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  • Aiveen Connolly

Myth Busting: Glycaemic Index

First of all, what is the Glycaemic Index? The Glycaemic Index (GI) was designed to more accurately describe the effect that different foods have on your blood sugar levels. It was originally developed to help people with diabetes to control their blood sugar levels, but it can also be beneficial for regular exercisers and elite athletes too. The index is essentially a ranking of foods from 0 to 100, based on their immediate effect on blood glucose (blood sugar) levels. It is recommended by the World Health Organisation (WHO) that GI values should be used as a guide for people's food choices.

The use of GI is a very debatable topic among exercise and health professionals, but in some studies the GI nutritional recommendations show an improvement in exercise performance and enhances exercise capacity.

In the performance nutrition industry, it can be useful to classify foods into high GI and low GI. High GI foods are rapidly absorbed and cause a rise in blood sugar levels. The higher the GI, the higher the blood sugar levels will be after eating that food. Examples of high GI foods are baked potatoes (GI 85) and white rice (GI 87). Low GI foods are slowly absorbed and cause a smaller rise in blood sugar levels. Fibre and fat content of food tends to lower the GI of a food. Foods high in fibre take longer to digest and thus produce a slower rise in blood sugar levels. Examples of low GI foods include porridge, lentils, beans, fruit, vegetables and dairy products.

Research has shown consumption of low GI meals 2-4 hours pre exercise provides a slower release of energy and improves the ability to last longer in endurance activities and delays fatigue. Researchers have found cyclists who ate a low GI meal containing lentils (GI 29), 1 hour before exercise, managed to keep going for 20-minutes longer compared to cyclists who consumed high GI foods.

The GI value of any food item is affected by many factors, including preparation of the food, food processing and what other foods are eaten at the same time. The factors that influence the GI of a food include the size of the food particles, the biochemical make-up of the carbohydrate, the degree of cooking and the balance of fat, sugar, protein and fibre.

However, it is important to note that there are some limitations to using the GI. The biggest limitation of using GI is that it does not take into consideration the portion size you are eating. For example, watermelon has a GI of 72, which would be classified as a high GI food. If you measure out an average slice of watermelon or 120g of watermelon, this amount only contains around 6g of carbohydrates which is not enough to rapidly raise blood sugar levels.

Another limitation is some high fat foods have a low GI. For example, crisps/chips have a lower GI compared to a baked potato. It is best to avoid selecting foods only by their GI and to consider the total amount of carbohydrates, fat, fibre and salt. Avoid consuming large amounts of saturated and trans fats as there are many benefits of replacing saturated fats with unsaturated fats. Also, the GI value tells us nothing about other nutritional information. For example, whole milk has a GI value of 31 but it also has a high fat content so whole milk may not be the best choice for weight loss or weight control.

In summary, the GI is a useful way to categorise carbohydrates into high GI and low GI. High GI foods reduce a rapid rise in blood sugar while low GI produces a slow rise in blood sugar. Low GI foods are a great option to consume 2-4hrs before exercise to help improve endurance and delay fatigue. However, there are a number of drawbacks with the GI that need to be taken into consideration when determining what foods you want to consume.




Thomas DE, Brotherhood JR, Brand JC. Carbohydrate feeding before exercise: effect of

glycemic index. Int J Sports Med. 1991 Apr;12(2):180-6. doi: 10.1055/s-2007-1024664.

PMID: 1860741 .


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