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Amino acids are left behind when your body breaks down protein. They perform important functions, such as growing and repairing muscle, tissue and cells. The 23 amino acids are divided into three categories. Essential amino acids are the ones your body can’t make on its own. Nonessential amino acids are the kind that your body does make on its own. Conditional amino acids are typically created only in times of illness or stress. Certain foods are among the richest sources of amino acids.