People with darker skin may need far more winter vitamin D than guidelines say

Official vitamin D recommendations from several international agencies are based predominantly on data from White populations. Whether those numbers hold for people of Black or South Asian descent living at high latitude has never been properly settled, and the authors describe developing recommendations for these groups as an urgent need. Winter is the critical season, because it is when blood levels of 25-hydroxyvitamin D are hardest to sustain and diet does most of the work. This analysis set out to estimate, from randomised trial data, how much vitamin D these populations actually need to stay above accepted thresholds.

The researchers pooled individual participant data from 10 randomised controlled trials of vitamin D3 supplements or fortified foods, identified through a systematic search with predefined eligibility criteria. The combined dataset covered 677 participants of Black or South Asian descent, aged 5 to 86, all living at latitudes of 40 degrees north or higher. Rather than working from study averages, they modelled each individual's wintertime blood level of 25-hydroxyvitamin D against total vitamin D intake. From that model they derived the daily intakes needed to keep 97.5% of people above blood thresholds of 25, 30 and 50 nmol/L.

To keep 97.5% of people above the 25 nmol/L threshold, South Asian participants needed an estimated 23.9 µg of vitamin D per day and Black participants 24.1 µg. For the higher 30 nmol/L threshold the figures rose to 27.3 and 33.2 µg respectively. Reaching 50 nmol/L in 97.5% of individuals demanded 66.8 µg per day, and at that intake the model predicted the top 2.5% of people could reach blood levels of 158 nmol/L or more, a level linked to potential adverse effects in older adults in supplementation studies. The age-stratified estimates for the lower thresholds did not exceed the tolerable upper intake levels for vitamin D.

These requirement estimates sit substantially above the equivalent figures for White individuals, and above what several international agencies currently recommend. The authors point out that existing recommendations rest predominantly on data from White participants and on aggregate analyses rather than individual-level data, which this study was designed to improve on. That makes it a direct challenge to a one-size-fits-all approach to vitamin D guidance at high latitude. The authors are clear that much more work is needed in dark-skinned populations, on both the dose-response relationship and the characterisation of risks and benefits for actual health outcomes.

If you have darker skin and live somewhere with a weak winter sun, this is a reason to think standard vitamin D guidance may underestimate what you personally need to avoid low blood levels through the darker months. It is worth knowing that the thresholds here are blood concentrations rather than proven health outcomes, so the higher intakes are about maintaining status, not a demonstrated benefit. The finding that very high intakes could push a small fraction of people toward blood levels linked with potential harm in older adults is also a reminder that more is not automatically better.