When it comes to healthy eating, you may have heard the term glycaemic index (GI) mentioned alongside carbohydrates, blood sugar and diabetes. But what exactly does GI mean, and should you be choosing foods based on their GI rating?
Understanding the glycaemic index can help you make more informed carbohydrate choices and build balanced meals without having to eliminate carbohydrates from your diet.
People without diabetes generally do not need to worry about the glycaemic index because the body naturally manages blood sugar fluctuations after eating.
What is the glycaemic index?
The glycaemic index is a system used to classify carbohydrate-containing foods according to how quickly they raise blood glucose levels after eating.
GI values are generally grouped into three categories:
Low GI: 55 or less
Medium GI: 56–69
High GI: 70 or more
Low-GI foods tend to cause a slower and more gradual rise in blood glucose, while high-GI foods are digested and absorbed more quickly and can produce a faster rise.
Importantly, GI only applies to foods that contain significant amounts of carbohydrate. Foods such as meat, fish and most oils don't have a meaningful GI value.
Why does the glycaemic index matter?
The amount and type of carbohydrate we eat can influence blood glucose levels. Choosing lower-GI carbohydrate foods more often may help produce a steadier blood glucose response.
For people managing diabetes, understanding GI can be particularly useful as part of an overall healthy eating pattern. However, GI is only one piece of the nutritional picture.
A food's GI does not tell you whether the food is healthy overall.
For example, a food can have a low GI but still be high in saturated fat, salt or calories. Conversely, some nutritious foods may have a moderate or high GI.
Examples of low- and high-GI foods
Lower-GI choices include:
Rolled oats
Most beans and lentils
Chickpeas
Some wholegrain breads
Many fruits
Non-starchy vegetables
Plain yoghurt
Higher-GI foods can include:
White bread
Many highly processed breakfast cereals
Some rice products
Potatoes, depending on the variety and preparation
Sugary foods and drinks
What affects a food's GI?
Several factors can influence how quickly carbohydrate is digested and absorbed.
Processing
Generally, more highly processed carbohydrate foods can be digested more quickly. Grinding, milling and other processing methods can change the structure of a food and influence its GI.
Cooking
Cooking can soften starch and make it easier to digest. The cooking method and duration can therefore affect a food's GI.
Ripeness
The ripeness of fruit can also affect its carbohydrate composition and digestion. For example, a riper banana generally has a higher GI than a less-ripe banana.
Fibre
Fibre can slow digestion and carbohydrate absorption. Foods naturally rich in fibre, such as legumes, whole grains, vegetables and many fruits, can therefore be useful components of a balanced diet.
What you eat with it
GI describes an individual food under standardised testing conditions, but we rarely eat foods in isolation.
Combining carbohydrate with protein, healthy fats, fibre and other foods can influence the overall blood glucose response of a meal.
The bottom line
The glycaemic index can be a useful tool for understanding how carbohydrate-containing foods may affect blood glucose in people with diabetes. Choosing high-fibre, minimally processed carbohydrate foods and combining them with nutritious sources of protein, fats and vegetables can support a balanced eating pattern.
However, GI shouldn't be used in isolation. Portion size, overall nutrition, meal composition and individual health needs all matter.
For people with diabetes or other conditions requiring careful blood glucose management, individual advice from an accredited dietitian or healthcare professional can help translate these principles into an eating pattern that suits their needs.