Keratin

Vitamin A supports the structural integrity of keratin. Nails are made of keratin, a structural protein with exceptionally high sulfur content. Cysteine and methionine are the two sulfur-containing amino acids most critical for keratin integrity. Trace minerals act as cofactors and coenzymes that activate keratin production proteins; without them, structural proteins cannot function even when amino acid supply is sufficient. The number-one symptom of sulfur deficiency is brittle nails.