The Glycemic Index – What it is and why it’s relevant to diabetes

The Glycemic Index – What it is and why it’s relevant to diabetes

When navigating foods to control your diabetes, it can be confusing. There’s carbohydrates, protein foods, good fats, bad fats, vegetables, fruit but fruit is sugar and no single food group contains just one type of nutrient.

No wonder managing your diabetes can be difficult. Then to add to the mix, you’re healthcare practitioner may have mentioned something called the glycemic index.

I imagine sometimes it can begin to feel like researching what to eat is a full time job.



I’ve previously written about the types of food which will and will not effect your blood glucose levels which you can find by clicking this text. 

The glycemic 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 GI. A food with a score of 100 will be the quickest acting carbohydrate and thus be high GI. 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.

As a result, 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 GI compared with a food which is low GI.

GI Diet

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 values of all the GI foods and 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 GI. Research has shown lower GI 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 glycemic 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 GL in action is watermelon. watermelon is high GI 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.

Fat & Protein – Zero GI but need to be considered

I have also encountered patients who begin eating foods with a medium to low GI score when the reality is the food is high GL. A prime example of this is chocolate. This is simply because carbohydrate is not the only macronutrient contained within these foods. Chocolate is made alongside fat and some protein. Fat and protein help slow down 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 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 rice is the lowest GI rice available.

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 suck them. Mash potato will quickly be ready to swallow whereas a raw potato will not break down without chewing. 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 most 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.

Also consider fat may slow down absorption making a food lower GI but it doesn’t mean it is good for you. I have already mentioned chocolate but roast potatoes will be low GI because they are coated in fat but are not the most healthy foods to be eating.

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 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.






8 thoughts on “The Glycemic Index – What it is and why it’s relevant to diabetes”

  1. I have just been diagnosed with borderline type 2. Most of my family members have Type 2. My daughter has Type 1. I had gestational whilst pregnant with her. My family are not overweight. I am 5’9″ and weigh 9 stone. I mostly eat fruit and vegetables, fish, chicken. Prior to my diagnosis I have developed GERD and have followed a diet to prevent phlegm. This has meant giving up dairy products and switching to Almond milk etc. I wonder if being very ill with GERD has prompted Type 2.

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