Relative Energy Deficiency in Sport (RED-S) is a serious but often overlooked condition affecting athletes of all levels. Caused by chronic under-fuelling, it impacts performance, hormonal balance, bone health, immunity, and mental well-being. This guide outlines the early warning signs—such as fatigue, menstrual irregularities, and frequent injuries—and offers practical steps for recognising, diagnosing, and addressing RED-S before long-term damage occurs.