What Casuses Brittle Nails?

Suddenly my nails have become brittle. They have always had mild ridges on them, especially the thumbnails. I never wear fingernail polish. I take a multivitamin and try to have a healthy diet with vegs, fruits, fish, sardines every week, take extra flax seed oil. I lose weight easily since a year ago. I have a mild cough, infrequent and dry. Loss of appetite, I make myself stop to eat. No fatigue. Do these point to anything in particular? I m 63 and in otherwise good health.