Nutritional Basics, Part 1 - "Where does my energy come from?"

fuelit’s sports nutrition blog

with Dr. Tom Kirk of

Welcome to the first of fuelit’s blogs aiming to build your sports nutrition knowledge and enable you to make informed choices on your training and nutrition. Whether you’ve recently taken up a new sport and are confused by the many sports nutrition products out there, or you are an experienced athlete who would like to understand more about how your training and nutrition interact, we hope these blogs will be an interesting and informative introduction to the world of sports nutrition. Over the coming weeks, we’ll discuss what happens to your body when you start to exercise, what role food and drink play in enabling you to train and compete to your best and delve a little deeper into the different nutritional choices currently available; all with the aim of giving you the information and support you need to make your own mind up about how to fuel your sport effectively and perform at your best.
We all feel the changes our bodies make even within the first few seconds of beginning to exercise, but what is actually happening during this time, and how can nutrition affect this? This first post will look at this transition from resting to exercise, and what is happening inside your body.
At rest, most of your energy expended is used simply to stay alive, from fuelling the muscles that control your breathing and keep your blood pumping to the energy used by your brain to control every function of your body. Your resting energy needs are provided almost entirely by ‘aerobic’ means, which is the breakdown of your body’s fat stores (and sometimes a little carbohydrate too) with the presence of oxygen. The amount of oxygen used for this is pretty low, at around 0.25 litres each minute for an ‘average’ adult.
When you begin to exercise, oxygen use increases rapidly. Depending on how well trained you are, and how much effort you are making, this oxygen need could increase by up 10-25 times, placing a huge strain on your body. Fortunately, we are well adapted to meet this demand.
When you move from rest to exercise, your body rapidly increases your breathing and heart rate in order to provide the fuels and oxygen needed in your muscles. As long as the exercise intensity is not too hard, your body reaches a steady state within a few minutes where the oxygen supply and breakdown of fuel stores (predominately a mixture of fats and carbohydrates, see below) match energy needs.

Some of the factors affecting fuel use:

  • Intensity (higher intensity demands more carbohydrates)
  • Training status (more endurance-trained athletes will use more fat at a given intensity)
  • Duration (longer duration exercise uses more fat)
As the duration goes on, your body’s limited carbohydrate stores begin to deplete. If you’re well fed, you should have around 2000-2500 kcal of carbohydrates (glucose and glycogen) stored in your muscles, blood and liver (see ‘The body’s fuel tanks’ below). As one of the body’s main priorities is to maintain blood glucose concentration and glucose supply to the brain (which, unlike muscles, cannot use fat as a fuel), as these carbohydrate stores deplete your body will become more reliant on fat (of which there is an essentially unlimited supply).
Despite the vast stores of fats in even the leanest of us, intense exercise is heavily reliant on carbohydrates (see the figure opposite) so you need to provide your body with carbohydrates from external sources (food and drink) to continue exercising at medium to high intensities. This is especially true when exercising for a long time, as your body’s own glycogen stores are depleted. As a result, one of the key strategies to improve endurance exercise performance is to maintain a high level of carbohydrates available to your muscles. Many studies have found that eating or drinking carbohydrates during endurance exercise lasting over an hour improves your performance, as you are able to maintain a high intensity for longer before exhausting your carbohydrate stores. You’ve likely ‘hit the wall’ before, and this is exactly what happens when you run out of carbohydrates – we’ll talk more about this soon!
As you move from low to high intensity exercise
your body relies more on carbohydrates and less
on fat stores. (Reproduced from Romijn et al., 1993)
I hope this has set the scene a little about future articles in this series and given you an insight into what’s going in inside your body as you start to exercise. In the next post, we’re going to talk in a little more detail about why you need carbohydrates and how to maximise and maintain your limited glycogen stores.

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