Vitamin D facilitates skeletal growth and strength, working with calcium and phosphorus to be absorbed better. It also strengthens your immune system by increasing the activity of "natural killer cells."
You can get this vitamin by eating fatty fish (catfish, salmon, mackerel, sardines, tuna) eggs, and meat. In more developed countries, staple foods such as milk, flour, and margarine are artificially fortified with vitamin D. Mushrooms are the only vegan/vegetarian food source of vitamin D.
Our skin cells actually make this valuable vitamin when they come in contact with UV rays from the sun, but research shows that darker skinned people living in temperate climates (like the United States) have lower vitamin D levels. The reason behind this is that melanin (the pigment of our skin) actually prevents the synthesis of Vitamin D. The darker complexion you have, the more likely your body isn't producing enough Vitamin D for itself!
A deficiency is not readily noticeable, so talk to your doctor about getting tested for Vitamin D levels - it's a simple blood test.
And if you still need some convincing, vitamin D has been linked to healthy hair follicle growth!