The Glycaemic Index

What is the Glycaemic Index?

The Glycaemic Index (GI) is a ranking of foods based on their overall effect on blood glucose levels. Slowly absorbed foods have a low GI rating, whilst foods that are more quickly absorbed will have a higher rating. This is important because choosing slowly absorbed carbohydrates can help even out blood glucose levels when you have diabetes.

Foods are given a GI number according to their effect on blood glucose levels. Glucose or white bread is used as the standard reference (GI 100), and other foods are measured against this. The effect on blood glucose levels of a portion of the test food containing 50g of carbohydrate is compared with the effect of the reference food (white bread or glucose) over a three hour period. It was previously thought that if you ate the same amount of carbohydrate, then whatever that carbohydrate was, it would have the same effect on your blood glucose levels. It is now known that different carbohydrate-containing foods have different effects on blood glucose levels. For instance, 30g of bread does not have the same effect as 30g of fruit or pasta.

What are the benefits of slow acting carbohydrates?

Because meals including low GI foods allow you to absorb carbohydrate more slowly, they help to maintain even blood glucose levels between meals and can therefore help you avoid ‘hypos’. The effect of a low GI meal can run into the following meal, which helps keep blood glucose more even during the whole day.

Slow acting carbohydrates will also reduce the peaks in blood glucose that often follow a meal, and this may have a role in helping to prevent or reduce the risk of getting Type 2 diabetes in those at risk. There are also benefits for weight loss. Low GI foods can help you to control your appetite by making you feel fuller for longer, with the result that you eat less. Research has shown that people who have an overall low GI diet have a lower incidence of heart disease.

Lower GI diets have also been associated with improved levels of ‘good’ cholesterol. One or two small changes can make all the difference. 

Does anything else affect GI?

Yes. Determining the GI of a meal is not so easy as reading a number off a chart. The addition of fat and protein slows down the absorption of carbohydrate. Chocolate has a medium GI because of its fat content therefore, and crisps and chips will actually have a lower GI than potatoes cooked without fat. Milk and other dairy products have a low GI because of their high protein content, and the fact they contain fat. The consequence of this is that if people were to confine themselves to low GI foods, their diet would be unbalanced and high in fat, which could lead to weight gain and increase their risk of heart disease. For this reason it is important not to focus exclusively on GI and to think as well about the overall balance of the diet.

Cooking method (frying, boiling and baking), processing, the ripeness of a fruit and the variety of a vegetable will also all affect a food’s GI rating. The structure and texture of a carbohydrate have an effect as well. Pasta and durum wheat have a low GI rating, whilst whole grains and high-fibre foods act as a physical barrier that slows down the absorption of carbohydrate. This is not the same as ‘wholemeal’, where, even though the whole of the grain is included, it has been ground up instead of left whole. For example, some mixed grain breads that include wholegrains have a lower GI than either wholemeal or white bread.

Can I get hold of a list of GI values for all foods?

There are books that give a long list of GI values for many different foods. This kind of list does have its limitations however. The GI of a food only tells you how quickly or slowly it raises the blood glucose when the food is eaten on its own. In practice, we usually eat foods in combination as meals - bread is usually eaten with butter or margarine, or as an accompaniment to a meal, for example; potatoes are often eaten with meat and vegetables. So cutting out all high GI foods is not the answer. The good thing is you can apply the GI concept so that you can lower the overall GI of a meal by including in it more low GI foods. You need to think about the overall balance of your meals, which should include starchy foods and be low in fat, salt and sugar.

The table below gives an indication of the GI rating for some common foods:

How can I get the benefit of GI?

Combining foods with different GIs alters the overall GI of a meal. You can maximise the benefit of GI by including a low GI food with each meal or snack, to lower the overall effect on your blood glucose levels. There are of course multiple combinations of foods that you could pick, but a few suggestions are given below:

Breakfast

• Use an oat-based breakfast cereal and eat some fruit
• Try Muesli, All Bran, Sultana Bran, Special K

Snacks

• Get into the habit of eating fruit as a snack (as well as part of a main meal)
• Yoghurt (choose low fat varieties)
• Popcorn
• Go easy on lower GI foods like chocolate and nuts, which have a high fat content, especially if you are trying to lose weight
• Rye bread and fruit loaf (again, watch amounts, especially if you are trying to lose weight)

Lunch

• Add baked beans to your jacket potato
• Try a lentil based soup
• Add variety with different breads, eg pitta bread and bread made with a substantial amount of mixed grains and pumpernickel
• Grilled chicken, salad, rice and peas

Evening meal

• Try basmati rice, sweet potato, buckwheat, bulgar wheat, pearl barley and noodles with your meal
• Include more vegetables with your meal
• Eat more pasta based meals
• Include more beans and pulses (dahl)

How strict should I be at applying the GI?

Eating to control diabetes isn’t just about GI ratings. It’s about eating a balanced and appealing diet, which you can sustain through time. It’s not all rye bread and pulses, it’s about adding variety, not imposing restrictions, in order to get the full benefits of low GI foods. Give some thought to the different starchy carbohydrates available — cereals, breads, potatoes, rice, pasta, yams, chappatis, pitta bread, cassava and other grains. A state-registered dietitian can give you more individual advice on food choices.

Further information

Visit www.glycemicindex.com (University of Sydney, Australia) for an online database of foods with their GI rating.  (Diabetes UK is not responsible for the content of external websites.)

The new glucose revolution by Dr Anthony Leeds, Professor Jennie Brand Miller, Kaye Foster-Powell, Dr Stephen Colagiuri (Published by Hodder and Stoughton, 2003) is available from most good bookshops.