The low down on glycaemic index, glycaemic load and insulin index

Post by Emily White

You hear so much in the nutrition world about glycaemic index, insulin index and glycaemic load and how you should avoid particular foods that fit into certain categories. Just walk down the cereal isle in the supermarket and you will see an abundance of packaging boasting that this particular product is ‘low GI’ and therefore the healthy choice. With all this confusion about these different terms it’s worth having a think about what in fact they actually mean. Should we be putting a lifetime ban on foods that have a high glycaemic index and on the contrary; does a low glycaemic index food mean it is a great healthy option? 

So firstly, the glycaemic index of a food is the level that indicates the effect that that food will have on a person’s blood glucose level.  Glycaemic load is similar in that it looks at how a food affects blood glucose levels but it also takes into consideration the amount of carbohydrate in that portion of food. The insulin index- as it sounds; is the measure used to quantify the effect certain foods have on insulin response. 

Looking at it a bit more closely imagine someone has a steady blood glucose of say 4-5mmol/L. When we eat something that is high on the glycaemic index or has a high glycaemic load (say a chocolate bar) we will see a high rise in blood glucose above 5mmol/L followed by a high drop off (to below 4-5mmol/L). This drop off is the action of insulin.  During the drop off where we are trying to restore blood glucose back to normal is where we experience fatigue and a ‘crash’.  The action of insulin converting glucose to glycogen for storage will also encourage the storage of carbohydrates as body fat. This is negative which is why we try to avoid these big peaks and troughs and why high GI food is considered ‘bad’. 

If however we eat a food that is much lower on the glycaemic index and has a much lower glycaemic load we will see a much more steady and proportionate rise and fall in blood glucose. This makes it much more easy for us to take in these CHO, with much less chance of crashes and storing body fat. 

However here is where it gets complicated.  You can get foods where the insulin index does not match the glycaemic index. For example you could have a food that has a high insulin index and a low GI. An example of this is milk (1, 2). The low glycaemic index means you have a small peak (blood sugar only rises slightly) but still quite a large drop off  (below 4-5mmol/L) due to the action of insulin. Therefore you are potentially still going to see increased fat storage and poor energy even though this food is ‘low GI’. Over time because such a large amount of insulin is being secreted, the ability of that insulin to be up taken effectively and utilized will be degraded.  This can result in reduced insulin sensitivity in the long run. Now I’m not saying drinking milk will give you diabetes, I am just showing the discrepancies between the different indices!

Other times we may see foods that have a high GI but low glycaemic load. An example is a carrot (1). They are considered high GI as they contain sugars that are digested rapidly.  However they do not contain a lot of carbohydrates so you would have to eat a lot of carrots (in the realm of dozens) to get an appreciable spike in blood glucose. 

The fact that a mere carrot could be put on the ‘do not eat’ list of anything shows the uselessness of the GI measure! Not to mention pasta is considered a ‘low GI’ food and therefore a healthy option which isn’t necessarily the case for many people most of the time (although that’s a debate for another day!).

Therefore the take-home message is that if something is natural, whole and unprocessed don’t be afraid of it just because it has a high GI. Similarly, if something is low GI don’t automatically assume it is going to be the best option for you. 

Overall it is the total quality of what we eat that is important, not just picking and choosing indices (such as GI, GL, ID). But hey, that’s just me and my somewhat bizarre belief that if something is natural, whole and unprocessed it’s probably going to be superior over food that’s not (irrespective of the lists they find themselves on)… I know, I’m weird like that. 

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1. Atkinson, F. S., Foster-Powell, K., & Brand-Miller, J. C. (2008). International Tables of Glycemic Index and Glycemic Load Values: 2008. Diabetes Care, 31(12), 2281-2283. doi: 10.2337/dc08-1239

2. Holt, S. H., Miller, J. C., & Petocz, P. (1997). An insulin index of foods: the insulin demand generated by 1000-kJ portions of common foods. The American Journal of Clinical Nutrition, 66(5), 1264-1276.