There are 21 different Amino Acids which form animal proteins. They can be classified according to whether they must be provided whole in the diet (essential) or can be manufactured in the body (non-essential). A third group may have some capacity for manufacture by the body but may become essential in some circumstances such as rapid growth, stress, or illness. These are termed the conditionally essential amino acids.
Not only building blocks for proteins, but amino acids also play a variety of essential roles in their own right. Adequate dietary supply of amino acids in the form of protein is critical for maintaining health, growth, and tissue repair.