You do not consume the recommended amount of vitamin D. This happens when you are a strict vegetarian. We are only bothered about our skin, but are we bothered about our bones. Application of sun screen lotions, staying indoors, clothing habits, pollution and minimal exposure to direct sunlight are the major reasons of vitamin D deficiency.
You step out of the house, get into your car, reach your office building and work in an air-conditioned environment that has closed doors and windows. Your exposure to sun light is limited. Remember, your body makes vitamin D only when your skin is exposed to sunlight.
If you are dark skinned, the pigment melanin in your skin reduces the ability to make vitamin D in presence of sunlight. Dark skinned people are more at a risk of vitamin D deficiency. Medical problems such as cystic fibrosis, Crohn’s disease, etc, can influence your digestive tracts ability to absorb vitamin D from the food that you eat.
With age, your kidney may not be able to convert vitamin D to its active form thus causing a deficiency. Obesity can cause vitamin D deficiency. If you have a body mass index of 30 or greater, you may be vitamin D deficient.