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 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 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 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.
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 to slow down 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. So 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.
Note: If you were to restrict yourself to eating only low GI foods, your diet is likely to be unbalanced and may be high in fat and calories, leading to weight gain and increasing your risk of heart disease. 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.
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 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.
Note: The amount of carbohydrate you eat has a bigger effect on blood glucose levels than GI alone.