Apart lack of sunlight, what causes low vitamin d levels?

Insufficient intake: There are two ways to get vitamin d. Your body makes it when you are exposed to the sun, or you can take it in your diet. Vitamin d tends to be found in fish (usually cold water) and eggs. There's some in fortified milk, but not enough. Over all, it's pretty hard to get enough from your diet unless you eat huge amounts of certain fish or take a supplement, so i recommend supplements.

Hypovitaminosis D: Vitamin d deficiency can be caused by inadequate intake, inadequate sunlight, impaired absorption, and impaired conversion to the active form by the body. Within each of these categories there can be many causes.