For the body to move, jump, run, or cycle, it needs energy. But where does this energy come from? The answer lies in nutrition — more specifically, in carbohydrates, which are converted by the body into a highly efficient fuel: glucose.

In this article, we explain in a simple and direct way how the body produces energy, how it stores carbohydrates, and why it is essential to ensure a continuous supply during physical exercise, especially in long-duration training sessions or competitions.

It all starts with digestion: transforming carbs into glucose

When we eat, carbohydrates (bread, rice, fruit, pasta, etc.) are digested and converted into glucose, the main energy substrate for muscles and the brain. Each gram of glucose provides approximately 4.1 kcal — vital energy for movement and mental focus.

But the body doesn't use all glucose immediately. What is not needed at the moment is stored as glycogen, in two strategic locations:

●      Liver: to maintain stable blood glucose levels between meals or during exertion.

●      Muscles: for local use during intense muscle contraction.

The energy currency: ATP

The glucose that enters muscle cells is used in a process called glycolysis, which culminates in the production of ATP (adenosine triphosphate) — the "exchange currency" of cellular energy.

ATP is what allows muscles to contract, propel the body, and sustain effort. The more intense the activity, the greater the energy demand — and the faster glycogen stores are depleted.

Reserves have limits: and they run out

The body can store about 2500 kcal in the form of glycogen. However:

●      The muscle reserves are used exclusively by the muscle where they are stored.

●      The liver reserves can release glucose into the bloodstream, fueling various tissues — including the brain.

During prolonged exercise, these reserves diminish, and if not replenished, fatigue, a drop in pace, and difficulty concentrating set in.

How to avoid energy crashes?

The solution is simple and effective: replenish carbohydrates during exercise. Especially in training sessions or competitions lasting more than 60 minutes, it is essential to ensure an external supply of glucose to maintain performance and prevent the depletion of internal reserves.

Understanding how the body produces and uses energy is the first step to training smartly. By ensuring a continuous supply of glucose — both from reserves and through supplementation during exertion — you are protecting your performance, your focus, and your ability to go further.

Provide your body with the fuel it needs

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