Nutritional Basics, Part 2 - "Metabolism and Fuel Choice"

In the second of our articles by Dr Tom Kirk of Custom Cycle Coaching we’ll continue to explore the theme of how your body’s metabolism and fuel choice changes during exercise and how nutrition can influence this. As you may remember from last time, our bodies mainly use two types of ‘fuel tank’ to provide the energy needed while exercising; the larger stores of body fat in the muscles and adipose tissue (your ‘fatty’ tissue) and the much more limited carbohydrate stores in our muscles and liver. Many of the developments in sports nutrition over the last decades have focussed on how we can use these stores most effectively, such as:
  • Training the body to preferentially use fat stores as a fuel, sparing our limited carbohydrate stores
  • Supercharging our initial carbohydrate stores so that they last longer
  • Providing additional carbohydrate fuel during exercise to enhance performance
  • Optimising the type and timing of carbohydrate intake to maximise performance
  • Re-fuelling after exercise to improve recovery in situations where you will be training or competing again in a short time (hours or days)


As we saw in the last article, the amount of carbohydrate we use while playing sports depends on the intensity that we exercise at as well as how long we’re exercising for. In short events or training sessions, lasting under 1 hour, as long as our initial stores are high (i.e. you are not on a very low carbohydrate diet or have done an exhausting session recently) there’s no need for additional carbohydrate intake. However, our initial fuelling status is important and ‘carbohydrate loading’ is popular in endurance athletes in an attempt to maximise glycogen stores before long events. We have known this is important since the 1960’s when Swedish scientists showed that people following a high carbohydrate diet for several days were able to increase their muscle glycogen stores compared to eating a normal mixed diet or a low carbohydrate diet. Importantly, this was associated with a much longer time to exhaustion (189 min. compared to 126 min. on the mixed diet and just 59 min. on the low carbohydrate diet) when cycling at a fixed, moderate intensity (approximately marathon pace for a trained athlete) [1].

Running out of carbohydrates during endurance exercise can lead to very serious consequences.


In endurance events maintaining high carbohydrate stores is very important and in extreme events, such as the Ironman triathlon (check out the video!) and the Tour de France it is recommended to eat over 10g of carbohydrates per kilogramme of body weight each day to ensure that glycogen stores remain high enough [2], that’s the equivalent of 2.8kg of cooked pasta!
Manipulating your carbohydrate stores can also be important in determining training adaptations. Recently, limiting carbohydrate intake during and around training, for example by training first thing in the morning before breakfast or training twice a day, described as ‘training low’ can improve certain endurance training adaptations, such as the ability to use fat as a fuel [3]. This spares our limited carbohydrate stores, either for higher intensity periods in a race, such as climbing a hill, or so that you can last longer before the dreaded ‘bonk’! However, in competition carbohydrate is still a very important fuel source and is even required for efficient fat metabolism, so while this can be a useful training strategy in some situations, in endurance sports you should always aim to compete with well stocked carbohydrate stores. We’ll look into strategies like this to enhance training adaptations in further detail in a later article.

As well as long duration exercise, carbohydrate is very important for high intensity efforts, such as the repeated bursts of high intensity sprinting in team sports. This depletes glycogen stores more rapidly than you might expect from the overall distances run [4]. As a result, carbohydrate ingestion has been shown to be beneficial to sports performance in a variety of sports, from steady state endurance sports to intermittent and highly skilled sports such as football and tennis lasting an hour or longer .

Now you have a better idea of what happens to our bodies during exercise and why fuelling correctly is so important in many situations. In the next article we’ll look at what happens when we eat during exercise, looking at digestion of different types of foods before we move on to more specific fuelling recommendations for before, during and after important training sessions and competition.

  1. Bergstrom, J., et al., Diet, muscle glycogen and physical performance. Acta Physiol Scand, 1967. 71(2): p. 140-50.
  2. Burke, L.M., B. Kiens, and J.L. Ivy, Carbohydrates and fat for training and recovery. J Sports Sci, 2004. 22(1): p. 15-30.
  3. Hawley, J.A. and L.M. Burke, Carbohydrate availability and training adaptation: effects on cell metabolism. Exercise and sport sciences reviews, 2010. 38(4): p. 152-60.
  4. Krustrup, P., et al., Muscle and blood metabolites during a soccer game: implications for sprint performance. Medicine and science in sports and exercise, 2006. 38(6): p. 1165-74.

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