I know a lack of Iron can cause nails to become brittle... have you been dx with being anemic with in the last year? - and protein is needed for strong healthy hair, a daily can of tuna if often suggested.

BUT - the best way to find out what your body may be lacking is to have blood work drawn on your next yearly check up.
And as always 6 to 8 cups of water (per day) does the body good.