Glycemic Index - Useful or Not?

Over the past 20 years nutritional science research has shown that controlling carbohydrate, not fat is the key to health.

Researchers analysed different carbohydrate containing foods and began to classify them using the glycemic index (GI). The following explanation of the GI is taken directly from

The glycemic index (GI) is a ranking of carbohydrates on a scale from 0 to 100 according to the extent to which they raise blood sugar levels after eating.

The problems with the glycemic index:

1. Accuracy of Testing

To get a food’s GI score, researchers used healthy subjects, who ate 50 grams of a carbohydrate-containing food and then tested their blood every 15 minutes to see by how much and how quickly their blood sugar increased over a 2-hour period.

The higher and faster the blood glucose rose, the higher the GI score the food was given. Glucose, the most simple sugar is assigned a GI score of 100 and everything else is bench-marked against this.

2. Some of the Problems

  • The tests are performed when the participant is fasted, meaning the affect of other foods are not taken into account.

  • Blood glucose is only measured 2 hours post food consumption – not long enough given digestion can last over 24 hours.

  • The testing method only tells us how QUICKLY a carbohydrate is turned into glucose, not how MUCH is turned into glucose.

  • We metabolize and digest carbohydrates at different rates, meaning even with group testing GI is very general and not specific to the individual meaning its accuracy is compromised.

  • Different carbohydrates are metabolized by us in varying ways, for example some people cope well with vegetables but not with grains.

These are just a few of potentially lots of concerns about the accuracy of the testing. When we look at the practicalities of using the GI in every day life we will be able to make a more informed choice as to whether or not the glycemic index is worthwhile using as a dietary tool.

3. Practicality

  • The glycemic index is measured using 50g carbohydrates portions, which aren’t always applicable to real life – we are unlikely to eat in 50g portions, for example raw sugar versus broccoli, meaning there is no practicality on how best to eat certain foods.

  • You may also miss out on potentially beneficial foods as they have a high GI score. Parsnips have a GI score of 97 whilst a snickers bar has a score of 55.

  • We rarely eat only one carbohydrate source at a time or carbohydrate-only meal, and fats and protein can slow down absorption of carbohydrate meaning real-life GI scores would be different to lab scores.

  • By only looking at a blood glucose response, we ignore the insulin response in relation to the carbohydrate.

    • A food that raises insulin quickly, but for a short time only is potentially less likely to aid fat storage than a food that keeps insulin elevated for longer, given insulin is one of the primary hormones responsible for fat storage.

    • Prolonged insulin elevation thanks to slow-release, low GI foods may lead to higher fat storage than high GI foods such as fruits as although fruit raises insulin dramatically, the effect is short-lived.


There is far too much variability and too many mistakes in the measurement and application of the glycemic index for it to be used as a stand-alone dietary advice tool.

Since the accuracy of the measurement isn’t guaranteed, and just the impact on blood sugar over a 2-hour period in a lab situation, not taking into account the food quality, I wouldn’t suggest that it should be used in real life and be taken too seriously.