Ever wanted to know how best to fuel your athletic endeavours?
We took your top questions to Ted Munson a top Performance Nutritionist to produce this series of blogs.
Exercise Nutrition: How does my body use carbohydrates and fats during exercise?
People often wonder if they should be relying on burning fat or using carbohydrates during exercise, and this is really a question of the intensity and your goals for the workout.
When we’re exercising at low intensities, like walking – we predominantly use fat as a fuel source. We have ample fat storage and 1g of fat contains almost three times more energy that carbohydrate (9 kcal vs 4 kcal).
As exercise intensity increases our body shifts to using glycogen and available glucose in the blood to create energy to fuel efforts.
Carbohydrate is the dominant fuel for high intensity endurance performance. Outside of exercise, they provide us with ready, available energy for day-to-day tasks. When we consume carbohydrate and it is not used as immediate energy, it is stored in the muscles and liver as glycogen. It’s then converted into glucose quickly and can therefore be used as fast energy.
Fat | Carbohydrate |
Preferred fuel source for low intensity exercise | The main fuel for higher intensity exercise |
Oxidized much slower than glycogen, meaning that it does not supply energy rapidly | Muscle and liver glycogen stores can only sustain performance for up to 90 minutes of endurance exercise |
Less available for fuelling high intensity exercise | Provides fast energy to be used by the working muscles |
Becoming reliant on using fat as a fuel can prevent the body from utilising carbohydrate | We can only consume 60-90g of carbohydrate per hour during exercise |
How long can I train before needing to take on some form of energy?
We can store enough glycogen in our body to last around 60-90 minutes of moderate – high intensity exercise. After this performance will start to be impacted and if you don’t slow down you will eventually run out of energy. You may have heard of or experienced ‘hitting the wall’ – this is where glycogen stores are fully depleted, and the body is unable to draw upon this pool of energy. This makes fuelling during endurance exercise key to maintaining endurance capacity and performance. When fuelling during exercise, it’s important to start fuelling early and not when you’re tired as your body uses more energy than can be absorbed.
Here’s the recommended times and fuelling amounts you should consider when planning your nutrition strategy:
Exercise | Time | Carbohydrate Requirements |
Brief Exercise e.g 5 km run | < 45 mins | Not needed |
Sustained high intensity exercise e.g races, track sessions or time trials | 45-75 mins | Small amount of around 30g of carbohydrate or mouth rinse early if high performance is required in the final 30 mins |
Prolonged endurance exercise e.g 80km bike ride or 20km trail run | 1 – 2.5 hours | Up to 60 g/h |
Ultra endurance exercise e.g 50km run | >2.5 – 3 hours | Up to 90 g/h |
So for example a 70 kg cyclist going out for a 2.5 hour moderate intensity training ride could aim to consume:
Hour 2: 500ml bottle of Performance Energy drink + ½ cereal bar = 53g carbohydrate.
Hour 1: 500ml bottle of Performance Energy drink + 1 energy gel = 52g carbohydrate.
What types of carbohydrates should I use when training/racing?
We now know that we need to fuel differently depending on the type and duration of exercise. But what kind of carbohydrates work best? Carbohydrate comes in a variety of forms. Sugars, including glucose, sucrose and fructose are all forms of carbohydrates that you’ll see in sports products. They are all metabolised differently, affecting performance output.
We can only consume 60g carbohydrate per hour from glucose alone, however by using a glucose-fructose mix, you can consume more carbohydrates – up to 90g per hour. This is because glucose and fructose use different transporters when digesting.
During exercise you ideally want to keep carbohydrate intake simple, and spaced out as this will pass through the gut easiest so look for easy to digest carbohydrates such as drinks and foods such as dates or cereal bars.
Using a variety of sources of carbohydrates may also help combat stomach issues often experienced by endurance athletes, as digesting is increased
How much carbohydrate should I eat per day?
Balancing some low and high carbohydrate days can actually aid performance and our body composition, this is called ‘Fuelling for the work required’. Some sessions could be performed without carbohydrate (e.g having breakfast after low-intensity morning training) with lower total carbohydrate intake for the rest of the day, whereas for harder effort and long endurance sessions, carbohydrate may be increased in the day and consumed in greater quantities during the sessions. The amount of carbohydrate required depends on your daily exercise demands, goals and body weight:
Intensity | Exercise | Carbohydrate Recomendations |
Light | Rest day or a recovery session | up to 5 g per kg of body mass per day (g/kg/d) |
Moderate | Moderate exercise e.g 1 hour session or strength | 5-7 g/kg/d |
High | Endurance session 2-3 hours in length | 6-10 g/kg/d |
Very high | Ultra-endurance or sessions 4-5 hours in length | 8-12 g/kg/d |
The types of carbohydrate you consume outside of training should ideally look to optimise your health and you should focus on consuming nutrient dense foods like vegetables, fruit and fibre rich foods. Try to avoid replenishing your carbohydrate stores with low quality foods, as this likely won’t aid your health and performance in the long run.
Looking for an all in one solution to fuel performance and hydration?
In step 2 of our Fuelling Performance guide to exercise nutrition, we discuss hydration and its role in exercise.
