The process by which the body converts food and other substances through chemical reactions into energy, building materials, and waste products. After digestion breaks food into absorbable units, metabolisation in cells transforms glucose into ATP, amino acids into proteins, and fatty acids into membranes or stored fat. The liver plays the central role, processing nutrients arriving from the gut and detoxifying foreign compounds. The rate and pattern of metabolisation vary with age, genetics, diet, activity, and health status. Understanding these transformations is fundamental to nutrition science, pharmacology, and the management of metabolic diseases.