Failure to consume foods that contain vitamin D and limited sun exposure cause low vitamin D levels, states WebMD. People with dark skin and those who are obese may also have vitamin D deficiencies. In some cases, the body is unable to use available vitamin D properly.
Fatty fish, cheese, eggs, beef liver and fortified milk are major sources of vitamin D, explains WebMD. Those who do not consume enough of these foods may be deficient in vitamin D. This is particularly true of vegans, who do not consume animal products.
The body produces vitamin D when exposed to the sun, so those who live in northern latitudes, wear clothing that blocks the sun, or do not spend much time in the sun are prone to developing a vitamin D deficiency, states WebMD. People with dark skin have more melanin in their skin, which interferes with the body's ability to produce vitamin D.
As people get older, their kidneys may have difficulty converting available vitamin D to a form that is usable by the body, says WebMD. The intestines of those with cystic fibrosis, celiac disease and Crohn's disease may not be able to absorb available vitamin D, and those who are obese may have low levels of vitamin D in their blood because the fat cells absorb it.