Keratin Keratin: a fibrous protein that makes up the structure of nails and hair. Related Articles: Glossary: KeratinisedMenopause and Brittle NailsWomen going through the menopause frequently report brittle nails...7 free! 10 free! 99 free! What is the ‘deal’ here?What is the difference between ‘pure UV gel’ and a ‘hybrid’ and where are ‘7 Free’?Pseudomonas