All You Need To Know About GI

by DeanMcKillop 3672 views Nutrition

All You Need To Know About GI

The GI (Glycemic Index) was created in the early 1980’s by a professor out of The University of Toronto in an effort to improve the dietary recommendations of carbohydrate intake in people suffering with Diabetes. Due to the omission of insulin in type 1 diabetics and the resistance in type 2 diabetics, it was proposed that by minimising the speed in which blood glucose rises within the blood, it would help support diabetic patients attempting to control their body composition due to decreased circulating blood glucose levels.

Sounds great in theory…

But what is the GI exactly?

The GI is a measurement of blood glucose elevation speed and its consequential retraction, following a 12hr fast and the consumption of 25-50g of carbohydrates from any given single nutrient food type.

Using pure glucose (sugar) as a reference point and a measurement score of 100, all other food groups then are measured against glucose and given a relative score.

Below are some of the current GI scores given to popular food types: 

Low <55 Medium 56-69 High >70
Grapefruit, Apples, milk, bananas, most vegetables, yoghurt, pears, custard, oats Ice cream, pinto beans, orange juice, brown rice, sweet potato, cous cous, pineapple White bread, white rice, carrot, potato, bagels, donuts, dried dates, honey

In essence, the higher the GI score the faster blood glucose will rise, which can be good in some instances such as speeding up glycogen replenishment but it can also be bad depending on the situation at hand and the persons physiological status.

Following the table above, someone who is looking to control their diabetes or someone trying to minimise blood glucose surges, should consume carbohydrate sources that are found within the first 2 boxes for the majority of their meals and only very limited smaller portions of the high GI products.

But there are some application flaws here…

Remember, GI is a measurement of the speed in which blood glucose rises after 12 hours of fasting and 25-50g of carbohydrate from one serve of a single nutrient food/meal.

So I ask… when have we ever completely fasted for >12 hours, whereby we then only eat 25-50g of carbs from 1 single food? Never, right?


Sure, we may fast for 6-8 hours while we sleep overnight, and perhaps we only ate 2 hours prior to bed time so we are now 8-10 hours deep into a fast, but when have you ever then consumed only 1 serve of 25-50g of carbs from one food at a time?

I have certainly never eaten 500g to 1kg of carrots for breakfast alone (25-50g of carbs)… have you?

Similarly, it is highly unlikely that within a westernised diet that a breakfast meal will contain only 1 single nutrient, of which, the moment an additional nutrient is introduced, the GI of the food consumed is altered.

For example

  • Protein consumption alongside a carbohydrate will slow gastric emptying.
  • Fat consumption alongside a carbohydrate will slow gastric emptying.

Which means the moment we consume a carbohydrate alongside a protein or a fat (unsaturated), regardless of its GI prior to being combined with another macronutrient, the GI has now been altered and slowed down due to the gastric emptying speed of the secondary nutrient.

Why am I referencing breakfast you may ask?

I am solely referencing breakfast because it is the one true time we are all fasted, which is a requirement for GI to be considered applicative. 

Knowing this, it could be argued that utilising GI is somewhat irrelevant as the science of how GI is determined versus the reality of its application, do not match up in principle and practice.

In saying that, this does not mean that GI is useless but instead it should be utilised more appropriately to determine the cause and effect of a serve of a particular carbohydrate post consumption.

Which is where Glycemic Load (GL) comes in

The GL of a food takes into account both the speed in which blood glucose rises and the amount of carbohydrates consumed within a meal as well, which is far more indicative of its effect on blood glucose levels in a standardised diet.

In lay terms, the GL is determined by both the speed a food type will rise blood glucose as well as the total serve of carbohydrates it contains, which is far more important when considering the effect it has on the body.

Let’s look at the example of a GI of 100 food

A food with a GI of 100 yet only 10g of carbohydrates per serve has a GL of 10 and a food with a GI of 10 but 100g of carbohydrates in a serve will also have a GL of 10.

So where we would normally avoid the high GI food due to its expected effect on blood glucose, it may be pertinent to consider the serving size as having greater weight than the type of carbohydrate after all as both of these scenarios yield the same GL despite being at the opposite ends of the spectrum in GI.


Similarly, where a low GI food is normally promoted as healthy and great to eat, consuming a large quantity of such a food can be considered as equally stressful as consuming a smaller intake of a higher GI food as the GL places the food into the context of its application instead of looking at it in isolation.

You see, the great thing about GL as opposed to GI, is that GL has real world application opportunity, whereas GI is primarily only relevant in a laboratory setting.

And therein lies the problem with the majority of nutritional advice in today’s society…

We take scientific finds of tremendous specificity and attempt to apply them to the general population without reference to the necessary specification of their application.

While the score of a carbohydrates GI may provide insight into the simplistic nature of its digestive speed, it fails to recognise its dose dependent relationship in real world application.

Instead of focusing on the GI of a food, consider its total GL as this is a far more accurate indication of a carbohydrates effect on blood sugar levels.


Exercise Scientist

I completed my Exercise Science Degree at the University of QLD and have worked in the fitness industry for over 8 years, including a short stint at the Brisbane Broncos in 2010 as a student. I also hold my Level 2 Strength and Conditioning Coach accreditation (ASCA) and have competed in 1 bodybuilding season, placing 2nd at the IFBB u85kg Nationals.

ViewDean's Articles