Optimal fuelling for ultra marathons: insights from current research

Carbohydrates

Carbs are king when it comes to fuelling ultra marathons. They are the primary source of energy for the body to convert into glucose for immediate fuel. Onboard glycogen stores (if fully stocked) can provide up to 2,000 calories of energy (from about 500g glycogen). As most ultras far exceed this requirement, to sustain performance a continual supply of carbohydrates is necessary.

For most ultra runners the optimal amount of carbohydrate per hour is ~ 60-90g. Yes, this has changed significantly in recent years thanks to a clearer understanding of the role of multiple transportable carbs (more below). Compared to fats and proteins, carbohydrates are metabolised faster and more efficiently, making them the key macronutrient for sustaining prolonged high- and moderate-intensity efforts. While fats and proteins have a role to play in long-term energy provision and muscle repair, respectively, they are not as effective as immediate energy sources during the race. They also increase gastric emptying time, which could lead to nausea. Prioritising carbohydrate intake, therefore, helps maintain optimal energy levels, supports endurance, and enhances overall performance in ultra marathons. It does not mean we must avoid protein and fats (the role of other macronutrients in ultra races needs further research - it may turn out to be significant!).

Multiple transportable carbs

The term "multiple transportable carbs" refers to the use of more than one type of carbohydrate in order to utilise distinct transport mechanisms in the gut, allowing for higher levels of absorption and utilisation. Typically, the body can absorb about 60 grams of glucose per hour because it uses a specific transporter protein (SGLT1); at higher rates this transporter becomes saturated. This was where the original maximum recommendation came from (and hung around in running lore years after cyclists proved otherwise). However, when combined with other types of carbs like fructose (which uses the GLUT5 transporter) or sucrose (which is broken down into glucose and fructose), the total absorption capacity increases. This is akin to using two lanes to enter and leave a roundabout instead of queuing in a single lane—more cars (or carbs) can pass through in the same time. By utilising these different transport pathways, runners can absorb, and use, something more in the region of 80-120g of carbohydrates per hour (e.g. 60g glucose and 20-60g fructose), enhancing energy availability, endurance and performance during races, and recovery afterwards. 

Companies such as Maurten and SIS have found benefits from 120g of carbs per hour - a number now seen as the target for a hard day on a grand tour in cycling. Whilst running advice tends to follow the research, the tiny margins and high stakes of Grand Tour cycling often mean their teams are ahead of published research. So it is certainly possible I will have to revise this article in a few years. I personally find 80g/hour works really well for me, for now. 

Salt

Salt is vital for maintaining electrolyte balance and can help prevent hyponatremia during ultras. However, relying on salt tablets can be problematic due to the risk of consuming too much salt, which can lead to gastrointestinal distress and dehydration. Instead, consuming salty foods or drinks based on taste (eat what you crave) can be a safer and more effective strategy (Costa et al., 2019). This approach allows runners to respond to their body's natural cravings, which may be indicative of their immediate electrolyte needs, thus aiding in maintaining homeostasis. Learning to listen to these cravings helps runners avoid the adverse side effects of excessive salt intake, such as nausea, bloating, and (rarely) impaired kidney function. By consuming salt through food and drinks, runners can better regulate their intake.

Runners working closely with a qualified sports dietician or sports scientist may be able to dial in both salt and water needs for a race, but for most people the advice on salt - just like water - is to listen to your body. You can get a free 20-minute consultation with the sports scientists at Precision Fuel and Hydration here. At home sweat testing is insufficient due to inaccuracies in (some) testing devices and the fact that variations in diet, hydration levels, climate etc might make one day’s test less reliable for another in different conditions (Baker, 2017). However, these variations within a single individual tend to be small, so accurate sweat testing will give you a great ballpark. Accurate testing is available in many countries through Precision Fuel and Hydration’s Sweat Test Centres.

Water

Drinking to thirst, rather than aiming for a specific amount of water per hour, can help to maintain optimal hydration and performance (with the exception of those working 1-1 with a qualified sports dietician). Athletes should quench their thirst upon arrival at aid stations and then fill their bottles to drink from between stations. Drinking a larger volume every 20 minutes leads to better absorption than frequent small sips (Mears et al., 2020), however if the technicality of the terrain makes sipping easier it becomes the preferred option.

Drinking to thirst helps prevent both dehydration and the perhaps more dangerous condition of hyponatremia, which occurs when the body's sodium levels are diluted by excessive water intake. 

Dehydration can impair physical and cognitive performance, leading to fatigue, dizziness, and decreased endurance. Conversely, hyponatremia can cause symptoms ranging from nausea and headache to, in severe cases, confusion, seizures, and even death. 

By listening to their body's signals and drinking to satisfy thirst, runners can better regulate their fluid balance and support homeostasis.

In summary, carbohydrates remain the foundation of endurance fuelling, with recent research advocating for higher intakes of multiple transportable carbs to maximise absorption and utilisation. Equally important is the balanced intake of salt and water. Relying on salty foods and drinks rather than salt tablets allows runners to respond to their body's electrolyte needs. Similarly, drinking to thirst helps maintain optimal hydration levels, promoting better physical and cognitive performance.

As a full-time professional endurance coach I have the opportunity to stay up to date: constantly reading, questioning and learning. I adapt as the science does so that my athletes can benefit from the best advice. You can join the team here.

My knowledge comes from wide reading, personal and coaching experience, and a certificate in “Nutrition, Recovery and Training Adaptations” as part of a diploma through FC Barcelona (two pieces of coursework and a timed exam). That course was written by Asker Jeukendrup who has a great website here: https://www.mysportscience.com/

I deliberately don’t address general nutrition here as I’m not a qualified dietician (whilst coaches need to have expertise and experience in lots of fields (strength, physiology, psychology, biology…) it’s important to acknowledge the limitations of that reach!). If you’re looking for advice about your diet outside of racing, or more specific advice for training you should seek a registered dietician. This is even more important if you have, or are recovering from, any form of disordered eating.

References

Baker, L.B. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Med 47 (Suppl 1), 111–128 (2017). https://doi.org/10.1007/s40279-017-0691-5

Jeukendrup, Asker. (2024). '120 grams per hour', My Sport Science. Available at: https://www.mysportscience.com/post/120-grams-per-hour (Accessed: 24 May 2024)

Mears, S. A., Boxer, B., Sheldon, D., Wardley, H., Tarnowski, C. A., James, L. J., & Hulston, C. J. (2020). Sports drink intake pattern affects exogenous carbohydrate oxidation during running. Medicine & Science in Sports & Exercise, 52(9), 1976-1982. https://doi.org/10.1249/MSS.0000000000002334

Costa, R. J. S., Knechtle, B., Tarnopolsky, M., & Hoffman, M. D. (2019). International Journal of Sport Nutrition and Exercise Metabolism, 29(2), 130-140. https://doi.org/10.1123/ijsnem.2018-0255

Viribay, A., Arribalzaga, S., Mielgo-Ayuso, J., Castañeda-Babarro, A., Seco-Calvo, J., & Urdampilleta, A. (2020). Effects of 120 g/h of carbohydrates intake during a mountain marathon on exercise-induced muscle damage in elite runners. Nutrients, 12(5), 1367. https://doi.org/10.3390/nu12051367