Keratin is a tough insoluble fibrous protein that is the main structural constituent of epidermal tissues like hair, nails, horns, hooves, feathers, claws, and bills. Keratin contains large amount of sulfur especially cysteine. Eating a lot of carotene rich food like carrots is important for making good quality keratin.
Keratin is not digested by the enzymes in the stomach and thus cannot be hydrolysed and used for nutritional purposes. Keratin can be broken down slowly by soil bacteria and so can be used as fertilizer. It is also used in coating pills that are meant to dissolve only in the intestine. Another benefit of keratin in the body is in the formation of protective calluses in the skin. Constant rubbing and pressure will cause the thickening of the outer cornified layer of the skin called epidermis to form calluses. This is for protection and is usually found among athletes, manual laborers, and musicians playing stringed instruments. These calluses are keratinized epidermal cells. Keratin in the epidermis also makes the outer layer of the skin almost waterproof and together with collagen and elastin, keratin helps make the skin strong, tough and pliable enabling it to protect the body from foreign matters.
Mutations in the keratin genes may lead to diseases like epidemolysis bullosa simplex, icthyosis bullosa of Siemens, epidermolytic hyperkeratosis, steatocystoma multiplex, keratosis pharynges, and rhabdoid cell formation in large cell lung carcinoma with rhabdoid phenotype. Some cancers like carcinomas, mesotheliomas, thymmas, sarcomas and trophoblastic neoplasms are determined by keratin expression.