As skin tans, it darkens to protect against harmful ultraviolet (UV) radiation, but the increasing pigment in the skin also blocks sunlight-triggered vitamin D production. In fact, according to a 2016 study, the impact of a suntan on vitamin D synthesis can be enough to cause or perpetuate vitamin D deficiency. The study, which was presented at ENDO 2016, the annual meeting of the Endocrine Society, evaluated 986 people between 13 and 82 years old who lived in Recife, Brazil. All of the participants had high rates of daily sun exposure and didn't regularly use sunscreen or take vitamin D supplements. Researchers evaluated each participant's skin color and gave them a score based on the Fitzpatrick phototype scale, which is used to estimate the response of different skin types to UV light. Higher scores on this scale indicate deeper skin color and a 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. Finally, they measured everyone's vitamin D levels and compared them with their skin phototype and sun index scores, finding that:
Overall, 72% of participants had deficient vitamin D levels: their average vitamin D level was 26.06 ng/ml, while the cutoff for normal vitamin D levels is 30 ng/ml. Even among those with very high daily sun exposure, most had vitamin D levels that are considered sub-optimal.
The participants with vitamin D deficiencies tended to be older and to have lower sun index scores than those with normal vitamin D levels.
The study’s findings indicate that, even if you get a lot of sun, a suntan may prevent your vitamin D levels from staying at optimal concentrations (although, staying out of the sun too much isn't good either, according to the study). If you’re looking to raise your vitamin D levels, you may be better off sticking to moderate sun exposure and boosting your vitamin D intake by eating eggs, fortified cereals and milk, or taking a vitamin D or fish oil supplement. Of course, talk with your healthcare practitioner before adding any new supplements to your health regimen.