Carbohydrates are an important type of macronutrient, providing the human body with a main source of energy. However, not all carbohydrates impact the body in the same way. Glycaemic index (GI) is a marker of how a food will raise blood sugar levels.
Different carbohydrate containing foods and drinks are broken down, digested and absorbed at different rates. The higher the GI, the more quickly the food is absorbed, giving a more rapid rise to blood sugar levels. It can be a helpful tool to look at the GI of foods to help with making healthier choices and for people who need to manage blood sugar levels, such as those with prediabetes or diabetes. However, it isn’t just as simple as choosing lower GI foods to help improve blood sugar levels.
We will explore how to apply the GI of foods into making better choices for overall health, the factors that can impact the GI of foods, as well as the overall importance of using GI as a guidance for healthier eating.
How is GI calculated?
The GI of foods is measured on an index of 0 to 100. It is measured against a reference of pure glucose (sugar), which has a GI of 100.
It has been calculated based on tests carried out in participants, where certain foods containing 50g of available carbohydrates were given. The blood glucose of these participants was measured at regular intervals within 2 hours of consuming these foods. From this time, an average glucose response of that food is calculated and converted into a GI score, out of 100.
Anything with a GI of 55 and below is classed as low GI, and this includes the majority of fruits and vegetables, milk, pulses, beans and nuts. Generally, these foods lead to a slower rate of digestion with a more steady and gradual rise in blood sugars. A GI of between 55-70 is classed as medium, which includes foods like honey, basmati rice and wholemeal bread, having a moderate impact on blood sugars. Higher GI foods above 70 include potatoes, short grain rice and white bread. These foods cause a more rapid spike in blood sugar on average, followed by a quick crash.
How can knowing the GI of foods be helpful?
Research has shown that choosing lower GI foods more regularly in our diet can help support long term blood glucose levels, in those with type 2 diabetes. Although there is less evidence for people with type 1 diabetes, it’s still advised to choose majority of lower GI foods, to help prevent sharp rises and falls in blood sugars after meals.
However, not all low GI foods are typically healthy choices. Many lower GI foods can contain a higher number of fats, as fat actually slows down absorption and digestion of the carbohydrates. As an example, many milk and white chocolates have a lower GI, though we would not recommend frequent and large consumption of these foods!
This is why it is important to think about the overall nutrients in the food, and not just the GI.
What can impact the GI of foods?
Although the GI of a food is based on previous research, there are several factors that can influence the GI, increasing or even decreasing the number. This is important to keep in mind when consuming these foods.
The cooking and preparation method
If starches such as pasta and potatoes are overcooked, this can lead to the GI increasing, as the food has been broken down more, meaning it is more readily digestible. However, it’s been shown that cooling and reheating starches can actually lower the GI, as it helps to develop resistance starch. This is a starch that resists digestion, acting like a fibre. The development of resistance starch in these foods means the foods are digested slower, bringing down the overall GI.
The processing of the food
If a food has undergone more processing, this can increase the GI of the food. Processing can be helpful to extend the shelf life of foods or alter the texture and flavour, but it is important to consider the impact this can have on the GI of the food item. This can be because the fibres have been broken down more, making the carbohydrates easier to digest. As an example, white bread is a more processed form of wholegrain bread, and therefore the white bread has a high GI, vs the wholemeal load which has a medium GI.
Other macronutrients found in the food
Foods that contain a higher amount of protein and/or fats, will naturally bring the GI of that food down. This is because these macronutrients take a longer time to digest and break down, meaning the digestion of the sugars from the foods are also slower. As an example, many milk products, which do contain a certain amount of sugars from the milk sugar known as lactose, are also lower GI, as they contain both proteins and fats.
Fibre content of foods
Choosing higher fibre carbohydrates will provide us with a number of health benefits. Firstly, higher fibre, wholegrain foods take a longer time to digest, as fibre is not able to be broken down by our gut enzymes. Therefore, this slows down the breakdown of the sugars in the food, and reduces the blood sugar spike in response. Another great reason to include higher fibre foods in the diet is because the fibre is a nutrient source for the microbiome residing in the gut. A higher fibre diet has been shown to have a positive impact on blood sugar control overtime, which can be linked to the action of these gut microbes.
The ripeness of the food
As a fruit ripens, this will increase the GI content, as the starches from the fruit are broken down more into simple sugars. For example, a green banana can have a GI of 40, whereas ripe bananas increase in GI to around 65.
Healthier GI swaps
If you are looking to make some healthier food swaps to choose lower GI options, here are some to consider:
- Choose a wholegrain bread over a white bread
- Swap peeled and mashed potato, for roasted potato with skin on
- Choose a wholegrain cereal such as Shredded Wheat or oats, instead of refined cereals such as Cornflakes
- Swap couscous for a whole grain such as quinoa or bulgar wheat
- Choose a lentil or bean based pasta instead of regular wheat pasta
Limitations of GI
Although knowing the GI of foods can be helpful, there are several drawbacks that need to be kept in mind, if you are using GI as a guidance of food choices.
Portion sizes are not considered
GI is ranked on foods per 50g of carbohydrates. However, portion sizes of foods can differ, which can alter the actual carbohydrate content of foods. So someone may eat a portion of food that provides less than 50g carbohydrates and will therefore have a lower GI than stated. This is where the glycaemic load (GL) is important (see below for more information).
Food combinations are not considered
The GI of foods are based on that food in isolation. However, it is not typical to eat these foods in isolation, as they are often part of a main meal. For example, we would likely eat pasta with a sauce, vegetables and a type of protein, rather than just pasta on it’s own. Adding foods that contain a rich source of fibre (such as vegetables) and proteins (such as chicken or fish, or legumes) can help lower the GI of the total meal, as these slow down the digestion of the carbohydrates.
Individual responses are not considered
Although GI of foods are based on an average database of the people tested, it does not account for differences in individuals. Such as their gut microbes, which can impact the way our blood sugar responds to certain foods, or genetic differences and sensitivity to insulin, which can be impacted by many factors including age, gender and weight.
Low GI foods doesn’t mean healthy
Certain foods that have a lower GI than others, such as milk chocolate, which has a GI of 45. Based on this, you would think that consuming chocolate regularly may be ok due to it’s GI. Similarly for sausages, which have a GI level of 28. However, these foods are still high fat and processed and therefore are not foods we suggest consuming in large amounts regularly!
What about Glycaemic Load (GL)?
Unlike GI, the GL of foods considers both the GI and the portion size of the food. Foods with a high GL is over 20 have a significant impact on blood sugar, medium is between 11-19, and low is a GL of 10 or less, which has minimal impact on blood sugar.
This can alter certain foods that have a certain GI, but then when adjusted for portion, can change. For example, watermelon has a GI of around 76, but a GL of around 5 per serving. Looking at the lower GI foods is better than just GI alone, as it considers portion sizes.
Focusing just on GI can lead to overconsumption of higher fat foods, that can naturally be lower GI, but are high in calories. This may contribute to weight gain, which in turn can complicate the control of blood sugar, as well as increasing risk of heart disease.
If you have diabetes, it isn’t only GI that needs to be considered. Think about the overall food choices, food combinations and meal timings, focusing on higher fibre, wholegrain foods, which are also low in saturated fats, salt and added sugar.
How to use GI or GL in food choices
Using lower GI carbohydrate foods more often such as wholegrains like quinoa and brown rice can be helpful, as well as foods high in fibre such as legumes, nuts and seeds, fruits and vegetables. Higher GI foods may be helpful to consume right before or soon after higher intensity exercise for a quick release of energy, or when someone is experiencing hypoglycaemia (low blood sugar). If you want to consume high GI foods, combining it with lower GI foods can help to overall reduce glucose spikes and better manage overall blood sugar control.
Glycaemic index can be a valuable tool for looking at how different carbohydrates influence blood sugar levels. It is important for diabetes management and overall weight control, though glycaemic load is often a more practical measure, when it comes to looking at realistic portion sizes.
Ultimately, focusing on a minimally processed, whole food diet, rather than only following a low GI diet, is key for long term health and wellbeing. If you want to make changes to your diet to support your blood sugar levels, reach out to our diabetes dietitians today. Learning how to make informed dietary choices around carbohydrates will help optimise your energy levels, metabolic health and overall well-being.
