Contrary to popular belief, white spots and lines on the fingernails are not caused from a vitamin deficiency. The white spots and lines are not a sign that the person has a vitamin deficiency either.
Trauma to the fingernail is what causes those white spots and lines. The trauma can be so slight to the fingernail that you never even realized that a trauma occurred. Something as simple as popping a top on a can of soda can cause a trauma to the fingernail.
It takes approximately 6 months for a fingernail to grow from the base to the finger tip, several traumas can take place within those months leaving multiple white spots and lines on a fingernail.
If you have one fingernail in particular that constantly has white spots or lines, that fingernail is consistently the recipient of trauma. Quite often it is the fingernail on the index finger. If you can pinpoint the traumatic event that is occurring to the fingernail(s), and eliminate it, then the fingernails will quit having white spots and/or lines on them.