Homeostasis is the process by which a cell, organ, or the whole body keeps its internal conditions stable despite a changing outside world — holding temperature, blood sugar, fluid balance, pH, and countless other variables within narrow limits. The classic example is blood glucose: after a meal, insulin drives sugar into cells; between meals, glucagon releases it from stores — the seesaw never resting. Negative feedback loops — sensors detecting deviation, effectors correcting it — do the work throughout physiology. Claude Bernard discovered the principle, Walter Cannon named it. In nutrition, homeostasis explains why the body resists dietary change: weight, appetite, and metabolism are all defended set-points, which is why lasting change demands persistent effort.