Lack of shine in toenails may be caused by malnutrition or some chronic diseases, or fungal infections. The main component of the toenail is keratin, and when there is a lack of protein, amino acids and certain vitamins and trace elements, the toenail may become lustrous due to lack of nutrition. It is recommended to supplement with foods with high protein content, such as milk, eggs and some soy products, as well as some vitamin complexes. Also, chronic wasting diseases can cause toenails to lose their luster. It is possible to have fungal infections, after which the toenails may become thick, brittle or gray, in addition to lustrous.