Hormonal

A single bout of resistance exercise increases mixed muscle protein synthesis (MPS) and breakdown (MPB) rates, with MPS elevation persisting for at least 48 hours, but net protein balance remains negative in the fasted state because MPB also increases.

If you want to build muscle, do not train in a fasted state. This study shows that while exercise boosts protein synthesis for 48 hours, it also boosts breakdown. In a fasted state, your body breaks down more protein than it builds, resulting in a net loss. To turn this exercise stimulus into muscle growth, you must consume protein to shift the balance from negative to positive.