Protein is an essential nutrient that provides amino acids for growth, repair, and maintenance of the body's tissues. It is made up of 20 amino acids, 9 of which must be obtained through food. Animal proteins like meat and dairy are considered "complete" as they contain all essential amino acids, while plant proteins like grains are "incomplete." The amount of protein needed varies from 0.4-1g per pound of body weight depending on activity level and goals like muscle building or fat loss. High protein diets have not been shown to harm health if calories are managed. Good sources of protein include meat, eggs, dairy, nuts, and legumes.