Tan skin may provide some protection against the sun's harmful UV rays, but this increase in pigment blocks vitamin D synthesis and limits the skin's ability to produce vitamin D, according researchers in Brazil.
"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," said study author Dr. Francisco Bandeira, of the University of Pernambuco Medical School in Recife, Brazil.
For the study, researchers examined nearly 1,000 males and females from Recife who were between 13 and 82 years old. All had significant daily sun exposure and none routinely used sunscreen or took vitamin D supplements.
Using the Fitzpatrick skin phototype scale, which is a numerical measure of skin color and type used by dermatologists, the researchers assessed the response of different skin types to UV light. Generally, higher scores indicate darker skin tones and the tendency to tan, not burn.
The participants' sun index was also calculated by multiplying the number of hours of sun exposure they got on a weekly basis by the fraction of exposed skin.
The researchers compared the participants' sun index scores and skin type with their blood level of vitamin D. Most of the participants with very high daily exposure to the sun had lower-than-normal serum vitamin D levels.
Overall, 72 percent of the participants were deficient in vitamin D. Those lacking this nutrient tended to be older and have lower sun index values, the study found.
"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," Bandeira said in a news release from the Endocrine Society.
The findings were presented Saturday at the annual meeting of the Endocrine Society, in Boston. Studies presented at meetings are usually considered preliminary until published in a peer-reviewed medical journal.
The U.S. Office of Dietary Supplements, part of the U.S. National Institutes of Health, provides more information on vitamin D.
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