Nutrition refers to the activities by which living things obtain raw materials from the environment and transport them into their cells. The cells metabolize these raw materials and synthesize structural components, enzymes, energy-rich compounds, and other biologically important substances. All the elements and compounds taken into a living thing are nutrients.
Animals, including humans, are heterotrophic organisms, and their nutrients consist of preformed organic molecules. The organic molecules rarely come in forms that are readily useful, so animals must process the foods into forms that can be absorbed. This processing is called digestion.