What is the Glycaemic Index?
The Glycaemic Index (GI) is a ranking of carbohydrate containing 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 have a higher rating. This is important because choosing slowly absorbed carbohydrates, instead of quickly 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 is used as a standard reference (GI 100) and other foods are measured against this; a food portion (containing 50g of carbohydrate) effect on blood glucose levels over three hours, is compared to the effect of 50g of glucose.
What does the research say about low GI diets?
Research looking at the effect of GI on blood glucose control varies.
- A review of research (a cochrane review) showed that HbA1c can be lowered by 0.5 per cent in people with diabetes who adopted a low GI diet (although the majority of studies were related to people with Type 2 diabetes).
- Some studies have shown that people who have a high GI diet tend to have higher HbA1c levels.
- Two recent trials have shown following a low GI diet has no benefit compared to following the American Diabetes Association dietary education.
Slow acting carbohydrates will also reduce the peaks in blood glucose levels that follow a meal, and this may have a role in helping to prevent or reduce the risk of developing Type 2 diabetes. Research has shown that lower GI diets have also been associated with improved levels of ‘good’ cholesterol and a lower incidence of heart disease.
Does anything affect GI?
Yes, in theory choosing a food on the basis of their GI enables people to keep their blood glucose levels constant, however we know determining the GI of a meal is not as easy as reading a number off a chart.Factors that may affect the GI of a food include:
- Cooking methods: frying, boiling and baking.
- Processing and the ripeness of fruit and certain vegetables.
- 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 either wholemeal or white bread.
- Fat lowers the GI of a food. For example chocolate has a medium 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 of their high protein content, and because they contain fat.
The consequence of this, is that if you were to restrict yourself to eating only low GI foods – your diet would 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. For this reason it is important not to focus exclusively on GI and to think about the balance of your meals, which should be low in fat, salt and sugar and contain plenty of fruit and vegetables.
Can I get a 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. 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 margarine 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.
It is important to note the amount of carbohydrate you eat has a bigger effect on blood glucose levels than GI alone. Looking at the GI values of pasta and watermelon you might expect that a typical portion of watermelon would affect your blood glucose levels more than pasta. This is not the case because it is the amount of carbohydrate that is the most important factor in determining your blood glucose levels. The ‘Glycaemic Load’ (GL) takes into account the Glycaemic Index (GI) of the food and the amount of carbohydrate in a portion and gives you a more accurate picture of what really happens when you eat carbohydrates.
The following equation shows you how the Glycaemic Load (GL) is calculated.
Glycaemic load = GI x g of carbohydrate
|GI of 50g of carbohydrate
|Typical portion size
||2 inch slice|
|Amount of carbohydrate in a portion
||38x70 = 27
|72x6 = 4|
If you were to eat a typical portion of about 200g of cooked pasta, it would provide you with 70g of carbohydrate. Despite having a low GI it would have a bigger impact on your blood glucose levels than if you were to eat a typical portion (2 inch slice) of high GI watermelon, which would provide you with only 6g of carbohydrate. The Glycaemic Load of a portion of pasta is 27 and of watermelon it is only 4.
What is the advice if you want to choose lower GI carbohydrate containing foods?
- Instead of instant cooked rice, choose basmati or easy cook rice. You could also try pasta or noodles.
- Switch baked or mashed potato for sweet potato or boiled new potatoes.
- Instead of white and wholemeal bread, choose granary, pumpernickel or rye bread.
- Swap frozen microwaveable French fries for pasta or noodles.
- Instead of rice snap breakfast cereals or cornflakes, try porridge, natural muesli or wholegrain breakfast cereals.
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 switching to a low GI option food with each meal or snack. There are multiple combinations of foods that you could pick, but a few suggestions are given below:
- Try an oat based breakfast cereal, for eg porridge.
- Add sliced fruit to wholegrain breakfast cereals.
- Add baked beans to your jacket potato and serve with a large green salad.
- Try a bean based or vegetable soup.
- Eat a variety of different breads, for eg grainy or pumpernickel bread, instead of white or wholemeal bread.
- Consider boiled potato or sweet potato instead of mashed potato with your meal.
- Choose basmati or easy cook rice instead of long grain rice.
- Include plenty of vegetables with your meals.
- Include more beans and lentils in your meal, try adding them for eg in casseroles and curries.
- Get into the habit of eating fruit.
- Low fat yogurt.
- Go easy on lower GI foods like chocolate and nuts, which are high in fat and calories, especially if you are trying to lose weight, so save them for occasional treats.
How strict should I be in applying the GI concept?
Eating to control your diabetes isn’t just about GI ratings and should not be used in isolation. Choosing foods solely on the basis of their GI without regard to their content of energy, saturated fat or salt is unlikely to result in a healthy diet.
Although some research has shown that low GI diets help in controlling blood glucose levels, the amount of carbohydrate you eat has the biggest influence on your blood glucose levels after meals.
The overall balance of your diet is also important and it should be low in fat, salt and sugar with plenty of fruit and vegetables. A registered dietitian can give you more individual advice on food choices so ask your doctor to refer you.