Histidine is an alpha amino acid that is used as a building block during the production of proteins. It is considered to be an essential amino acid, which means that it must be obtained from the diet since the human body cannot produce it. Foods that contain high amounts of histidine include beef, lamb, cheese, pork, chicken, turkey, soy, fish, nuts, seeds, eggs, beans, and whole grains

It is thought that histidine and other amino acids used in skin care products can assist the skin in making important proteins, such as collagen and elastin. Collagen is important for skin strength, while elastin is important for skin elasticity. Both collagen and elastin levels in the skin decrease with age. Therefore, topical application of ingredients that stimulate their production, such as histidine, may help to decrease signs of aging. Isoleucine may also be used as a skin/hair conditioning agent.