Proteins are composed of molecules called amino acids, which, when joined in different combinations, make up most of the tissues in our bodies: muscles, organs, skin, nails, and hair, for example. They also make up many of the biochemicals, like enzymes and hormones, which help our bodies grow, heal, digest, and detoxify. There are twenty-three amino acids in the human body, nine of which are essential. Animal sources of protein -- such as eggs, dairy products, and meat -- contain all nine essential amino acids. Plant sources of proteins -- such as beans, nuts, and seeds -- lack some of the essential amino acids.
Individual needs for protein can vary greatly depending on age, activity level, genetics, or chronic/acute illness. Those of us who do not meet our bodies' need for protein through our diet can experience symptoms of protein deficiency, which include: weakness, poor muscle tone, fatigue, dry and brittle hair and nails, slow wound healing, weakened immune system, blood sugar imbalances, and even depression.