At the cellular level, the key energy currency is ATP—made from nutrients (carbs, fats, proteins) plus oxygen. Glucose is the fastest-available fuel, but sharp rises and drops can cause fatigue. The goal is stable energy: complex carbs with protein and healthy fats. Long-term energy also depends on sleep, regular activity, and muscle mass because hormonal balance and mitochondrial efficiency respond to these factors. Persistent low energy warrants evaluation for anemia, thyroid issues, or glucose dysregulation.