Spending time in the sun increases the levels of vitamin D in your body, but getting too deep of a tan stops its production and leads to deficiency.

A new study has found that people who have dark skin and are exposed to the sun on a regular basis are often deficient in vitamin D.

Brazilian researchers found that as skin tans, the darker tone protects itself against harmful ultraviolet (UV) radiation, but as the pigment deepens, it blocks the synthesis of vitamin D, limiting the body's ability to produce the essential vitamin.

Our bodies make vitamin D, often called the "sunshine vitamin," when the skin is exposed to sunlight. But many Americans are deficient in vitamin D, and others live on the brink of deficiency even in the sunniest of months. Studies have linked low vitamin D levels with compromised immune systems, heart disease, diabetes, depression, and a host of other ailments.

"Our research showed that, in a large sample of individuals living in a tropical region located 8 degrees south of the equator with very high rates of sun exposure and extremely high UV irradiation, most people had serum vitamin D below 30 ng/ml (nanograms per milliliter), the cutoff for normal," said lead study author Dr. Francisco Bandeira, chief of the Division of Endocrinology and Diabetes at the University of Pernambuco Medical School in Recife, Brazil.

Bandeira and his colleagues evaluated 986 people between 13 and 82 years of age who lived in Recife, Brazil. All study participants had high rates of daily exposure to sun and didn't use sunscreen regularly or take vitamin D supplements.

They found that volunteers who had greater sun exposure had skin that was more tanned and also had less vitamin D deficiency than other participants, but most of those with very high daily exposure still had serum vitamin D levels lower than normal.

Overall, 72 percent of the volunteers were deficient in vitamin D.

"Our findings suggest that skin tanning, which is a natural protection against the harmful effects of UV irradiation, limits the progressive rise in serum vitamin D towards optimal concentrations," Bandeira said.