What is the likely cause of vitamin d deficiency?

No sun, poor intake: Vitamin d deficiency can happen either when you get an adequate intake through your diet or have inadequate exposure to uv light from the sun. Some foods, such as cows milk in the United States, are supplemented with vitamin d to help boost intakes. You can get a good dose from sun exposure by keeping your arms and legs exposed to direct sunlight without sunscreen for about 10 to 15 minutes during the months of the year when you can get uvb light from the sun. In indiana that is true from mid april until early november.