Vitamin D helps your body absorb calcium. Without enough Vitamin D, one can't form enough of the hormone calcitriol to absorb calcium from diet, affecting bone density. Low levels are linked to bone pain, muscle weakness, and fatigue.
Most people in cities spend their days indoors and wear full clothing when outside, preventing sunlight exposure which is the primary source of Vitamin D.
Generally, levels between 30-100 ng/mL are considered sufficient. Levels below 20 ng/mL are considered deficient.