Vitamin D is one of the most common nutrients that Americans are deficient in. If you are not spending much time out in the sun, that may be one of the reasons why. Vitamin D comes directly from the sun, though you can also supplement your diet with it as well. If you deficient, you may have a drop in energy and may have a slow-moving immune system. In addition to this, you may also have alopecia or hair loss. There are studies that connect this type of hair loss to a Vitamin D deficiency.