A question you might wonder if you read the label on sports drinks is “why do they contain fructose?”
Is it because of taste? Partly, in some cases.
But the main reason is there are legitimately performance-related reasons for this addition.
This post discusses what those are and why sports drinks contain fructose.
Carbohydrates as a Fuel Source
Fat and carbohydrate are our two major fuel sources during exercise.
The energy we can obtain from fat is pretty much endless during exercise, even for lean individuals.
Carbohydrates on the other hand are relatively limited. We store far less glycogen in terms of total energy than fat. Our liver and muscles are only capable of storing so much.
And carbohydrates ARE our bodies best fuel source during exercise. Obviously, the carbohydrate vs fat debate is more complex overall, but for exercise intensity above a certain threshold, carbohydrates do improve performance.
Single Carbohydrate Sources
As a starting point, to get the most benefit from carbohydrates consumed during exercise, it makes sense that you want them to be quickly digested and absorbed.
Therefore, something like straight glucose makes sense over a lower GI complex carbohydrate for example. Although it is a little bit more complex since maltodextrin is technically a complex carbohydrate but happens to be absorbed at a similar rate to glucose.
The main issue with just using a single source like glucose or maltodextrin is that we are limited in how much we can absorb in a short space of time.
Glucose seems to have a maximal absorption rate of ~1.1g/min. This, therefore, limits the oxidation of glucose as a fuel source to ~1.1g/min after glycogen stores are depleted.
To simplify, this means that you are limited to being able to take on just over 60g of glucose per hour during exercise. Anything above this probably is not providing any additional benefit. Going beyond that number probably increases the risk of GI distress too.
Multiple Carbohydrate Sources
Different carbohydrate sources are absorbed through different pathways.
Glucose is absorbed via the glucose transporter SGLT1.
Fructose is absorbed via a different intestinal transporter called GLUT5.
By consuming multiple carbohydrate sources that are compatible with each other, you can metabolise more total carbohydrates.
Since carbohydrate can be a rate-limiting factor in certain endurance activities, it can help improve performance if you are able to increase your absorption and oxidation rates.
How Does Fructose Fit In?
While glucose seems to be limited to ~60g/hour, fructose seems to be limited to around 30g/hour.
This means that theoretically, you should be able to take on about 90g of carbs per hour to maximize performance.
It also explains why most products typically contain a 2:1 ratio of glucose to fructose.
Interestingly, size does not seem to effect absorption rates. Most people would assume larger people could absorb more carbohydrates per hour, but that does not seem to be the case.
It obviously depends on individual tolerance too.
Some people get GI symptoms at lower amounts. To build up to being able to take on 90g of carbs per hour without symptoms takes time, and in some cases, being lucky in terms of tolerating them well.
Certain individuals seem to be able to exceed this marker too. For example, a study on mountain runners found benefits from 120g of carbs per hour, in comparison to 90g per hour.
A study from Jorn Trommelen and colleagues highlighted the difference in carbohydrate oxidation after the consumption of glucose, glucose + fructose and glucose + sucrose.
The results are below, with credit to his Instagram @nutritiontactics.
As expected, the two options that had multiple carbohydrate sources significantly outperformed.
The glucose + fructose option slightly outperformed the glucose to sucrose option too though. Which also adds further weight to why fructose can be beneficial in sports drinks.
In terms of race outcomes, there is research supporting this as well.
One time-trial study compared water-only, glucose only and glucose + fructose. The glucose + fructose option outperformed the glucose only option by 8% and the water only option by 19%.
That is a massive improvement in performance, and it is relevant for a lot of athletes.
The reason why fructose is often included in sports drinks and sports gels is simply that it improves performance. By utilising a combination of multiple carbohydrate sources, athletes can absorb and utilise more total carbohydrates.
While this has been known for a relatively long period of time now by those in the sports nutrition space, when this was discovered, it was a pretty massive breakthrough for sports nutrition since it can genuinely improve performance by a significant margin.