Dr. Antony Young of the University of King’s College London described how sunscreens, even when used optimally to prevent sunburns, do not prevent the formation of vitamin D in the skin. The study he described in his lecture was remarkable for the stringency of its design. He and his colleagues recruited lightly pigmented volunteers from Poland to enjoy a week’s winter holiday on the sunny island of Tenerife.
They divided the volunteers into three groups: one (the ‘control group) was advised to enjoy their holiday and to continue to use their sunscreen products as they usually do. The other two groups were given tubes of sunscreen to use 3 times during the day, and instructed to cover all their sun-exposed skin with it 3 times each day. The tubes were weighed after applications to measure compliance. One of the treated groups used a product that primarily screened UV-B (SPF ~15), while the other applied a product that also screened UV-A, with an SPF >30. Hours in the sun were clocked for all Tenerife participants using wristband solar chronometers; exposure times did not differ between the 3 groups; all clocked many hours in the sun. A fourth group remained behind in Poland.
Virtually all members of the control group in Tenerife experienced sunburns, indicating that if they did use sunscreens, their practices were ineffective. In contrast, no one in the two treatment groups developed a sunburn. But here is the key point: their sunscreen consumption averaged over 3 oz/day (or ~100g/day). In other words, each day they used the equivalent of a commonly-sized tube of commercial sun screen! It is safe to say that few people probably use this quantity of sunscreen in their sun protection routines, yet this approximates the quantity that is employed when testing laboratories generate the products’ SPF ratings.
Indeed, numerous earlier clinical studies have shown that, ordinarily, most people apply much less sunscreen than recommended. In one study, it was estimated that the average quantity of an SPF 15 applied on a sun holiday resulted in a SPF of no more 3(!), affording little protection against sunburn. Moreover, sunscreen usage was correlated with more time spent in the sun, suggesting little net benefit to the way sunscreens are typically used.
As expected, blood levels of vitamin D in the Tenerife control group – all of whom experienced sunburns – rose dramatically, while those in the control group that stayed behind in the Polish winter, declined. Both sunscreen-treated groups in Tenerife also experienced a generous bump in the levels of vitamin D in their blood, albeit not quite as high as in the control group. Yet, this study also demonstrated that even when highly effective sunscreens are used correctly – with quantities that protected these light skinned person from burns even in intense tropical sunlight – vitamin D levels still increased substantially.
Bottom Line: This study demonstrates that sunscreens can be used to protect the skin from solar toxicity without risking vitamin D deficiency as a side effect. Moreover, since very few individuals use these quantities of sunscreen in actual practice, it is highly likely that most peoples’ sunscreen usage does not interfere with the skin’s ability to produce vitamin D in other settings. On the other hand, the way most people use sunscreens is ineffective for sun protection.