In an effort to estimate just how much vitamin D dark-skinned individuals can make during exposure to sunlight, UK researchers exposed a group of people of South Asian ethnicity to several different doses of UV radiation:
Farrar MD, Webb AR, Kift R, et al. Efficacy of a dose range of simulated sunlight exposures in raising vitamin D status in South Asian adults: implications for targeted guidance on sun exposure. Am J Clin Nutr. 2013;97(6):1210-6.
This same research group had previously determined that laboratory exposure to UV irradiation equivalent to casual summer sun exposure produced 25(OH)D levels great than 20 ng/mL in almost all white adult participants, but failed to do so in any of the adults of South Asian ethnicity. This study was designed to determine how much more UV exposure those of South Asian ancestry would need to achieve vitamin D blood levels similar to those seen in people with lighter skin types.
Researchers recruited 60 adults of Indian, Pakistani, and Bangladeshi ethnicity with skin type V. People with skin type V have a fairly dark skin tone from increased melanin production. This extra melanin protects them from UV radiation, reducing their likelihood of developing non-melanoma skin cancers, but means they must spend much more time in the sun than those with lighter skin to produce the same amount of vitamin D.
The participants were divided into six groups and exposed to UV radiation three times per week for six weeks. The UV lights were designed to mimic the summer sun with 95% of the light as UVA radiation and the remaining 5% as UVB. The groups were divided by the amount of radiation they received, measured in standard erythema doses (SED):
These doses were equivalent to anywhere from 15-90 minutes of peak summer sun exposure in Manchester, United Kingdom (53.5˚ N), the larger the number, the more minutes in equivalency. The six week study design was chosen to represent the length of school summer vacation, the time period where most people in the UK receive the majority of their vitamin D-producing UVB radiation. Participants were dressed in T-shirts and knee-length shorts during UV exposure to simulate casual, summer clothing. The study was conducted during the winter months to avoid confounding with any UVB exposure from the sun.
At baseline, all participants had 25(OH)D levels less than 20 ng/mL. Ninety percent had levels less than 10 ng/mL and eighteen people had levels less than 5 ng/mL, the latter a serious deficiency state which can cause rickets and osteomalacia. Baseline parathyroid hormone (PTH) was also measured.
After this six week simulated summer, 25(OH)D levels rose significantly in all six groups, but only six participants achieved blood levels greater than 20 ng/mL, with no one specific SED group showing significant improvement in vitamin D levels.
The authors noted that in this follow-up study, even with a three-fold increase in UV exposure, those of South Asian ethnicity are not able to make sufficient vitamin D at northern latitudes wearing casual clothing. Those receiving the larger doses of UV radiation were left with an average vitamin D blood level of only 15 ng/mL.
From an evolutionary perspective, those with skin type V evolved this darker skin as protection from the intense UV exposure they received in the tropics. While this increased melanin production was protective for South Asians living in South Asia, it makes it very difficult to produce adequate vitamin D in the sunlight of northern latitudes. Based on the studies by these authors it will be difficult, if not impossible, for those with darker skin to achieve a natural vitamin D level from sun exposure alone, particularly if they do not commit to getting full-body sun exposure.
It is therefore imperative that those with the darker skin types living in areas of the world far from where their skin types evolved supplement with oral vitamin D year round to avoid the startling deficiency states observed in this study. If they want to make their own vitamin D, they likely have to expose more skin and get even longer periods of sun exposure.