Fatigue is very great tiredness — the exhaustion that sleep does not fully fix, distinct from ordinary sleepiness. As a verb, to fatigue is to tire someone out. In nutrition, fatigue is a cardinal symptom of deficiency: iron-deficiency anaemia starves tissues of oxygen, B12 and folate deficiencies cripple red-cell production, and inadequate energy intake simply leaves the body unfuelled. Chronic fatigue also shadows diabetes, hypothyroidism, and coeliac disease — the body failing to convert food into usable energy. Athletes know fatigue as the limiter of performance, managed through carbohydrate fuelling, hydration, and recovery. The symptom is nonspecific — a hundred conditions produce it — which is why clinicians pair it with tests rather than assumptions. Fatigue is the body reporting, in its bluntest language, that something in the energy equation is wrong.