As skin tans, it darkens to protect itself against harmful ultraviolet (UV) radiation, but the increasing pigment blocks vitamin D synthesis, limiting the skin’s ability to produce more vitamin D, a new study from Brazil finds. The results were presented at ENDO 2016, the annual meeting of the Endocrine Society, in Boston.
“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 Francisco Bandeira, MD, PhD, associate professor of medicine and chief of the Division of Endocrinology and Diabetes at the University of Pernambuco Medical School in Recife, 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.”
Bandeira and colleagues evaluated 986 people between 13 and 82 years of age, with roughly equal numbers of males and females, living in the city of Recife, Brazil. All study participants had high rates of daily sun exposure and did not regularly use sunscreen or take vitamin D supplements.
The researchers evaluated each participant’s Fitzpatrick skin phototype scale to estimate the response of different skin types to UV light. In general, higher Fitzpatrick scale scores indicate deeper colour and tendency to tan rather than burn in the sun.
They also calculated each participant’s sun index, the number of hours of sun exposure per week multiplied by the fraction of body surface area exposed.
They measured everyone’s serum vitamin D levels and compared them with their skin phototype and sun index scores.
Although the individuals with greater sun exposure had skin that was more tanned and less vitamin D deficiency than other participants, most of those with very high daily exposure had serum vitamin D levels below the normal cutoff.
Overall, 72 percent of participants had vitamin D deficiency, and their mean vitamin D level was only 26.06 ng/ml. The participants with deficient serum vitamin D tended to be older and have lower sun index values than those with normal levels.
Low levels of total vitamin D and bioavailable vitamin D can help predict a person’s risk of major adverse cardiovascular events such as a heart attack, stroke, heart failure or death, according to a first-of-its-kind study from the Intermountain Medical Center Heart Institute in Salt Lake City.
“Our study found that low levels of both total vitamin D and bioavailable vitamin D appear to be associated with poor cardiovascular outcomes,” said lead author Heidi May, PhD, MSPH, a cardiovascular epidemiologist with the Intermountain Medical Center Heart Institute.
The study evaluated 4,200 participants between the ages of 52 and 76. A quarter of the study participants were diabetic and 70 percent had coronary artery disease.
Clinicians tested participants’ vitamin D metabolite levels, which included components of vitamin D that are formed during metabolism, to determine the metabolites’ association with future major adverse cardiovascular events. Bioavailable vitamin D results from vitamin D being absorbed into the blood stream without binding to surrounding proteins.
During metabolism, only 10-15 percent of total vitamin D is available in the body to act on target cells, as most are bound to vitamin D binding proteins. Therefore, evaluating whether the proportion of vitamin D that can be used may be important, as only unbound vitamin D, such as bioavailable vitamin D, is available to act on target cells.
The study tested many different types of vitamin D, but found that measuring total vitamin D and bioavailable vitamin D were the most accurate in predicting harmful cardiovascular events.
“This study is the first research that evaluates the association of vitamin D metabolites with cardiovascular events,” said Dr. May. “And evaluating usable vitamin D could mean the difference on the amount of vitamin D prescribed, if it’s prescribed at all.”
The study expands on the results of several observational studies, including some performed at Intermountain Healthcare, but researchers recommend conducting more studies on non-Caucasian populations because past research shows vitamin D metabolites affect Caucasian and non-Caucasian races differently.
Results of the study were presented at the American College of Cardiology Scientific Sessions in Chicago.