By Hannah - All Bodies Director and Accredited Practising Dietitian
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Should you fuel up with carbs or should you workout fasted? This is the question on every body’s lips.
The answer: I’d love to say "yes just fuel every workout with carbs" and although 90% of the time that may be true, it’s not that simple and you’re probably reading this because you want to know why you should eat before a workout.
To get to the heart of this we need to dive into fuel sources and energy systems.
1. Energy systems are metabolic pathways that your body uses to convert fuel sources (i.e. fats, carbs & amino acids) into energy. Your body will draw upon different fuel sources and energy systems for different types of training based on the demands of the workout.
2. Fuel sources aren’t equal. Your body is most efficient at using carbohydrates as a fuel source, meaning that you tend to be able to maintain higher intensities for a longer duration (aka workout harder for longer).
When carbohydrates are digested, they are stored in your muscles and liver in the form of glycogen. Your body draws on these stores during exercise to fuel your workouts.
Fats are stored as triglycerides predominantly in adipose tissue (fat cells) and some in your muscles. Your body can store greater amounts of energy in the form of fats. However, your body isn’t as efficient at converting stored fats into energy to be used in workouts.
There are 2 reasons why:
The rate at which your body can produce energy varies significantly between energy systems.
Processes that use carbs as the major fuel source such as glycolysis, glycogen breakdown oxidation and blood glucose oxidation can produce energy much faster than Fat oxidation or amino acid (protein) oxidation.
Therefore, if you’re about to do a sprint (requiring large amounts of energy to be produced quickly) your body would struggle to produce that energy fast enough if it was only fueled by fats.
‘Efficiency’ refers to how much oxygen is required to produce the equivalent amount of energy.
To produce the same amount of energy from fats, compared to carbs, fats will require about 5-6% more oxygen – much less efficient! This means that your heart needs to work that much harder to get the oxygen to your working muscles and so you are likely to fatigue earlier.
So with that in mind, if your workout is ones that elevates the heart rate with repeated efforts of higher intensity work or sustained moderate work, you’ll likely benefit from fueling the session with carbohydrates.
Ensuring you have adequate carbs in the tank may not look like a banana directly before the session (although this can be helpful!), it is more likely ensuring your diet contains adequate carbs across the day.
If you are an athlete and you consume a low carb diet but then add a banana before a 90mins football session, you’ll likely run out of carbs in the tank and be back to burning inefficient fats. This is where your fueling becomes highly individualized.
Some things to consider when determining how and when to fuel your sessions:
Your body size and muscle mass
Your training age/fitness level
Heat or altitude
Your body composition or weight goals
Your training and fitness goals
So yes, I would say that most people would benefit more from fuelling their sessions with carbohydrates in the tank, however, as with anything in nutrition, it’s not black and white.
For some more guidelines on the specifics of pre-training fuel have a read of our blog post Nutrition for sports performance Part 1
If you have any questions on any of the above please don’t hesitate to reach out!
Graph image: maughan, R.J. and Gleeson, M. (2010). The Biochemical basis of Sports Performance (2nd edition). Oxford University Press, Oxford.