Vitamin D deficiency can occur for a number of reasons:

You don't consume the recommended levels of the vitamin over time. This is likely if you follow a strict vegetarian diet, because most of the natural sources are animal-based, including fish and fish oils, egg yolks, cheese, fortified milk, and beef liver.

Your exposure to sunlight is limited.
Because the body makes vitamin D when your skin is exposed to sunlight, you may be at risk of deficiency if you are home bound, live in northern latitudes, wear long robes or head coverings for religious reasons, or have an occupation that prevents sun exposure.


You have dark skin.
The pigment melanin reduces the skin's ability to make vitamin D in response to sunlight exposure. Some studies show that older adults with darker skin are at high risk of vitamin D deficiency.


Your kidneys cannot convert vitamin D to its active form.
As people age their kidneys are less able to convert vitamin D to its active form, thus increasing their risk of vitamin D deficiency.


Your digestive tract cannot adequately absorb vitamin D.
Certain medical problems, including Crohn's disease, cystic fibrosis, and celiac disease, can affect your intestine's ability to absorb vitamin D from the food you eat.


You are obese.
Vitamin D is extracted from the blood by fat cells, altering its release into the circulation. People with a body mass index of 30 or greater often have low blood levels of vitamin D.


Vitamin D Deficiency: Symptoms, Causes, and Health Risks