The physiological process by which absorbed nutrients are taken up by tissues and incorporated into the body’s structures and energy systems. It follows digestion and absorption as the third stage of nutrition: the gut delivers amino acids, sugars, fatty acids, vitamins, and minerals to the blood, and assimilation distributes and transforms them, building proteins, storing glycogen, and maintaining every cell. Hormones regulate its pace and direction, with insulin promoting storage after meals and stress hormones mobilizing reserves between them. Inefficient assimilation underlies wasting diseases, where food is eaten but the body cannot use it, while its smooth operation is what makes growth, healing, and daily energy possible. The term captures nutrition’s ultimate purpose: not merely eating, but becoming.