Sports Nutrition Basics – Fuel Sources During Exercise and Sport

One of the most important things for any athlete to understand in the area of sports nutrition is the basics of nutrition as a fuel source during exercise. Understanding this will help to make sense of how we eat for training, how we prepare for competition, how we optimise recovery from training and competition, and how we gain and lose weight.
What I will describe here is what we refer to as the macronutrients. As the name suggests, these are the nutrients that we eat in large quantities, but importantly they are the ones that we can use as an energy or fuel source. Energy is measured in calories or kilojoules - they are two different measurements like centimetres and inches are two measurements of distance. It is the total energy (regardless of which nutrients it comes from) that determines whether we are gaining, losing or maintaining a stable weight and body fat.

Energy sources during exercise and sport

The way humans produce energy is not that different to a car. Both burn a fuel (or mixture of fuel types) with oxygen in an “engine”, produce carbon dioxide and water as a result, and produce energy they need to power all parts of them. The main difference is that a car has one central engine to produce all the energy it needs, whereas the human body has trillions of mini “engines”, inside every cell of the body. This makes sense if you think about it – the more engines you have, the less important it is if one engine fails.
So what are the “fuels” for the human body? We produce energy primarily from four nutrients in food:

  1. Carbohydrate (every gram produces about 17 kilojoules or 4 calories of energy)
  2.  Protein (every gram produces about 17 kilojoules or 4 calories of energy)
  3.  Alcohol (every gram produces about 27 kilojoules or 7 calories of energy)
  4.  Fat (every gram produces about 37 kilojoules or 9 calories of energy)

Each of these energy sources has advantages and disadvantages. From an exercise perspective though, only two are really used (protein is occasionally burnt as a fuel source in exceptionally long ultra endurance exercise and won’t be discussed here), carbohydrate and fat.
Fat is a very useful energy source (and the one you are predominately burning as you read this) because we can store very large amounts of it (both in our muscles and in our body fat cells), and it will provide energy for an incredibly long period of time. For example people that go on hunger strikes can last up to two months or more provided they have adequate water. However the downside to fat as a fuel source is that it doesn’t produce energy very quickly, and requires a lot of oxygen to do so.
Carbohydrate is the body’s preferred fuel used during intense exercise, because it produces energy more quickly than fat and requires less oxygen (and in some cases no oxygen) to do so. The downside is that carbohydrate is only stored in muscle and a small amount in the liver, and there’s only so much we can store at once. Endurance athletes that carbohydrate load are deliberately eating to maximise the amount of carbs stored in the body prior to competition.

Fuel choices during exercise

A look at the graph below will start to give you an idea of what fuels are used and when. As you can see at low intensities of exercise fat is the main fuel source. There’s plenty of it (even in very lean people), we don’t require a lot of energy quickly at this intensity and so we can take in more than enough oxygen to meet the demand. Once the intensity starts to rise you can see that carbohydrate starts to play a role. At 60% of maximum intensity you’re starting to breathe more heavily, to get in enough oxygen to keep up with the demand for energy. Just like a car takes on more air and fuel when you put you foot down, we start to burn more fuel and consume more oxygen.

Once you get up to 80% of maximum intensity you can see that carbohydrate has become the main fuel source used, although some fat is still being burnt. By now you’re breathing heavily, and struggling to get enough oxygen in to keep up with demand. Carbohydrate doesn’t need as much oxygen, and produces energy more quickly, so it’s the fuel of preference. However remember that carb stores are limited, so this intensity won’t last forever (a post on how your brain paces exercise and fuel use is still to come).
Once your intensity gets to a level that exceeds the body’s ability to provide oxygen, your starting to exercise “anaerobically”. This means producing energy without oxygen. Generally speaking it’s carbohydrate that is used to do this (more on anaerobic energy in a future post). The drawback from anaerobic exercise is that it produces lactic acid. Lactic acid itelf is not a problem (in fact research now shows it’s actually beneficial), but through a series of chemical reactions in the body this process also makes your blood and the inside of your cells more acidic. This is not a good thing because blood that’s too acidic causes damage to the body. So the limit to anaerobic exercise is that the body can only tolerate a certain level of acidity in the blood, and your brain will ultimately stop you from working so hard that you cause yourself any harm.
As you improve your fitness with training, you’re able to get more oxygen in (ie. your VO2max increases), and so for the same power output or running speed you are actually exercising at a lower intensity and burning more fat and less carbs. That’s an advantage because it means you can save the carbs for an even faster pace when it matters later in a competition.
That’s enough talk about fuels for today, I’d love to hear if you have any comments, questions or suggestions for sports nutrition topics you’d like to hear more about. The next post will be about how we can eat to improve our fuel stores and maximise sporting performance.


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  6. Hi. Your article is realy clear and informative. I like this. To make it more complete, can you explain about when the fat is realy burnt ? Some says when glycogen is zero, and another says when insuline is zero (baseline). This is ambiguous and prone to misleading. And number 2, what is the energy source used when we do heavy lift excercise in the morning on empty stomach ? Im sure alot of people need your answer about this 2 questions. Hope you still on this blog. Thank you

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