What Is The Glycaemic Index? | How To Measure Low & High GI Foods


By Personal Trainer & Health Consultant |

William Slatter

The glycaemic index (GI) is a way of quantifying how consuming different carbohydrates can influence an individual’s blood glucose levels.  It assigns carbohydrates and carbohydrate-rich foods a score between 0 and 100, with higher scores reflecting a carb which quickly gets digested, and causes a sharp increase in blood glucose which then quickly subsides.

A low GI carb however is more slowly digested, so this produces a slow and steady rise and fall in blood glucose, and these are often referred to as ‘slow release carbs’.

Low GI carbs have a score of 55 or less.

Medium GI scores are between 56 and 69.

High GI scores are those exceeding 70 points.

Knowing more about the GI of foods has benefits for all groups, however it is particularly important for those who have a prior diagnosis of diabetes!

Why Is Glycaemic Index Important?

If your blood glucose levels fall too low, then you’ll feel tired and sluggish, which will affect your ability to perform not only physical, but also mental tasks.  If blood glucose rises too high however, then your body signals to the pancreas to secrete more insulin to get glucose out of the bloodstream, however the body will then store some of this excess sugar as fat.

Also, high blood glucose will trigger a large release of insulin, which may actually reduce blood glucose levels to below required levels.

gi glycaemic index✓ Based on this, low GI foods can ensure your body has an adequate energy source from blood glucose, but as the
levels are more consistent, rather than rapidly rising and falling, the body is less likely to store sugar in the blood as fat.

This can support the notion that diets containing mostly low GI foods can reduce the risk of developing cardiovascular disease, type II diabetes, and metabolic syndrome.

Due to the way that different carbohydrates cause different changes in blood glucose, a low GI food (below 55 points) should keep you feeling full for longer, which will help you avoid snacking and consuming extra calories.

✓ High GI foods however are more likely to cause a quick glucose rise, followed by a crash, which often leads to people snacking over and over again.

So in theory:

300 calories of low GI carbohydrates will keep you feeling fuller for longer
>>  Than 300 calories of high GI carbohydrates!

GI Scores of Everyday Foods

The following table is adapted from 2008 research investigating GI scores of common foods (2).  Another comprehensive database of GI scores can be found on the glycaemic index website (3).

We have just included some every day foods so you can get a sense of what their GI scores are, however head over to the website for anything not mentioned below!

Lucozade Original 95
Rice cakes 87
Cornflakes 81
Porridge (instant oats) 79
White Potato (boiled) 78
White bread 75
White Rice (boiled) 73
Brown Rice (boiled) 68
Pitta Bread 68
Popcorn 65
Sweet Potato (boiled) 63
Coca Cola 63
Porridge (rolled oats) 55
Banana 51
Orange juice 50
Orange 43
Baked beans 40
Chocolate 40
Milk (full fat) 39
Milk (skimmed) 37
Carrots (boiled) 39
Apple 36
Nutella 33
Kidney Beans 24
Peanuts 7

Glycaemic Index Can Change

Depending on what other foods are paired with a singular carbohydrate, the GI ranking may reduce!

How GI Scores are calcuated

To calculate a food’s GI score, the carbohydrate (with no other food) is fed to a group of 10 or more people who have fasted the night prior to testing, which minimises the effect of other stomach contents on the way the body digests the carbohydrate in question.

These individuals then provide small blood samples at 15-30 minute intervals over the next few hours after consumption to determine blood glucose levels and track how quickly it changes.  From this information, the food can be assigned a GI score.

When do GI scores change?

Although the above is a good method when it comes to ensuring consistency across GI scores, in the real-world it is rare to just eat a carbohydrate by itself, without some other food alongside it.

1) GI reduces: when paired with protein/fats

The effects of fat and protein on GI have been explored in scientific research, and there is fairly substantial evidence that fats and protein reduce the glycaemic response to carbohydrate intake (4).

What this means is that although carbohydrates are assigned GI scores, it is worth remembering that these scores can change depending on the other components to your meals.

2) GI reduces: when combined with vinegar/lemon juice.

3) GI increases when carbohydrates are cooked

Interestingly, cooking carbohydrates for longer can actually increase GI, as can ripeness of fruits with a lot of carbohydrates in them, such as bananas. 

For this reason, the GI score is a great guide, but it is worth learning how other foods can influence the way the body digests these carbohydrates.

Glycaemic Index vs Glycaemic Load

Glycaemic Index can tell us how quickly a food will impact on blood glucose, however a similar measure is glycaemic load (GL).

low gi snack foods

GL is another way of looking at what carbohydrates do to blood glucose levels, but it also incorporate the amount of carbohydrates in a food as well as how quickly it can raise blood sugar levels.  This allows you to work out how much a food will raise blood sugar levels, rather than simply knowing how quickly blood sugar will rise.

Calculating the Glycaemic Load

Simply multiply the grams of carbohydrate in a food, by the food’s GI score, and then divide by 100.

GL above 20 is considered high.

GL 11-19 is moderate.

✓ GL below 10 is low.

The benefit of glycaemic load is that it can help influence portion sizes, which is arguably more important than just knowing how quickly a food can influence blood sugar.

Take Home Message

The GI score of a food is a great way of telling you whether it’ll give you a quick energy boost, or whether it’ll slowly release energy into your system to keep you going throughout the day.  It is important to remember however that eating carbohydrates alongside proteins and fats can change a food’s GI score, so use GI as a guide rather than getting too caught up in the numbers.

Consider looking at glycaemic load rather than glycaemic index, as the glycaemic load can put this information in perspective of portion sizes, which may be more useful in the long run.

Key notes:

The glycaemic index is a way of quantifying what different carbohydrate sources do to the sugar levels in your blood stream.

Low GI foods are digested more slowly, and can help you feel full for longer than the same number of calories from a high GI food.

Eating carbohydrates alongside fats and protein can change the GI score of the carb source, so the glycaemic index should be used as a guideline, but values can fluctuate.


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