge oats
• apples
• grapefruit
• milk
• yoghurt
• bread with whole, intact grains.

How can you tell what’s low GI and what’s not?

There is no way to estimate what GI a food will have, as many factors influence it including:

• the type of starch in the carbohydrate (amylose is more slowly digested than amylopectin)
• the type of sugar (glucose is rapidly absorbed, whereas sucrose raises blood sugar levels only moderately)
• whether and how the food is processed or cooked
• whether there is any fat accompanying the carbohydrate, as fat slows the rate of stomach emptying and so slows digestion
• the presence of any viscous fibre accompanying the carbohydrate. This increases the viscosity of the contents of the intestines, slowing down the interaction between digestive enzymes and starch and so slowing digestion e.g. barley, oats
• the acidity of the food. Acid foods like vinegar, lemon juice, vinaigrette dressing and acidic fruit slow down stomach emptying.

How to find the GI of a particular food

There are two ways you can find out the GI of a food:

1. The first is by visiting the Sydney University’s Glycemic Index website where you can easily search their food database.

2. The second is through a food labelling program that helps consumers identify the GI values of foods in the supermarket. Known as The Glycemic Index Symbol Program, it uses an easily recognisable GI symbol on foods. To find out more about how the program works, visit the GI Symbol Program website which will tell you which foods are low GI and what the GI of any particular food is.

What is Glycaemic Load?

The effect of a carbohydrate food on blood sugar levels is not only determined by its GI or carbohydrate quality but also by the amount of carbohydrate in a particular food. Glycaemic Load is the term used to describe the overall effect of these two factors on blood sugar.

For example:

• The GL of a potato (GI around 90) containing approximately 18 grams of carbohydrate is 18 x 90% = 16.
• The GL of an apple (GI around 40) containing approximately 15 grams of carbohydrate is 15 x 40% = 6.

From this we can see the potato will produce a blood sugar rise three times that of the apple. When foods have similar amounts of carbohydrate, GI will have the greatest influence on blood sugars. When the amount of carbohydrate in a portion differs the best way to predict blood sugar effects will be via the GL.

Another example:

The GI of watermelon is high (GI = 72), but its Glycaemic Load is relatively low (GL = 7), because the quantity of carbohydrate in a slice of watermelon (150 g) is minimal, as it contains a lot of water.

GL and GI are similar concepts, except that GL takes serving sizes into account. It is calculated by taking the number of grams of carbohydrate in a serving, multiplying by the GI, and dividing by 100.

It is best not to try to keep your blood sugar levels down limiting the total amount of carbohydrate you consume. This can lead to a diet that is higher in fat and often higher in kilojoules, which can cause weight gain.

Some nutritionists use the following cut-off figures for a food's Glycaemic Load:

Low - Less than 10 points

Medium - 11-19 points

High - Over 20 points

Alternatively aim to keep the total GL per day under 100.

To read the latest about GI and GL you can subscribe to The Glycemic Index Foundation’s blog GI News or read their FAQs.