Event Fuelling (Part One): Our Body Energy Systems

Now I was about to write a gem of an article on how to fuel for an event, particularly with the Crossfit Open in full swing. However, I had a realisation as I started writing that without knowing the background to how our body sources energy, confidently fuelling a session is not going to be as effective or intuitive.

So here goes, an article written for everyone who exercises – no matter your age, training level or goals!

Firstly, what is energy sourced from?

Energy comes from a tiny molecule with a very big name – Adenosine Triphosphate (i.e. one adenosine molecule attached to 3 phosphate molecules) OR what we will call, ATP.

Energy is released when the bond between the phosphate molecules breaks. There is only enough ATP stored in your muscles for around 2-3 seconds of work – so that is where your energy systems come in to replenish ATP stores.


Creatine Phosphate System

This system uses Creatine Phosphate as it’s fuel source.

It can provide the highest of power outputs with high rates of energy production, however is depleted within 10 seconds. For this reason, it is used for max intensity efforts of very short duration.

As the muscle creatine phosphate levels decline so rapidly, passive recovery for at least 30 seconds and up towards 3-5 minutes is essential.

As creatine is the limiting factor, it is for this reason that increasing creatine stores in the muscle (through supplementation) can be a real benefit to performance.

Anaerobic Glycolysis

This is the predominant energy system for efforts lasting 5-30 seconds and repeated efforts with a short recovery time.

This source of energy breaks down carbohydrate stored in your muscle (glycogen) to pyruvate and then to lactate (or lactic acid). For each glucose molecule broken down, 2 molecules of ATP are produced.

As lactate accumulates in the muscle, the pH falls and has a variety of effects – of course one of them being that all too familiar discomfort! This is where buffering supps can come in handy such as beta-alanine and bicarbonate. We specialise in implementing these, so contact us for more info on if they could be of benefit to you.

Despite the discomfort, this energy system produces a high rate of ATP but really needs an active recovery to get rid of the waste products.

The Aerobic System

For efforts moving beyond 30-60seconds, the body requires oxygen to create ATP – so instead of being broken down to lactate, pyruvate gets oxidised to carbon dioxide and water.

This is a much slower process, but generates a lot of energy that is really unlimited. It can use carbohydrate (glycogen in muscle and liver), fats (triglycerides in muscle and fat stores) and some proteins (amino acids). It can also be replenished by consuming food/drink during exercise.

Oxidation of one molecule of glucose results in 38 molecules of ATP, while one molecule of triglyceride can result in over 100 molecules of ATP. The amount of protein you use is proportional to how much protein is in your daily intake.

The amount of power that can be generated by the aerobic system varies greatly between individuals and is characterised by VO2max. This is influenced by your training level, genetics, gender, age and body composition.

How this Translates in Energy for Endurance Events

Although endurance events such as marathon, triathlon, road cycling, etc relies on the aerobic system, the anaerobic metabolism still makes a significant contribution. This energy system kicks in when there is a high energy demand such as hill climbs, intermediate sprints or finishing sprints.

So although it makes sense to be efficient in fat oxidation, for reaching top-end speeds and pushing the limits of performance, carbohydrate stores (muscle glycogen) are key to maximising power output.

This is why our advice is focused on driving adaptations to training to improve fuel use while also balancing the need to hit those top-end speeds.

How this Translates in Energy for Team Sports of Events with Repeated Efforts

Team sports or any events with multiple efforts, the anaerobic system is essential. However, the aerobic energy system will still be important in recovery following each effort. During this recovery, the aerobic system replenishes the fall in ATP, increases the Creatine Phosphate stores and oxidises that lovely lactate build-up.

So What Next?

Understanding what energy system your body uses for your chosen exercise will then help you decide what kind of fuelling will be best pre- during- and post.

This really is just a summary of a very complex topic, but we hope you have gain some knowledge and with it some confidence to go and smash those goals! Stay tuned for Part 2 coming up soon!

In the meantime, if you have an event you would love some individual nutrition planning for, head on through to our memberships page and straight on to our Event Package.

Alicia Edge


Alicia is the head Advanced Sports Dietitian at Compeat Nutrition. She is also a mum and triathlete, so advice extends beyond the basics and is instead focused on providing effective and achievable nutrition for both training and racing.

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