How Many Grams Of Carbohydrate Should I Eat During Exercise?

One of the most effective ways to enhance performance as an athlete undertaking prolonged intense exercise is through fuelling, by which I am specifically referring to the strategic intake of carbohydrates during a session.  In this article, I’ll cover the key aspects you need to know when deciding how many grams of carbohydrates you should consume per hour during exercise to maximize performance. This is one of the most common questions I get asked as a nutritionist and an area where the textbook guidelines that are commonly shared might not be the best approach for you.

Background – Why fuelling’s important?

Carbohydrates, particularly glycogen (the name for carbohydrates when stored within the muscle and liver), is the predominant fuel source the muscle will use for any moderate to high-intensity exercise, as illustrated in the graph below from a classic study back in 1993 (1).

As you can see, the higher the % of VO2 Max that an athlete is exercising at, the more energy they are expending (as denoted by the larger bars with increasing exercise intensity), and as illustrated by the lightest green bar, the more muscle glycogen that is being used. Plasma glucose, or blood glucose, is typically regulated by the breakdown of liver glycogen. Muscle triglycerides and plasma-free fatty acids (FFA) show the contribution of fat as a fuel. Fats contribution is highest at moderate exercise intensities but is reduced at high intensities. As you can see, the harder you work, the more carbohydrate that is utilized; with high exercise intensities (i.e., 85% VO2 Max), a very larger proportion of fuel is derived from carbohydrates.

When rested and well fuelled, we typically have anywhere in the region of 1500-2500kcals of glycogen stored within the body, equivalent to around 375g - 625g of carbohydrate with around 60-100 grams in the liver and the rest distributed around the muscles. The more trained we are, the greater our capacity to store glycogen and, therefore, the greater amount of work we can do before we become fatigued. The graph above illustrates the different amounts of glycogen an athlete can store based on how well-trained they are (2).

Generally, when exercising at a moderate to high intensity, we can burn through a large proportion of our glycogen in as little as 60-90 minutes. However, this time frame depends on various factors, including the individual athlete's metabolism (i.e. the rates at which they use fat and glycogen), their diet around training, exercise intensity, environmental conditions in which they are exercising, and the type of exercise they are performing.

 When glycogen stores reach critically low levels (as shown by the red bar on the left of the graph above), fatigue sets in, and we typically see a sharp reduction in performance. Our rating of perceived exertion climbs through the roof; our muscles struggle to function, we see a large drop in exercise economy, our blood glucose drops too low levels, and we blow up/bonk/hit the wall—a generally unpleasant experience.

Alongside maximizing glycogen stores before exercise through diligent fuelling in the 6-24 hour period before a hard session, we can also use fuelling (i.e. carbohydrate intake) during the session to prolong the point at which we become fatigued and allow us to maintain our performance.  

The graph above shows what happens if you don’t fuel (3). This is from a study in which athletes exercised at a fixed moderate intensity for as long as they could whilst consuming only a placebo (i.e. water). As you can see from the vertical axis, which shows the percentage contribution of the different fuels, the use of muscle glycogen reduces to very low levels (due to depletion of these stores) over the course of the 3 hours. As exercise duration progresses, there is an increased reliance on other carbohydrates (i.e., blood glucose derived from liver glycogen). At 3 hours, the contribution from carbohydrates drops to low levels (likely due to both liver and muscle glycogen stores reaching low levels), at which point the athlete fatigues as they cannot continue riding at the fixed pace. 

The graph above is from the same study, using the same participants, exercising for as long as they could at the same exercise intensity, but in this trial, participants were provided with additional carbohydrates in the form of a sports drink. As you can see, they were able to go for significantly longer (1 hour longer) compared to the placebo group on the other graph, and this is likely to be explained by the maintenance of carbohydrate use at higher levels late into the exercise task, with other carbohydrates (i.e. those taken in from the sports drink and liver glycogen) preventing the increase in fat use and potentially delaying the point of fatigue. In effect, taking in fuel can enhance our capacity to exercise and allow us to exercise harder for longer.

How much does taking in carbohydrates during exercise enhance performance?

How much does fuelling during exercise enhance performance? This will be very context-specific and be influenced by a whole host of factors, from the demands of the exercise task and your individual metabolism to the duration of the activity. In 2014, two researchers completed a systematic review of a significant majority of the data to date on carbohydrate supplementation and exercise performance (i.e. how quickly you can complete an exercise task such as a cycling time trial) and capacity (i.e. how long you can ride at a fixed pace before you blow up/fatigue) across a broad range of durations (4). In total, they examined over 60 studies, which tested close to 700 athletes, and found that over 80% of the studies (50 studies) showed a significant performance benefit from carbohydrate supplementation, which, in the world of sports nutrition, is pretty substantial.

As you can see in the figure below from the study (4). When taking all the studies together and plotting the general trend, they showed the performance improvement seen in the carbohydrate trials compared to the water/placebo trials. The key takeaways from this graph are that as exercise duration increases, the impact of fuelling during the ride-on performance is more significant. For exercise durations of 1-2 hours, performance increased by around 5%; this climbed to as high as 10% when exercising for longer than 2 hours and when high rates of carbohydrates were fed (more on that later). These figures don’t necessarily directly translate into real-world performance, but a 10% improvement in performance, even in the lab, is enormous!

How many grams of carbohydrates should I be eating to maximise performance?

So you now know how carbohydrates improve performance and to what degree they can improve performance, but how much should we be taking on? The optimal dose of carbohydrates to consume during exercise will depend on several factors, which we’ll get into below. The table below illustrates the current sports nutrition guidelines that you may have seen or heard of before (5). However, as is often the case with guidelines that generalize things into easy-to-follow advice, they aren’t always the best approach or relevant to everyone, so I wouldn’t recommend blindly following these.

In line with the performance data and the guidelines above, the longer an athlete exercises, the greater the recommended carbohydrate intake to maximize performance. For exercise durations less than 1-2 hours, typically only small amounts of carbohydrates are required to optimize performance, as providing you are well-fuelled pre-exercise, you should have sufficient glycogen available to fuel that activity without carbohydrate availability limiting performance. For short-duration exercise, there is some evidence that carbohydrate provides benefits beyond simply providing more fuel (as fuel availability is unlikely to be limiting to performance) and that merely having a small amount of carbohydrates in the mouth is detected by receptors within the oral cavity, which stimulate reward centres within the brain that help make exercise feel a little easier and therefore improve performance, particularly in the fasted state or when glycogen stores are low.

With any exercise task below two hours, where the recommendations for carbohydrates are pretty low, there are very few issues with taking on these very modest amounts of carbohydrates; they are unlikely to cause any stomach issues, and their impact on performance is relatively small, so for the vast majority of athletes, there isn’t an issue with following these. At these intakes, the type of carbohydrate you consume has a much smaller impact, too, with most normal foods containing carbohydrates easily able to achieve these, compared to higher intakes, which are only possible with specific formulations.

However, as you get beyond 2+ hours, there is a significant chance of depleting glycogen stores, and the additional carbohydrates consumed during exercise can significantly affect performance through its metabolic benefits (i.e., providing more fuel, allowing up to maintain carbohydrate use, and preventing the drop in blood glucose)—as we previously covered. There are also a lot more potential risks when it comes to fuelling exercise beyond 2 hours, and these strategies could have adverse effects on the athlete, so it’s predominantly these that I am challenging within this article.

Why the optimal dose for exercise of 2.5+ hours is probably not 90 grams an hour?

As you can see from the table above, when exercise duration gets beyond 2.5+ hours, the recommendation is to consume 90 grams of carbohydrates per hour from multiple transportable carbohydrates. However, having completed research in this area and having a pretty good understanding of the current research within this area, there are a few reasons why I think there are issues with this guideline and why there is a lot more context to consider, so the vast majority of athletes probably shouldn’t blindly follow it.

Carbohydrate dose and performance

One of the main issues with the research supporting the intake of 90g/hr is that very few studies supporting this guideline have compared multiple doses of carbohydrates to see if a lower or higher dose would be as/more/less effective. Researchers often compare intakes of one dose of carbohydrates against a placebo, which is then the evidence used to inform this guideline. Or, in this case particularly, compare 90 grams of glucose and fructose, which can be absorbed, with 90 grams of glucose alone, which has significant potential to result in stomach upset, which can negatively impact performance and skew the performance data towards the 90g/hr glucose and fructose trial.

One of the most comprehensive dose-response studies to date, which would address this issue, compared the intake of carbohydrate (CHO) doses ranging from 10 grams an hour up to 120 grams an hour during 2 hours of fixed-intensity cycling before conducting a simulated time trial to measure performance (6).

The study modelled performance based on their results which is shown in the figure above. As you can see from how I’ve butchered the figure, increasing your carbohydrate intake from low to moderate doses, we see more carbohydrate equals more performance, with the bulk of the improvements in time trial performance when athletes increased intakes from 0grams of carbohydrate an hour, up to around 60grams of carbohydrate an hour. There was then a sweet spot of around 70-80 grams of carbohydrates an hour (the study concluded this was 78g/hr), with the best time trial performance seen here. Increasing carbohydrate intake beyond this failed to improve performance further; in fact, it got worse. There are potentially a few different reasons for this, of which stomach issues induced by high intakes of carbohydrates (in athletes who may not be well accustomed to it) are likely a key one.

This isn’t the only study to suggest that smaller doses than the current guidelines may be as/more effective either. A study from the University of Stirling compared intakes of 39g per hour to 64 grams an hour (7). If more carbohydrates were better for performance, particularly when using a demanding exercise task that is multiple hours long (as they did), you’d expect to see the 64g/hr trial result in better performance, but this study saw no difference in performance between the two trials and similar metabolic responses. Similarly, a study from Baur et al. looked to address the limitation I mentioned earlier around the comparison of 90 grams of glucose-fructose, to an equal amount of glucose that is likely to result in gastrointestinal distress (8). They compared 60grams and 90grams an hour of glucose to 90grams an hour of glucose-fructose and found that there wasn’t a clear difference in performance between the 60grams an-hour glucose trial and the 90grams an-hour glucose-fructose trial, again suggesting that more modest intake of fuel might be equally as effective for longer duration activities.

Performance is complex and really difficult to measure in the laboratory, so there are always limitations, but this should illustrate that things aren't as clear-cut as you might imagine. In particular, I think there are only relatively modest improvements in performance from larger doses compared to more modest ones, which may be practically easier for athletes to implement in the real world.

As such, if you’re an athlete and looking to enhance performance, you’ll likely get a lot of the benefit from more modest doses, in the range of 40-70grams per hour. As such, this is where I would be aiming most of the time unless you have extensively trialled higher doses in training without adverse effects. We also see individual variability within these trials, so the optimal dose is likely specific to your circumstances, and the only real way to find this out is through your own experimentation.

Maximizing exogenous oxidation

When it comes to taking in carbohydrates during exercise, there is a limit to how much can physically be absorbed in the gut and utilized by the muscle (referred to as exogenous oxidation - i.e., the use of the carbohydrate you’ve taken in as a fuel rather than your endogenous glycogen stores), and this is a significant factor in influencing the dose that is recommended to athletes. Using carbohydrates like glucose, there appears to be an absorption limit of around 60grams per hour, but combine glucose with fructose, and because fructose uses a different transporter within the small intestine, you can potentially increase this absorption capacity to as much as around 108grams per hour! Allowing an individual to get in almost double the amount of carbohydrates.

These may sound like a lot of fuel, but it’s not uncommon for athletes to utilize carbohydrates at much higher rates than during exercise when we factor in glycogen use. It’s not uncommon for well-trained athletes to expend over 1000kcal per hour. Now factor in that a large proportion of this will be from carbohydrates, and even 90 grams of carbohydrates is only 360kcals, and it gives you some perspective.

One of the key things to bear in mind is that the peak rate of exogenous oxidation appears to be highly individual. Some athletes can utilize a lot, whilst some may not be able to. My understanding is that part of the reason that 90g/hr was chosen for the guideline is that it’s a dose of carbohydrate that results in high oxidative efficiency, which, in effect, means that a large proportion of what is taken in can be absorbed by the gut and utilized by the muscle. Much higher intakes than 90grams an hour, while possible, tend to have reduced oxidative efficiency i..e you might take in 120grams of carbohydrate an hour, but you may only be able to absorb and utilize 95grams an hour, leaving 25grams an hour unabsorbed, accumulating in the gut, and with time, likely resulting in significant stomach upset.

The figure below is from a study looking at the impact of feeding 120grams an hour of different forms of carbohydrates (drinks, chews, gels) using some very clever techniques; they could measure exactly how much of the carbohydrate that was taken in was actually used by the muscle (8). As you can see, despite taking on 2grams of carbohydrates per minute (120grams an hour), many athletes were only oxidizing around 1.6grams per minute/96 grams per hour, meaning that 24grams per hour was unaccounted for and likely accumulating in the gut.

If you’re a highly trained athlete who can oxidize more than 1.2 grams of carbohydrates per minute and may benefit from more fuel, then why not take on more? In contrast, if you have a low oxidative efficiency, you may be better off with a lower dose. It’s difficult to measure an individual's oxidation without expensive lab equipment that is off-limits to the vast majority of us, but it is another factor to consider, and if you're prone to stomach upset from high intakes of carbohydrates, this could potentially be one explanation for it.

The ratio of different carbohydrates appears to matter somewhat, too. Specific ratios of glucose and fructose appear to perform better than others, so this is also an important consideration when trying the fuel at high rates (10). With a maximum absorption capacity of around 60grams an hour of glucose, a 2:1 ratio of glucose and fructose (60g/hr glucose combined with 30g/hr fructose), appears to be pretty good when it comes to consuming less than 90grams an hour, but above 90grams an hour requires a ration closer to unity (i.e. 1:1 glucose to fructose)

Gut tolerance and gastrointestinal issues

During exercise, the gut is under significant stress, as blood flow is diverted away to the working muscle, and the stress of exercise can stress the gut further. High intake of carbohydrates during this time, particularly those that potentially leave carbohydrates accumulating in the put, can cause stomach issues, which can have a debilitating effect on an athlete’s performance. As the guidelines point out, for intakes above 60 grams an hour, ‘nutritional training is essential’. We know from recent research that it is possible to increase an athlete’s tolerance to carbohydrate feeding (although it doesn’t appear to do anything to increase absorption/oxidation) through the structured gut challenge, where, for example, an athlete might replicate their race day feeding for several training weeks and see a reduction in gut related symptoms such as feeling of fullness and bloating. Gut tolerance may be the key aspect of whether or not higher doses of carbohydrate intake result in improved or impaired performance, and this will be highly variable between individuals and require extensive testing/practice to establish/develop an individual athlete’s tolerance.

Exercise intensity, duration, and athlete standard

One of the issues with the guidelines is that they don't account for differences in athlete standards. A World Tour professional cyclist is likely to be expending in excess of 1000kcals per hour during racing, whilst an amateur athlete may struggle to sustain even half of that. Still, there is no differentiation in the guidelines for that. Whilst we don’t have masses of data comparing the impact of feeding on the super elite compared to a trained athlete, we do know that increased exercise intensity does seem to drive greater exogenous oxidation of carbohydrates, and therefore higher level athletes are more likely to be able to benefit from higher intakes.

In ultra-endurance events, i.e., those 12hours+, it’s simply not possible to sustain the same pace that you may be able to during shorter events of, say, 3 hours, and as such, the intensity and energy expenditure may be significantly lower. There is no recommendation to opt for lower intakes during this time, which is likely to be a more intelligent approach, particularly given the very high prevalence of gastrointestinal issues during ultra-endurance events that high intakes of carbohydrates may exacerbate.

Bringing it all together - How much should I be taking on?

This is the big question, and if I’m totally honest, it’s going to depend. Taking on carbohydrates during exercise is a fairly low-risk and cheap strategy in which there is the potential to add significant performance (or at least help an athlete achieve what they are capable of), so it is certainly a nutrition strategy that endurance athletes, particularly, should give a decent amount of focus to. Don’t consume enough carbohydrates, and there is the potential that you’re missing out on some extra performance/or you catastrophically blow up; take on too much, and you risk impairing performance through stomach issues, as well as it being more of a challenge to implement from a logistical perspective.

As always, your unique circumstances, including the type of event you're doing, your level of fitness, your tolerance to feeding, and whether it's a single or multi-day event, will all impact the best dose of fuel for you. Without extensive experimentation in training, I’d always err on the side of caution and adopt a lower dose that you can achieve consistently, particularly given that, based on the research we have to date, they may be close to as effective as higher doses.

Suppose you’re a world tour elite cyclist, spending vast amounts of energy daily. In that case, there is the potential you may see benefits to taking on intakes above 90 grams an hour, but these need to be very carefully managed with significant practice, which you likely have the time, budget and professional support to do. In contrast, if you’re an amateur athlete reading this a few days out from a major ride, I’d definitely look at using a much lower dose to ensure that it doesn’t have any adverse effects.

I think this quote from Dr Trent Stellingwerff nicely summarises things (4)…

"Each endurance athlete will have a unique “sweet spot” where he or she is able to absorb and oxidize a maximum amount of carbohydrate and fluids to improve endurance performance versus too many fluids and carbohydrates, which will cause gastrointestinal distress and decrease performance”.

As such, based on your own unique set of circumstances and physiology, you need to develop your own fuelling strategy that works for you in training. If you tolerate a lot of carbohydrates comfortably and notice it helps your performance, then by all means, look to hit the higher ranges, but if you regularly have stomach issues, aren’t exercising that hard, or just don’t see a performance impact from high intakes, modest intakes will likely be better for you.

Whatever you do, don’t try anything new at a key event/competition, as it’s a surefire strategy to ensure that it won’t work.

Thanks for reading. If you have any questions please don’t hesitate to drop a comment below. If you would like professional help developing a solid fuelling strategy, check out my services or contact me through the contact form.

References

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2. Hearris, M. A., Hammond, K. M., Fell, J. M., & Morton, J. P. (2018). Regulation of Muscle Glycogen Metabolism during Exercise: Implications for Endurance Performance and Training Adaptations. Nutrients, 10(3), 298. https://doi.org/10.3390/nu10030298

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8. Baur, D. A., Schroer, A. B., Luden, N. D., Womack, C. J., Smyth, S. A., & Saunders, M. J. (2014). Glucose-fructose enhances performance versus isocaloric, but not moderate, glucose. Medicine and science in sports and exercise, 46(9), 1778–1786. https://doi.org/10.1249/MSS.0000000000000284

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