Oral Presentation 18th International Congress on Photobiology 2024

How does natural sun protection and sunscreen use influence vitamin D synthesis?  (#18)

Antony R Young 1
  1. King’s College London, London, United Kingdom

Terrestrial solar UVB radiation (~295-315nm) drives cutaneous vitamin D synthesis but is also the main cause of erythema (sunburn) with overlapping action spectra for both endpoints. UVB converts 7-dehydrocholesterol (7-DHC) in skin into pre-vitamin D, which then undergoes UVR independent steps to become functional vitamin D. People with highly pigmented skins typically have poorer vitamin D status that those with lighter skins within a given latitude band. The inhibitory effect of melanin has not been quantified. A comparison of Fitzpatrick skin type (FST) VI (black) versus II (white) showed an inhibitory factor <1.5 which is less than melanin’s ability to inhibit sunburn [1]; the minimal erythema dose (MED) for FST VI is about 7-10 times greater than those with light skin.

UVB attenuation by sunscreens to prevent sunburn would be expected to inhibit vitamin D synthesis. Carefully monitored field studies show that correct sunscreen use (labelled sun protection factor (SFF) of 15), inhibited sunburn, and to some extent vitamin D synthesis, but allowed considerable enhancement of vitamin D status during a one-week holiday in which peak UV index was very high [2]. This is because sub-erythemal exposure is sufficient for good vitamin D synthesis. We estimated that the holidaymakers received about 1/10th of an MED per day through the sunscreen.

In summary, melanin and sunscreens are chromophores that attenuate UVB radiation. Melanin affords protection against erythema with a “SPF” of about 7-10 but has a modest inhibitory effect on vitamin D synthesis. The most likely reason for this is that there is sufficient 7-DHC above the melanin rich basal epidermis to enable vitamin D synthesis. In the case of sunscreens, the dose threshold for vitamin D synthesis is much lower than that for erythema. The lessons from these studies are that chromophore location is important and different end points have different UVR dose thresholds. They also show that sunscreen use (SFF 15) does not compromise vitamin D synthesis. We lack data on high SPF (50+) sunscreens.

  1. [1] Young, A.R., et al., Melanin has a Small Inhibitory Effect on Cutaneous Vitamin D Synthesis: A Comparison of Extreme Phenotypes. J Invest Dermatol, 2020. 140(7): p. 1418-1426 e1.
  2. [2] Young, A.R., et al., Optimal sunscreen use, during a sun holiday with a very high ultraviolet index, allows vitamin D synthesis without sunburn. Br J Dermatol, 2019. 181(5): p. 1052-1062.