Definition: The biochemical processes by which nutrients are broken down and converted into energy that's used by the body or stored for later use. Metabolism is measured in calories.
There are two types: anabolism and catabolism.
Anabolism is the buildup of molecules from smaller units, for example, building proteins from amino acids.
Catabolism is the breakdown of molecules into smaller units, such as breaking down lipids (fats) into individual fatty acids.