Do Sunscreens Block Vitamin D Production?
We received a question recently about some research that showed that people who applied sunscreen properly during a sunny break had an increase in vitamin D levels. The study was carried out by Dr Antony Young at the St John’s Institute of Dermatology in King’s College, London, however it has not yet been officially published in a peer-reviewed journal (due later this year, according to a reporter from the Sydney Morning Herald). This study seems to turn on its head the traditional mantra that sunscreens block vitamin D production. Or does it?
Vitamin D is by the action of sunlight on skin. Sunlight consists of a broad spectrum of different kinds of light, both visible, and ultraviolet. Ultraviolet light is generally divided into UVA at wavelengths 400-320 nm and UVB at wavelengths 320-290 (for more background on UV light, see this site from the Skin Cancer Foundation). UVA light penetrates the skin more deeply than UVB, and is considered to be the major cause of skin aging. UVB tends to only penetrate the upper layers of the skin and results in reddening and burning. A mnemonic to remember the general functions of UV light are UVA for aging, and UVB for burning. Both are implicated in skin cancer. Vitamin D is produced at wavelengths between 280 and 320 nm, corresponding to UVB light (see a recent study by Feldmeyer to look at the effect of wavelength on vitamin D production). Exposure to a little UVB light produces vitamin D, and excessive exposure causes skin reddening, then sunburn.
Sunscreens either filter UVB rays, or can be broad-spectrum and filter both UVA and UVB. The amount of sun filtered is expressed as a percentage. No sunscreen offers 100% protection. Sun protection factor sunscreen (SPF15) blocks only 93% of UVB light, and SPF30 blocks around 97%. Tests to determine the SPF factor are done in the laboratory. Volunteers wear a standard amount of sunscreen on a patch of skin and it is irradiated with UVB light for a certain amount of time. The amount of skin reddening is used to determine the SPF. The SPF number indicates the amount of time one can stay out in the sun before getting sunburnt. If it takes 15 minutes for someone to burn without sunscreen, SPF15 will allow that person to stay in the sun for around 4 hours until they start to burn.
Coming back to the study, volunteers were provided with either SPF15 sunscreen and were told how to apply it correctly, or they were left to their own devices regarding sunscreen use during a week-long vacation in sunny Spain. There was no information about whether the sunscreen was broad-spectrum and blocked out UVA light as well. Both groups had significant increases in their vitamin D levels when they returned, although the increase in the group provided with sunscreen and instructions was lower.
Vitamin D levels are a marker of UVB light exposure. When used correctly, sunscreens with high SPFs will filter out most of the UVB rays that produce vitamin D. Clearly, the subjects also were exposed to large amounts of UVB light that made it through the SPF15 sunscreen, and also likely had a high exposure to UVA light. Rather than showing that vitamin D synthesis occurs despite sunscreen use, the study highlights that despite using sunscreen, subjects increased their exposure to damaging UV light during their summer break.
Sunscreen filters out UVB light. The amount filtered depends on the SPF factor, how thickly it is applied, and how long the skin is exposed to the light. These factors also affect the amount of vitamin D synthesized. Black-and-white statements such as “Sunscreen blocks vitamin D” are not helpful to the average person because they create confusion about whether people should be concerned about vitamin D levels or concerned about exposing themselves to too much sun. In reality, there is a trade-off between increasing risk of vitamin D deficiency and increasing risk of aged skin and skin cancer (see review by Macdonald for a a review on all facets of the issue). On the other hand, vitamin D supplements are safe and can help prevent risk of deficiency in people who strictly follow sun-safe procedures. Perhaps there is an option for people to avoid skin damage and avoid vitamin D deficiency after all.-jb-
Elkins, L. Worried Sunscreen Blocks Vitamin D? Here’s Good News… Published in the Daily Mail, 3rdJune 2013, 23:51 GMT. http://www.dailymail.co.uk/health/article-2335478/Worried-suncream-blocks-vitamin-D-Heres-good-news-.html