On this page we cover:
- What is the glycaemic index?
- Low GI carbohydrate foods and drink
- Medium GI carbohydrate foods and drink
- High GI carbohydrate foods and drinks
- What affects GI?
- Glycaemic index and diabetes
- Ways to include low-GI foods
What is the glycaemic index?
The glycaemic index (GI) tells us whether a food that contains carbohydrate raises blood sugar levels quickly, moderately or slowly. This means it can be useful to help you manage your diabetes or prediabetes as part of a healthy diet.
Different carbohydrates are digested and absorbed at different rates and glycaemic index is a ranking from 0 to 100 of how quickly each carbohydrate-based food and drink makes blood sugar levels rise after eating them.
But it doesn't take into account your portion sizes which can have a bigger impact on your blood sugar levels.
Low GI carbohydrate foods and drinks
- 55 or below raises your blood sugar levels slowly. These are sometimes called ‘slow release’ carbs.
Examples: strawberries, raspberries, pears, apples, oranges, grapes, dried apricots, carrots, tomatoes, spinach, mushrooms, broccoli, unsweetened milk, unsweetened yoghurt, beans, peas, lentils, coarse oatmeal (such as steel-cut oats), rye bread, yam and plantain.
Medium GI carbohydrate foods and drinks
- 56 to 69 raises your blood sugar levels at a medium rate
Examples: cantaloupe melon, pineapple, overripe bananas, sweet potato, pumpkin, parsnip, broad beans, new potatoes, couscous, basmati rice, chapatti, plain popcorn.
Most fruit and veg are either low or medium GI and they also provide important nutrients such as vitamin C and fibre, so they are good to include in your daily diet.
High GI carbohydrate foods and drinks
- 70 or more raises your blood sugar levels quickly
Examples: Refined carbohydrates such as white bread, some types of wholemeal bread, regular potatoes (baked, boiled, fried and mashed), bakery items like doughnuts, bagels, sweets, some sugar-sweetened breakfast cereals, sugar sweetened drinks and watermelon (see watermelon example below).
Most whole fruit and veg are either low or medium GI, they also provide important nutrients such as vitamin C and fibre, so they are good to include in your daily diet.
What affects GI?
Cooking method, ripeness and processing
The GI of a food can also vary significantly depending on factors such as:
- Cooking methods e.g. frying, boiling and baking
- Processing e.g. milling, pureeing
- Ripeness or variety of fruit and vegetables.
Combining foods with different GIs alters the overall GI of a meal
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.
Protein, fat and fibre can also affect the GI of a food, because they slow down the digestion and absorption of carbohydrates.
- 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 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.
Glycaemic index and diabetes
There is some evidence to suggest a diet high in total fat, high GI and low fibre is associated with an increased type 2 diabetes risk.
Should I eat low or high GI foods if I have diabetes?
Research has shown that choosing low GI foods can particularly help manage long-term blood glucose (HbA1c) levels in people with type 2 diabetes.
Swapping high GI foods and drinks for slower, lower ranking ones are better for managing blood sugar levels in people with gestational diabetes too.
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 sugar levels steady after eating. In the long term this can help reduce the risk of complications of diabetes such as heart and kidney problems.
Other things to think about
However, GI is just one aspect of a healthy diet – other nutrients such as protein, fat and fibre are important too. Foods containing these nutrients can help you feel fuller for longer and keep blood sugar levels more stable. So to make a healthier choice, it's important to think of the bigger picture and look at food labels.
Paying attention to carbohydrate portion sizes, spreading carbs evenly throughout the day, and avoiding large portions all at once, can further help keep sugar levels stable between meals.
Here are some ways you can include lower-GI carbs in everyday meals:
- Choose brown rice or basmati rice instead of white rice. Or, try plantain, quinoa, bulgur wheat or barley for a change.
- Eat wholemeal chapatti and include dhal in your meals.
- Use new potatoes with the skin on instead of old potatoes (like Russet or Maris Piper).
- Instead of white and wholemeal bread, choose granary, pumpernickel or rye bread.
- Swap frozen chips for wholewheat pasta cooked al dente or noodles.
- Try coarse oatmeal or muesli (unsweetened, oat-based) instead of sugar sweetened breakfast cereals
Is it OK to only focus on GI?
GI values offer a general guide to help you choose foods and can be helpful as part of a wider dietary plan.
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 manage your blood sugar 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 free sugars and contain more fruit and vegetables, some wholegrains, beans and pulses, nuts and seeds, unsweetened dairy, fish and shellfish.
It's also worth bearing in mind that:
- 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.
- Not all high-GI foods are unhealthy choices – a baked potato for example has a high GI but they contain nutrients that are beneficial for health, especially in the skin, such as fibre.
Diabetes and glycaemic load
The amount of carbs you eat has a bigger effect on blood sugar levels than GI alone.
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 sugar levels after eating.
The glycaemic load is a measure that combines the glycaemic index rating and the amount of carboyhydrate in that quantity of food.
The most important thing to do is to watch your carbohydrate portion sizes – once you do this, you will get an added bonus for choosing low-GI alternatives.
Learn more about GI
There are books that give a long list of GI values for many different foods. And there is also the International Glycaemic Index (GI) Database. 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.
If you’re living with diabetes and would like to learn more about using the glycaemic index to help manage your blood sugar levels, speak to your GP about a referral to a registered dietitian.
