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New research suggests sunlight exposure guidelines may need revising. Shanina/Getty Images
  • Researchers used blood samples from 75 individuals to assess vitamin D levels regarding current sunlight guidelines, which scientists originally based on skin samples.
  • Their results suggest that current sunlight guidelines may need revising.
  • Further calculations and research are necessary before health experts can create new sunlight guidelines.

Vitamin D is a fat-soluble vitamin that plays a vital role in human health. It improves bone health and reduces the risk of chronic diseases, including:

Ultraviolet radiation (UVR) from sunlight is a major source of vitamin D, accounting for around 80% of a person’s recommended daily allowance (RDA). However, UVR from sunlight can also cause sunburn and skin cancer.

Sunlight contains two types of UVR: ultraviolet A (UVA), which makes up around 95% of the sun’s rays, and ultraviolet B (UVB), which comprises around 5% of the sun’s rays. UVA and UVB both cause tanning, skin aging, and the development of skin cancer, but only UVB leads to vitamin D production.

Current guidelines on sunlight exposure for vitamin D derive from 1982 research examining pre-vitamin D formation in the skin following UVR exposure. However, some researchers suspect these guidelines may need revising, as pre-vitamin D undergoes chemical changes before becoming vitamin D that our bodies can use.

Getting sunlight guidelines up-to-date is important to ensure people get enough vitamin D while not putting themselves at risk of sunburn and skin cancer.

In a recent study, researchers from King’s College London (KCL) in the United Kingdom conducted a study to test the current vitamin D guidelines. They compared levels of serum 25-hydroxyvitamin D3 — the gold standard for assessing vitamin D — in healthy volunteers following UVR exposure with suggested levels from current guidelines.

They found that the current guidelines did not predict vitamin D production from UVR exposure accurately. However, adjusting vitamin D calculations could fix this.

“The old vitamin D spectrum will work if we subtract 5 [nanometers (nm)] from each wavelength, so the whole spectrum is shifted 5 nm to the left,” Professor Antony Young of Experimental Photobiology at KCL, and lead author of the study, told Medical News Today.

The study appears in PNAS.

The researchers recruited 75 young, healthy volunteers in the winter and spring of 2011–2014 when ambient UVB levels were low and skin gained some protection from clothing.

The volunteers underwent full or partial body exposure to five different artificial UVR sources with varying levels of UVB radiation spectra on five occasions 3–4 days apart. Before, during, and after exposure to each spectrum, the researchers measured their blood vitamin D levels.

The team then used various statistical models to determine the relationship between UVR dose and blood vitamin D levels, also known as the “action spectrum.” They then compared this with the action spectrum for pre-vitamin D3 used to create current guidelines for sun exposure.

The researchers found that blood vitamin D levels following UVR exposure did not match the suggested current guidelines.

To explain their results, the researchers say that the action spectra for cutaneous pre-vitamin D3 and serum vitamin D are likely different, as vitamin D3 degrades before forming vitamin D, making it hard for spectroscopy to predict vitamin D levels accurately from pre-vitamin D3 alone.

“Pre-vitamin D is converted into vitamin D in the skin by heat then enters the blood circulation,” said Professor Young, “It has to undergo two enzymatic steps before it becomes functional biologically. We used the gold standard to assess vitamin D status, which is 25-hydroxyvitamin D3.”

“The original study looked at pre-vitamin D in skin that does not take into account the later steps in the process. We also used low-exposure doses that are recommended for safer sun exposure. The higher the UVR dose, the more photochemistry happens, with possibly different results. So a comparison is a bit like comparing apples with oranges. All I can say here is that our data have more biological and physiological relevance,” he added.

The researchers added that there might also be an error in the action spectrum for pre-vitamin D3. To test this, they incrementally reduced the pre-vitamin D3 action spectrum by 1 nm to see how their results would change. They found that a 5 nm shift, known as a “blue shift,” corrected the old model.

The researchers explained that before health experts can create new sunlight guidelines from these findings, further calculations are necessary.

“The revision of the guidelines will take more work,” said Professor Young, “This is done by “weighting” a given solar UVR spectrum with biological function, each of which has its own wavelength dependence.”

“To give an example of this: max UVB content (at solar noon) of solar UVR is about 5%, but this 5% will cause about 85% of the sunburn because the UVB is much more effective than the 95% of UVA at causing sunburn. The amount of UVB in the sun depends on the height of the sun. So these calculations shift minute by minute,” he added.

“Risk-benefit calculations need to be done with sunburn and vitamin D with our new spectrum,” he added. “This will tell us the safest time to be out in the sun at any time of day, season, or latitude. The good news is that vitamin D is produced at a much lower UVR dose than [what] causes sunburn. Many people deliberately seek sun exposure to get a tan.”

To conclude, the researchers say that sunlight guidelines according to blood serum vitamin D levels could include more refined assessments of the risks and benefits of sun exposure when compared to those based on pre-vitamin D levels.

The team also states that limitations to their study include a wide range of times in which volunteers had exposure to radiation. They also mentioned that there was a lot of interpersonal variation, although they expected this in vitamin D studies.

“What these authors have done is to expose humans to ultraviolet radiation that contained multiple wavelengths and measured blood levels of 25-hydroxyvitamin D, ” Michael F. Holick, M.D., Ph.D., and Professor of Medicine, who was not involved in this research yet authored the research behind the current sunlight guidelines, told MNT, “They did not measure pre-vitamin D3 in the skin.”

In future work, the researchers say that they will conduct risk-benefit calculations and model the effect of melatonin on different skin types. When asked what the main takeaway from the research was, Professor Young replied:

“I think that the main message is that the UVR doses for vitamin D production — and the sun is the main source — are very much lower than to cause a sunburn. Regular short times of exposure will give you good vitamin D status.”