The glycaemic index (or GI) is a measure of how quickly foods will increase your blood glucose levels. Therefore, only carbohydrate based foods are included on this scale which excludes both protein and fat based foods. Each food is given a score on a scale out of 100. This then tells us whether a foods is high, medium or low on the glycaemic index scale. A food with a score of 100 will be the quickest acting carbohydrate and thus be high glycaemic index. The lower the score, the more slowly this food will effect your blood glucose levels and the higher the score, the more rapidly the food will increase your blood glucose levels.
A score of 55 or less officially classes a food as low GI,
56-69 is medium and
70+ is considered high GI.
If we were to compare a snapshot of your blood glucose levels following a meal at any one time, more glucose will have entered the system with a high glycaemic index meal compared with a meal that is low glycaemic index.
The area under the curve demonstrates how much glucose is entering your circulation. Therefore, you can see less glucose entering your system at any single time with lower GI foods.
There are tables available which tell you the actual glycaemic index of foods. These can be found via a simple Google search or books are available on the subject.
I tend to avoid looking at the values themselves but rather stick to general principles. Typically, foods such as vegetables, wholegrain, fruit, milk, pulses, oats and basmati rice are low glycaemic index. Research has shown lower glycaemic index foods are particularly beneficial for those suffering with type 2 diabetes. There is less evidence in type 1 diabetes.
Glycaemic Load (GL)
One draw back of the glycaemic index is it does not factor in the amount of carbohydrate eaten. Glycaemic load factors in the amount of carbohydrate in a typical portion of food together with how quickly it raises blood glucose levels.
Like glycaemic index, glycaemic load rates foods based on a score.
Foods scoring 0-10 are considered low
Foods scoring 11-19 are considered medium
Foods scoring 20+ are considered high.
This is calculated using a simple equation. GL = GI x carbohydrate content in each serving / 100.
A prime example of glycaemic load in action is watermelon. Watermelon is high glycaemic index yet the amount typically eaten has a low carbohydrate content. Therefore, the GL will be low.
In the past, I have encountered individuals cutting out certain foods because they are high GI when in reality the amount they are eating would mean it is low GL. Therefore, the overall effect on their blood glucose levels would be minimal.
I have also encountered patients eating medium to low glycaemic index foods not realising these foods are high in carbohydrate. Therefore, the GI may be modest but the glycaemic load is high.
Therefore, the best strategy is to eat well portioned amounts of carbohydrate whilst ideally choosing the lower GI version.
Fat & Protein – Zero glycaemic index but need to be considered
Chocolate has quite a low glycaemic index score. This seems strange because chocolate is a very sugary food. However, chocolate does not only contain sugar. It also contains a lot of fat. Like the opposite of watermelon, chocolate has a lot of carbohydrate in each bar but it is quite slowly released into the body.
This is simply because fat and protein help slow down the absorption of foods. This means foods with carbohydrate, fat and protein will score lower on the GI scale. Chocolate has a high carbohydrate content per portion but a reasonable GI score. Therefore, chocolate will be low-medium GI but likely high GL.
This effect is particularly pronounced at meal times when we combine food groups. As mentioned, protein and fat both act to slow down the absorption of carbohydrate. When eating a balanced meal, the stomach literally has more to deal with and therefore it takes time to break down all the food which helps to slow down the rise in your blood glucose levels.
So watermelon will have a minimal effect on your glucose due to it’s low carbohydrate content. This is despite the fact the carbohydrate which is contained within it is quickly released. Chocolate on the other hand is quite slowly released but will effect blood glucose much more than watermelon due to its overall carbohydrate content.
So what should I do, Low GI, Low GL, a combination – Tell Me!
Circumstances when GI and GL may be beneficial is when choosing foods at meal times where many varieties may be available. Potatoes and rice are prime examples of this. As a rule, sweet and new potatoes are lower GI than standard jacket potatoes. Basmati and brown rice are low GI foods.
Foods that are generally medium to high glycaemic load include sugary foods like cakes, biscuits, sweets etc and any starchy foods. Vegetables and fruits tend to be lower GL.
How food is cooked
Avoid/minimise mashing or over cooking potatoes to the point when they are too soft/fork mashable because this increases the GI and GL. Softening potatoes begins to break down the starch in the food which allows the body to more easily absorb it. Therefore, a raw potato and a mashed potato will have two very different effects on your blood glucose levels despite a similar carbohydrate content.
An example I often highlight to patients is to picture what happens to foods when you put them in your mouth and swallow. Mash potato will quickly be ready to swallow whereas a raw potato will not break down without chewing. Not that I’m suggesting eating raw potatoes. It’s just to highlight the point.
The same principle is highlighted by breakfast cereals. Consider sucking a cornflake compared to a wheat biscuit. The cornflake will disintegrate whereas the wheat biscuit will stick around for a while. That said, even wheat biscuits become soft and moist once milk is added to them.
What is happening in your mouth is similar to what happens in your stomach, If a food is easy to swallow quickly without chewing much, the stomach won’t have much trouble absorbing it either. Therefore, it’s likely high glycaemic index.
Also remember fat slows down the absorption of carbohydrates. Therefore, very fatty foods may seem like a good option if looking only from a glucose standpoint. However, these may not be the best option from a health perspective. A takeaway curry would be a good example to highlight this point. It will be quite slow releasing due to its fat content. However, it will be high in calories and fat and is unlikely the healthiest meal.
Hopefully, you can begin to see the pro’s and cons of relying solely on GI and GL. Ultimately, focusing only on GI or GL without factoring in the portions on your plate, the balance in your diet, what your weight is doing and how fit you are, will not help your diabetes control. GI and GL are only additional tools alongside many others, which can help you reduce your blood glucose levels.
Personally, I wouldn’t go as far as calculating GI or GL for most foods. As I mentioned there are tables available on Google and there are books which you can buy highlighting the values of each food.
Generally speaking, the advice is likely what you would expect. Choose foods such as wholegrains, vegetables, salads, fruit, pulses, beans, lentils and brown/basmati rice and have a healthy balanced diet combining all food groups.
Avoid/minimise foods with a high sugar content such as chocolate, cakes, sweets and biscuits and eat a modest amount of fat.
Be consistent with this and you will likely see a reduction in your blood glucose levels.
Need more help
If you need more support we offer a range of 1:1 consultancy services and online programmes focused on helping you improve your glucose control and health for the long term. These include our type 2 diabetes recovery programme, type 1 glucose stability programme and winning weight loss programme. Go to the pages using the links above or in the headers or get in touch if you have questions.
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