Biotin

Brittle nails.
Iorizzo M, Pazzaglia M, M Piraccini B, Tullo S, Tosti A.
J Cosmet Dermatol. 2004 Jul;3(3):138-44.

 Nail brittleness is a common complaint characterized by weak inelastic nails that split, flake and crumble. It may be a consequence of factors that alter nail plate production and/or factors that damage the already keratinised nail plate. It is often idiopathic. It can also be caused by many dermatological and systemic diseases, nutritional deficiencies, drugs and traumas. Environmental and occupational factors that produce progressive dehydration of the nail plate have an important role in nail brittleness. Treatment of brittle nails is often difficult. Preventative measures, together with oral supplementation of vitamins (especially biotin), oligo-elements and amino acids, can be useful in improving nail strength. Cosmetic treatment affords camouflage and a degree of protection. PMID: 17134428

  Vitamins and minerals: their role in nail health and disease.
Scheinfeld N, Dahdah MJ, Scher R.
J Drugs Dermatol. 2007 Aug;6(8):782-7.

 Nail health and appearance are global concerns. We investigated the use of biotin vitamin E, alpha-tocopherol, vitamin C (ascorbic acid), vitamin A, retinoids, retinol, retinal, silicon, zinc, iron, copper, selenium, and vitamin B12 (Cyanocobalamin) in nail health and disease. The evidence that we adduce in this paper suggests that: 1) proper nail care seems to help maintain nail health; 2) no evidence supports the use of vitamin supplementation with vitamin E, vitamin C (ascorbic acid), vitamin A, retinoids, retinol, retinal, silicon, zinc, iron, copper, selenium, or vitamin B12 (Cyanocobalamin) for improving the nail health of well-nourished patients or improving the appearance of nails affected by pathologic disease; and 3) brittle nail syndrome appears to abate with supplementation with a 2.5-mg dose of biotin daily or a 10-mg dose of silicon daily, a useful form of which is choline-stabilized orthosilicic acid. PMID: 17763607