Our bodies need regular exposure to sunlight in order to synthesize optimal levels of Vitamin D, sometimes nicknamed the “sunshine vitamin”. At the same time, excessive and unprotected regular exposure to ultraviolet radiation (UVR) can significantly increase the risk of developing skin cancer.

In order to balance the two, there are public health guidelines on optimal sunlight exposure. However, a new study has found that these guidelines, based on three-decade-old research, may require a revisal in light of new findings.

A delicate balancing act

When the skin is exposed to the sun, the presence of UV rays triggers a biochemical reaction that transforms provitamin D3 (or 7-dehydrocholesterol) into previtamin D3, which then enters the bloodstream where it becomes converted to 25-hydroxyvitamin D3 by the liver, also known as 25(OH)D — it is this latter form of vitamin D that your doctor can measure when you take blood tests. Later, 25(OH)D is further broken down by the kidneys and other specialized tissues in the body into a biologically active form of vitamin D, which performs numerous important duties, including promoting bone health and immune function, regulating calcium and phosphorus levels, and much more.

The amount of Vitamin D produced for a given timeframe of sunlight exposure can vary greatly, depending on factors such as skin tone, location, time of the year (close to the equator there are more UVR frequencies that are involved in Vitamin D synthesis), time of day, whether or not there are sunscreen or skincare products applied on the skin, age, and air pollution.

For those with light skin or low melanin levels, basal cell carcinoma (BCC), squamous cell carcinoma (SCC), and melanoma — the three most common types of skin cancer, in this order — frequently occur in sun-exposed areas of the body, which suggests UV exposure may be to blame.

The most important UVR frequency band in Vitamin D synthesis is UVB, a shorter wavelength of ultraviolet radiation that has higher energy levels. UVB is also associated with sunburn, darkening, and thickening of the outer layer of the skin. Although it comprises only 5% of ultraviolet radiation, short-wavelength UVB radiation is responsible for over 80% of sunburns.

Three decades ago, scientists exposed human skin cells (ex vivo) to various UVB levels, finding for the first time an association between these short-wavelength rays and the production of Vitamin D in the human body. Ever since then, these findings have been used as the basis for official solar exposure guidelines, which advise sun exposure at least 3 times a week during daylight (between 10:00 A.M. and 3:00 P.M.) on 20%–30% of the body surface area for about 30 minutes per session.

But a new study from researchers at King’s College London suggests there’s a different relationship between UVB and Vitamin D, which may prompt the revision of current standards and guidelines.

The team of researchers measured blood vitamin D levels in 75 healthy young volunteers, before, during, and after partial or full-body exposure to five different artificial UVR sources with different amounts of UVB radiation. The goal was to find the sweet spot between the pros of solar exposure (Vitamin D synthesis) and the cons (risks of sunburn and skin cancer).

The Vitamin D levels were compared to those that would be predicted from the old ex vivo vitamin D research. This comparative analysis showed there were important discrepancies between the actual measurements in live subjects and the predicted values based on ex vivo lab tests.

“An improved action spectrum based on serum 25(OH)D₃, rather than cutaneous previtamin D₃, will result in more refined assessments of risks and benefits of solar exposure that should result in better public health advice,” the authors wrote in their study published in the Proceedings of the National Academy of Sciences.

The study is timely considering the Commission internationale de l’éclairage, the global technical committee that is in charge of setting UVR exposure standards will soon convene to discuss these findings and re-evaluate the wavelength dependency of Vitamin D.