Nutrition during physical exertion: essential scientific insights 

Nutrition during physical exertion: essential scientific insights 

If you’ve stumbled upon this article, chances are you’re curious about the optimal way to fuel your body during a race. You’re in the right place! This piece will cover the fundamental principles of nutrition and energy expenditure during physical exertion. Armed with this knowledge, you’ll be able to craft a nutrition strategy like a pro! 😉

Energy expenditure explained 

Let’s start with the basics. If you need to eat during a race, it’s because your physical exertion consumes energy. This energy comes directly from the foods you consume. There’s nothing mystical about it; the energy must originate from somewhere. Essentially, nutrition acts as the fuel your body requires to keep moving.

First, let’s estimate the amount of energy your body will need throughout your race. Consider this: a graph published by Minetti in 1994 illustrates the energy costs of running based on the slope of the terrain. For trail runners, imagine a road as a type of path—broader and composed of firmer, more cohesive materials than soil or pebbles.

Roads facilitate rapid movement, allowing for faster travel than even elite runners like Kilian Jornet, often with the aid of vehicles. For instance, it requires approximately 3.4 Joules (the unit for energy measurement, where 1 kcal = 4.184 Joules) to run one meter per kilogram on flat ground. This energy demand increases to about 10 Joules per meter per kilogram on a 25% incline.

However, determining your energy needs isn’t as simple as matching the caloric expenditure indicated by the graph. Humans utilize two main types of fuel sources, which adds a layer of complexity to the task—but don’t worry, it’s manageable!

The two primary fuels: Fats and Glycogen

The body relies on two types of substrates during exertion: fats (lipids) and glycogen. These are crucial for powering movement, as discussed earlier. Lipids store fats and glycogen stores carbohydrates, both ready to convert into energy.

Storage and breakdown differences:

  • Lipids store throughout the body in large quantities, serving as a plentiful energy source that can be tapped into during prolonged low-intensity efforts.
  • Glycogen stores only in your liver and muscles, with reserves much smaller (about 34 kcal/kg), quickly depleting and making them a critical but limited resource during high-intensity activities.

In an ideal scenario, one could potentially cover vast distances using primarily lipids, without needing to consume additional food. However, the body’s energy source allocation—lipids or glycogen—depends significantly on the intensity of the activity:

  • Low Intensity: Primarily uses lipids
  • High Intensity: Almost exclusively uses glycogen

During an exercise test on a track, our co-founder Guillaume Adam’s substrate usage provided a clear illustration of this dynamic. His body utilized a lower than average amount of fats, emphasizing how glycogen becomes the predominant source of energy as intensity increases.

Nutrition during physical exertion: Fats, carbohydrates

Implications for nutrition:

This predominance of glycogen, especially given its limited storage, underscores the importance of consuming carbohydrates during physical activities, even those of long duration and low intensity. Ensuring adequate glycogen stores is essential to maintain performance and prevent fatigue.

How to eat during effort?

You should think of your body as a car with a fuel tank that depletes as you move. Just like a car, you stop occasionally to refuel so you don’t run out of gas in the middle of your race. However, during a race, you won’t just refuel when you’re nearly empty; you’ll aim to replenish regularly in moderate amounts to continue running without vomiting all your nutrition back up.

Moreover, muscles cannot absorb a large amount of carbohydrates at once. It is often said that they are limited to a rate of 1g/min, and this can increase to 1.5g/min if you mix glucose with fructose. Therefore, you cannot ingest more than 60g – 90g of carbohydrates per hour of running, and with a lot of training, you may be able to increase this to 120g/h like some pros.

In the case of a medium-duration race (2h – 6h), it’s possible to sustain yourself only with personal nutrition in the form of bars or gels, but beyond this, it is advisable to vary your diet to avoid getting sick from foods that are too rich in sugars.

Getting used to eating during effort

Indeed, in adapting to the conditions of a race, improving how you handle nutrition is crucial. Getting accustomed to eating during a session will help your body adapt to digesting calorie-rich foods during physical effort and also enable you to familiarize yourself with various nutrition options before a race. Indeed, there are many different types of nutrition products on the market, and not all of them may suit you.

It’s important to try various brands and nutrition products while getting used to your training to find out which one works best for you. For example, on your long, low-intensity outings, experiment with eating and testing different products even if you don’t necessarily need to fuel up. This practice can ensure that you understand how to manage your nutrition optimally on race day, avoiding any gastrointestinal surprises and maintaining energy levels throughout.

Plan your nutrition strategy for your race 

Before a race, try to plan your nutrition so that you don’t have to think too much during the effort. To do this, look at the profile of the race and identify places that seem suitable for eating as well as the locations of aid stations along the course.

For eating, we advise doing so on uphill or less technical sections (for trail running), this way you can focus your attention on your nutrition without the risk of falling.

Once you’ve pinpointed the spots on the course where it would be ideal to eat (always regularly), all that’s left is to know the amount of nutrition you need to ingest.

For this purpose, we have created a planning tool that can give you information on the estimated energy expenditures for your race and the amount of carbohydrates you need to consume per hour.

This tool also provides an example of a nutrition pack with bars and gels to give you an idea (try to adjust based on the nutrition you have available).

Find your personalized nutrition plan here.

Happy racing!

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Mailis Durif-VarambonMailis grew up in the mountains, where she went hiking and biking every weekend. She loves outdoor activities where she can relax at the end of the day. At RunMotion Coach, she is responsible for communication management.