What is the glycaemic index?
The glycaemic index (GI) tells us whether a food raises blood glucose levels quickly, moderately or slowly. This means it can be useful to help you manage your diabetes. Different carbohydrates are digested and absorbed at different rates, and GI is a ranking of how quickly each carbohydrate-based food and drink makes blood glucose levels rise after eating them.
- The glycaemic index (GI) tells us whether a carbohydrate-containing food raises blood glucose levels quickly, moderately or slowly. This means it can be useful to help you manage your diabetes. Different carbohydrates are digested and absorbed at different rates, and GI is a ranking of how quickly each carbohydrate-based food and drink makes blood glucose levels rise after eating them.
- The GI index runs from 0 to 100 and usually uses pure glucose, which has a GI of around 100, as the reference. Slowly absorbed carbohydrates have a low GI rating (55 or below), and include most fruits and vegetables, unsweetened milk, nuts, pulses, some wholegrain cereals and bread
Research has shown that choosing low-GI foods can particularly help manage long-term blood glucose (HbA1c) levels in people with Type 2 diabetes. There is less evidence to support this in people with Type 1 diabetes, but we know that on a day-to-day basis choosing low GI foods can help keep blood glucose levels steady after eating.
- Not all low-GI foods are healthy choices – most chocolates, for example, have a low-GI because of their fat content, which slows down the absorption of carbohydrate.
- Combining foods with different GIs alters the overall GI of a meal. You can maximise the benefit of GI by switching to a low GI option with each meal or snack. Go easy on lower GI foods like chocolate, which is high in calories, especially if you are trying to lose weight. Save them for occasional treats.
- Eating to manage your diabetes isn’t just about GI ratings. Think of the bigger picture and choose foods high in fibre and wholegrains, as well as low in saturated fat, salt and sugar, as part of a long-term healthy diet.
Glycaemic index and diabetes – Q&A
What else affects GI?
- Cooking methods: frying, boiling and baking
- Processing and the ripeness of fruit and certain vegetables
- Fibre: wholegrains 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 wholemeal or white bread.
- Fat lowers the GI of a food. For example, chocolate has a low GI because of its fat content, and crisps will actually have a lower GI than potatoes cooked without fat.
- Protein lowers the GI of food. Milk and other dairy products have a low GI because they are high in protein and contain fat.
Is it OK to only focus on GI?
If you focus only on the GI of foods, without looking at other aspects, your diet could be unbalanced and high in fat and calories, which could lead to weight gain (making it harder to control your blood glucose levels) and increase your risk of heart disease. It’s important to think about the balance of your meals, which should be low in saturated fat, salt and sugar and contain more fruit and vegetables, wholegrains, pulses, nuts and oily fish.
The amount of carbs you eat has a bigger effect on blood glucose levels than GI alone. For example, pasta has a lower GI than watermelon, but pasta has more carbs than watermelon, so if you eat similar amounts of either of these two foods, the pasta will have more of an impact on your blood glucose levels after eating. The most important thing to do is get your portion size right – once you do this, you will get an added bonus for choosing low-GI alternatives.
It’s easy to include low-GI carbs in everyday meals:
- Choose basmati or easy cook rice, pasta or noodles. Or, try plantain, quinoa or bulgur wheat for a change.
- Eat wholemeal roti and include dhal in your meals.
- Use new potatoes instead of old potatoes – try sweet potatoes for a change.
- Instead of white and wholemeal bread, choose granary, pumpernickel or rye bread.
- Swap frozen chips for pasta or noodles.
- Try porridge, natural muesli or wholegrain breakfast cereals.
Educating yourself on GI
There are books that give a long list of GI values for many different foods. This kind of list does have its limitations. The GI value relates to the food eaten on its own and in practice we usually eat foods in combination as meals. Bread, for example is usually eaten with butter or vegetable-oil based spread, and potatoes could be eaten with meat and vegetables.
An additional problem is that GI compares the glycaemic effect of an amount of food containing 50g of carbohydrate but in real life we eat different amounts of food containing different amounts of carbohydrate.
Note: The amount of carbohydrate you eat has a bigger effect on blood glucose levels than GI alone