Why should you fuel with both glucose and fructose?

Glucose and fructose are two different types of sugars that can often be found in sports fueling products. Since they are both sugars and a source of carbohydrates, why is important to make sure that you are eating both during a training session?

Consuming both glucose and fructose sugars during exercise can help increase the amount of carbohydrates you can absorb into your system, thereby helping you stay fueled during your workout and avoiding that bonk!

Why? 

Let’s dive into how carbs are absorbed by your digestive system. Carbs are actually broken down first in your mouth. Certain enzymes in your saliva help to begin this process. As the carbs enter your small intestines, other enzymes work to further digest the long polysaccharide chains of carbs into their individual components, or monosaccharides. These monosaccharides are then small enough to be able to be absorbed into your body through the walls of your small intestine via transport proteins.  

You have a variety of different transport proteins that work to transfer different types of monosaccharides through your small intestine. Glucose and fructose are examples of two different monosaccharides and they each bind to their own unique transport proteins in the small intestine lining.  

These transport proteins can become oversaturated when there are too many molecules of a specific type of monosaccharide that is ingested, i.e. solely using glucose based sugars when fueling. For instance, if you have 200 molecules of glucose in your small intestines, but only 100 transport proteins that bind to glucose, all of those specific transport proteins will be busy and no matter how much more glucose you consume, you will NOT be able to absorb those carbs. However, if you were to include some fructose, then those transport proteins specific to fructose would be able to move those monosaccharides across the intestinal lining, so you would still be able to harness energy from fructose based carbs in addition to the glucose based carbs.

Many of our clients are high performing athletes whom we recommend consuming up to 90g of carbs per hour. At such a high amount of carbs, it is important to make sure that you are consuming different types of monosaccharides so that your body can absorb all of the carbs that you are ingesting. This can also help mitigate any GI discomfort that often results from consuming so many carbs.  Something an athlete may notice if they are consuming 60+ g carbs per hour solely from one sugar form is stomach sloshing as the increased sugars left in the intestine will pull water out of the blood stream and back into the intestine.

If you are looking to learn more about your specific fueling needs before, during, and after a training session, be sure to check out our 4 Week Sports Nutrition Education Program (SNEP) to calculate your unique nutrition requirements!


Reference: 

  1. Fuchs, C. J., Gonzalez, J. T., & van Loon, L. (2019). Fructose co-ingestion to increase carbohydrate availability in athletes. The Journal of physiology597(14), 3549–3560. https://doi.org/10.1113/JP277116
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