Glycemic index useful or useless?

A lot of foods or diets are marketed to athletes based on their glycemic index. A quick browse on the internet will provide you with reasons why athletes should eat low glycemic index foods, but you will also find plenty of articles on the benefits of high glycemic index foods. There are sports nutrition brands entirely based on low glycemic index and some that are completely based on high glycemic index. In this blog we will look at what this glycemic index really is, and we will discuss how useful this is.. or how un-useful...


What is the glycemic index?

The glycemic index (GI) is a relative ranking of carbohydrate in foods according to how they affect blood glucose levels. Carbohydrate sources with a low GI value (55 or less) are more slowly digested, absorbed and metabolised and cause a lower and slower rise in blood glucose and, therefore insulin levels. The GI refers to the increase in blood glucose in response to a certain amount of food compared with a standard (usually 50g of glucose). The glycemic index is determined by measuring the area under the glucose curve (AUC) of the food of interest as well as the reference food (glucose).

The GI measurements are usually based on the ingestion of 50 g of carbohydrate and measurements of blood glucose over a 2-hour period. The greater the glucose response is and the greater the area under the curve is, the greater the GI of a food is. A greater GI indicates rapid absorption and delivery of the carbohydrate into the circulation.


The reference food, usually glucose or white bread, has a glycemic index of 100. Foods are generally divided into low-GI foods, moderate-GI foods, and high-GI foods. Low-GI foods have a GI of 55 or less, moderate-GI foods have a GI between 56 and 69, and high-GI foods have a GI of 70 or higher. Apples or lentils, for example, result in a slow and small rise of blood glucose concentration, whereas white bread or potatoes result in a rapid rise in blood glucose concentration. The apples and the lentils, therefore, are classified as low-GI foods, and bread and potatoes are classified as high-GI foods.

Why do we measure a glycemic index?

Now we know what the glycemic index is, let’s look at what all the fuss is about. High glycemic index foods are more rapidly absorbed and deliver carbohydrate more rapidly to the muscle. This is great if you are looking for an energy source during exercise or if you are looking for rapid glycogen synthesis after exercise for recovery.


High glycemic index foods are more rapidly absorbed and deliver carbohydrate more rapidly to the muscle.

It has also been claimed that low glycemic index foods will cause less disturbances in blood glucose concentrations and this is by definition true (at least in resting conditions because during exercise carbohydrate feeding whether low or high glycemic index don’t seem to affect blood glucose). On the other hand, low glycemic index foods are often related to better health, weight loss and weight loss or sometimes to more sustained energy during exercise. We will look at these claims in more detail in future blogs. This blog will discuss the usefulness of the glycemic index concept in general.

Why the glycemic index is NOT so useful

The glycemic index is an interesting concept, but it has also been heavily criticized and has limited practical relevance. The GI works well if we are dealing with isolated carbohydrate sources: glucose has a high GI, fructose a low GI. In general, foods with large amounts of refined sugar (simple carbohydrates) have a high GI, and sugars with high fiber content and complex carbohydrates have a lower GI. However, some complex carbohydrates (starches), can have a high GI, and simple sugars like fructose a low GI. This makes it all a little confusing.

We hardly ever consume isolated carbohydrate sources and normally we combine different foods when we eat and it is rather unpredictable what will happen when we combine various foods with different GI. For example, adding relatively small amounts of fat to a high-GI carbohydrate can lower the GI of the food substantially.


Serving size and glycemic index

To establish a GI, comparisons are made between two carbohydrate sources and the amount of carbohydrate needs to be standardized. For example, if we want to know the glycemic index of watermelon, we compare 50g of glucose with 50g of carbohydrate in the form of watermelon. This is a lot of water melon: 650g of watermelon to be precise. Or here is another example: carrots. To get 50g of carbohydrate from carrots, a person would have to eat about 750g of carrots. That is a lot of carrots to eat in a single session. So is glucose response still high if we eat carrots in a normal amount? This has led to the development of the glycemic load which takes into account the serving size.

High variability in glycemic index

Another and perhaps the biggest issue with the glycemic index, is that it is highly individual. The GI for any given food might vary considerably between individuals. Tables provide average values, and these averages are not necessarily useful when applied to an individual (not everyone is average, clearly).

Maybe because of these individual differences GI values in the literature are not consistent. Foods that are low GI according to one source are high GI according to another and vice versa. A useful web site to consult is www.glycemicindex.com.

The GI for any given food might vary considerably between individuals.

How should the glycemic index be used?

Glycemic index is often used to categorise foods into “good” or “bad”. Typically, high GI are classed as bad and lower as good. This dichotomous thinking is problematic. Whether a food is good or bad (I prefer to say appropriate, or not) depends on the context. In some cases, high GI foods are appropriate. If we compare exogenous carbohydrate oxidation of a low and a high GI food, the oxidation rates of the high GI food are higher and this would also mean easier digestion, faster absorption, less likely to cause gastro-intestinal issues and more likely to be advantageous for performance. This was confirmed in a study where higher GI carbohydrate sources ingested during exercise, resulted in less GI symptoms and better performance than low GI carbohydrates. Also, a recent review showed no effect of pre-exercise low GI carbohydrate sources on performance compared with high GI (2).

In other situations, it may be advantageous to select mostly low GI foods to manage blood glucose levels better. At least this is often theorised but direct evidence between blood glucose levels, within normal ranges, and performance is lacking).

The reality is that foods should be selected based on the goals, taking into account many other factors such as tolerance, taste etc. For healthy athletes, the glycemic index should not be leading in these decisions.


The GI is a theoretical concept that is perhaps useful for educational purposes and in some specific situations, but it must be interpreted and used with caution and there are plenty of limitations.

In summary

In summary, the GI is a theoretical concept that is perhaps useful for educational purposes and in some specific situations, but it must be interpreted and used with caution and there are plenty of limitations. If the limitations and pitfalls are well understood, it might be a useful tool in some situations.

References

  1. Oosthuyse T, Carstens M, Millen AM. Ingesting Isomaltulose Versus Fructose-Maltodextrin During Prolonged Moderate-Heavy Exercise Increases Fat Oxidation but Impairs Gastrointestinal Comfort and Cycling Performance. Int J Sport Nutr Exerc Metab. 2015;25(5):427-438.

  2. Burdon CA, Spronk I, Cheng HL, O'Connor HT. Effect of Glycemic Index of a Pre-exercise Meal on Endurance Exercise Performance: A Systematic Review and Meta-analysis. Sports Med. 2017; 47(6):1087-1101.

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