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The Problems With The Glycemic Index

Thanks to some good quality nutritional science research over the last 20 years, more and more informed fitness, health and nutrition professionals are advising that controlling carbohydrate, not fat is the key to winning the weight loss war.

Realising carbohydrates are the problem, 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.

Sounds legit, until you dig a little deeper into the accuracy of testing and the practicality of GI in real life. We will look at these points individually and lead to a conclusion and advice based on the evidence…

The GI Diet, Glycemic Index, Vegetables, CarbohydrateThe Accuracy of Testing

To assign a food with its GI score, researchers use a group of healthy (disease-free, including diabetes) people. Participants eat 50 grams of a carbohydrate-containing food and their blood is tested every 15 minutes to see by how much and how quickly their blood sugar rises. The cut-off point for this measurement is 2 hours.
The higher and faster the blood glucose rises, the higher the GI score the food is given. The GI scale ranges from 1 to 100. Glucose, the most simple sugar is assigned a GI score of 100 and everything else is benchmarked against this.
Importantly, while GI scores are recorded in lab settings, these cutoffs were assigned in an arbitrary way.
Problems With GI Testing
  • 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.
  • Foods tested at different stages of their existence have different GI scores – the sugars in fruit crystallise during the ripening process, meaning a yellow banana has a higher GI than a freshly-yellow banana, but the GI testing can’t take this into account.
  • We metabolise 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 metabolised by us in varying ways, for example some people cope well with vegetables but not with grains, for example.

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 like we will be able to make a more informed choice as to whether or not the GI is worthwhile using as an accurate dietary tool.

Practicality of the GI in ‘Real Life’

  • GI is measured using carbohydrates in 50g portions, which aren’t always applicable to real life – we are unlikely to take in 50g of broccoli in a single sitting, for example. It takes little or no notice of portion sizes, meaning there is no practical advice on how best to eat certain foods.
  • Assuming a person follows the GI closely, they would potentially miss out on beneficial foods as they have a high GI score. Parsnips for example, have a GI score of 97 whilst a snickers bar has a score of 55. Snickers bars are obviously inferior to parsnips, but taking GI as gospel you wouldn’t think so.
  • 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. In English, 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.

These examples show that sometimes labs can only tell half the tale. There are practical implications that the method of testing for the GI scoring doesn’t take into account, such as the interference of other foods during the digestive process. This isn’t to be overlooked, as these seemingly small details can have large-scale affects on health and body composition.


The conclusion to be drawn is that there is far too much variability and too many mistakes in the measurement and application of the GI for it to be used as a stand-alone dietary advice tool. Unless the accuracy of the measurement could be guaranteed then I wouldn’t advise you to use it as it doesn’t take into account food quality, just the affect on blood sugar in a 2 hour window post consumption (in a fasted state), which isn’t applicable to real life to be taken seriously.


My dietary advice is based around food quality, not GI. I suggest you opt for vegetable and fruit carbohydrate sources – vitamin and mineral dense, natural and easily digestible. Try to avoid grain-based foods as these are linked to lots of digestive and metabolic disorders, not to mention hormone and energy fluctuations.

By sticking to sensible, natural carbohydrate sources you will keep your body fat levels in check and energy levels up!

The reality is, however, that hardly anyone actually does these things – we live in a world where there are increasing levels of obesity, diabetes, heart disease, medicinal drug use and overall declining wellbeing is the norm. It’s a trend that has to be reversed.

Thankfully, if you are diabetic, you can now use online services such as to sell your spare test strips. And as technology continues to advance, there will be more opportunities for those suffering with specific conditions to get better help.

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Owner of Personal Trainer, Father and fitness copy writer. Working hard making the world fitter and healthier!

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