You're obese.
The vitamin is stored in fat, and people who have a greater amount of fat tend to have less D circulating in their blood for use throughout the body. Chances are, the higher your BMI, the lower your D levels. (The good news: Increasing your D intake could help you lose weight. In a small diet study at the University of Minnesota, people who started with the highest levels of D lost significantly more weight—and more of it was dangerous abdominal fat.)

Next: How to make your vitamin D levels higher