Most of the time it's just caused by minor trauma to the nail bed. Do you work with your hands?

Medically speaking, those little white marks that sometimes appear as dots or lines on the fingernail are known as leukonychia. There are several possible causes for the discoloration, but the most common are mild trauma, a fungal infection, skin conditions, or nutritional deficiencies. For example, if you accidentally slam your fingernail into an object, a white mark will likely develop. But because a normal fingernail takes months to grow, by the time a blemish appears you may have forgotten what caused the trauma. White marks on your nails that are more linear could be a sign of a fungal infection. Gyms and nail salons are hotbeds for fungus and yeast, and are common places for contracting infections. These infections attack your nail directly, causing discoloration and even separation from the nailbed. If you suspect that's the cause for the white on your claws, see a dermatologist about a culture sample and diagnosis. Another culprit, although not as common, is psoriasis, a chronic skin condition that causes red, irritated blemishes and scaling throughout the body. In extreme cases, fingernails are affected by dead skin cells that accumulate in the nailbed and form thick layers, also known as "plaque." Treatment varies from prescription creams to ultraviolet light therapy. Finally, an old wives' tale fingers another suspect: your diet. In folklore, the most commonly cited cause for those white marks (and probably something your mom warned you about) is a deficiency of certain minerals in your body, such as zinc and calcium. Adding nutritional supplements or foods containing these minerals to your diet could clear up your white flecks in a few months. http://tinyurl.com/2gpear

I've always had lots of little white spots on my fingernails. Always. And I've always wondered the same thing. I've heard too much sugar, too much milk, not enough milk, trauma, etc., etc. And I always thought it was trauma, I thought, gee, I must hit my fingers on things ALL THE TIME. But a few months ago I started taking a daily multivitamin, and I now realize that I don't have those spots. So, my guess is that it IS a mineral deficiency.