Catabolism

Catabolism is the breaking down side of metabolism: larger molecules disassembled into smaller ones, releasing energy. Digesting food, mobilising fat from storage, breaking down glycogen and, when required, breaking down protein are all catabolic processes.

Its counterpart is anabolism, the building side, and the two run continuously rather than alternating. Your body is dismantling and rebuilding itself at all times, and what changes is the balance between the two. Net muscle gain is simply synthesis outpacing breakdown over weeks.

The word has been thoroughly abused by supplement marketing, which treats catabolic as a synonym for dangerous and sells products to prevent it. Catabolism is not a threat, it is how you access stored energy, and a body that could not do it would not survive a night's sleep.

The legitimate concern is narrower: losing muscle rather than fat during a large deficit or a long illness. That risk is real and it is addressed by two things with good evidence, adequate protein and continued resistance training, rather than by anti-catabolic formulas.

Cortisol is the hormone usually blamed. It is genuinely catabolic and it genuinely rises with stress and hard training, but the leap from a normal post-exercise cortisol rise to eating your muscle is not supported. Chronic stress and chronic under-recovery are the versions worth worrying about.

Fasted training gets caught up in the same anxiety. Training without breakfast does not meaningfully cost you muscle if your daily protein and energy intake are adequate, and daily totals matter far more than the state you happened to be in at seven in the morning.

In practice, eat enough protein, keep lifting through a diet, and ignore anything sold on the promise of stopping a normal metabolic process.