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Does sunblock prevent vitamin D absorption?

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Worries about limiting production of vitamin D shouldn’t stop you slapping on sunscreen.

"An ounce of performance is worth pounds of promises," said American actress Mae West. Chances are West wasn’t referring to work performance or the marketing hype of cosmetic firms. But, promises aside, how well do sunscreens perform in blocking damaging UV radiation and thus reducing vitamin D production?

The top layer of skin, the epidermis, contains a compound called 7-dehydrocholesterol (7-DHC). When your skin is exposed to the sun, the ultraviolet radiation (UVB rays) causes that compound to transform into a precursor of vitamin D; that in turn is converted into vitamin D. We need the vitamin for absorption of calcium, which is essential for bone growth.

Unfortunately, the same rays that produce vitamin D also damage skin cells, leading to tell-tale sunburn and, more worryingly, a greater risk of skin cancer. The golden rule before going out into the sun is to slip, slop, slap and wrap. That is, slip on a shirt, slop on sunscreen, slap on a hat and wrap on sunglasses.

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So, how exactly does slopping on sunscreen protect skin from sun damage?

Sunblock, as the name suggests, blocks sunlight from reaching our skin. Sunscreens typically combine organic and inorganic ingredients – the inorganic ingredients such as zinc oxide or titanium oxide reflect or scatter the UV radiation, and the organic ingredients absorb the radiation and dissipate it as heat. The net result is the radiation doesn’t reach your skin surface, averting cell damage.

But what happens to vitamin D production? Danish researchers recruited a group of fair-skinned adults, slathered them in varying amounts of 50+ sunblock and then tested vitamin D levels in their blood before and after they were exposed to very exact doses of UVB.

As you’d expect, the thicker the layer of sunblock, the less vitamin D their body produced when exposed to UVB, to the point that vitamin D production was completely halted when they followed the sunblock application instructions.

That might sound worrying from a vitamin D perspective, but how many of us carefully apply the prescribed thickness of sunblock to all our exposed skin, slapping more on it at the recommended intervals?

In 1995, Australian researchers conducted a study that nowadays might be deemed unethical. They randomly assigned 113 adults to apply what they were told was sunscreen to their skin daily during summer. However, unknown to the participants, half were supplied with a broad-spectrum sunscreen (SPF 17) and the other half were given a placebo cream that offered no sun-blocking ability.

Checks of their vitamin D levels at the beginning and end of the summer revealed no differences in vitamin D status between the two groups. Why didn’t the sunblock affect vitamin D status?

It’s possible the SPF 17 sunscreen didn’t offer a great deal of protection compared with SPF 50 cream used in more-recent trials. That may have allowed UVB rays to reach the skin and vitamin D to be produced.

It’s also possible that those applying the real sunblock didn’t have total cover at all times, leaving enough exposed skin to allow adequate vitamin D production. The reality is few people put on enough sunscreen to block all UVB light.

However, total sun avoidance can, indeed, become a problem. Massey University researchers found 84% of New Zealand-resident South Asian women, many of whom identified as completely avoiding the sun, had inadequate vitamin D levels.

Still, using sunscreen on your face – via your moisturiser and foundation – isn’t affecting the exposure of your hands, arms, or even the back of your neck to the sun. For most of us, the regular use of face products with built-in sunscreen is unlikely to significantly reduce vitamin D levels.

This article was first published in the October 6, 2018 issue of the New Zealand Listener.