Animals are heterotrophs and require food for fuel, carbon skeletons and essential nutrients.
Here we will examine where animals obtain food and how they convert food into chemically usable fuels and building blocks to maintain their body structure. An animal's diet must provide some essential nutrients. Many molecules used to build and repair cells can be synthesized from basic organic building blocks but some cannot. Adult humans can synthesize 12 amino acids. The remaining 8 amino acids must be obtained from the diet.