News, Views, & Reviews

• Sunscreen decreases vitamin D only when applied at 2 mg/cm² in controlled conditions.

• Normal sunscreen usage does not decrease previtamin D production.

Sunscreen has an important role in preventing ultraviolet (UV) exposure and subsequent DNA damage and carcinogenesis. As sun protection messages become more prevalent, so does the use of sun protection, including commercial sunscreens. Because the same UV-B light promotes the activation of vitamin D in the skin, active sun protection could lead to lower vitamin D levels. Fortunately recent studies have addressed this question.

In controlled conditions, consistent application of thick (2 mg/cm²) sunscreen at sun protection factor (SPF) 8 decreases vitamin D production when narrow-band UV-B is applied uniformly 20 minutes after application.1 This thick application is similar to the 2.2 mg/cm² used by the US Food and Drug Administration to measure the SPF of a particular sunscreen. However, this level of application is at least 50% more than what is used in studies of "real life" application by adults and children. It also does not account for the everyday waning of efficacy with movement, clothing, and friction, and the likelihood that most people do not reapply throughout the day.

One trial studying fair-skinned adults in northern Europe compared the use of an SPF 8 inorganic sunscreen on the upper body at thicknesses from 0.5 mg/cm² to 2 mg/cm². Following this, all patients were exposed to UV-B at 3 standard erythema doses, equaling 300 J/m². A standard erythema dose is 100 J/m², and in comparison, the "minimal erythema dose" for fair skin is often approximately 200 J/m2. These patients had serum 25-hydroxyvitamin D measured before the study, then were exposed to UV-B 4 times at a 2- to 3-day interval. Vitamin D levels were again measured 3 days after the final UV-B dose.

Serum vitamin D levels increased in an exponential manner with decreasing thickness of the sunscreen layer. The only group that did not see an increase in vitamin D level was that treated with 2 mg/cm² sunscreen thickness. This study used a relatively low SPF level at 8, so it is interesting to theorize whether a use of a higher SPF sunscreen at a thinner application level might also block vitamin D production.

• Psoriasis is associated with low vitamin D levels in all seasons

• Cutaneous lupus erythematosus is associated with low vitamin D levels and is more pronounced among patients with dark skin.

• Other autoimmune diseases such as rheumatoid arthritis have a less strong association.

• Patients with psoriasis at northern latitudes have a higher risk of low vitamin D year-round.

A yearlong case-control study in Italy followed 145 psoriasis patients, 112 patients with rheumatoid arthritis, and 141 healthy controls for levels of vitamin D, parathyroid hormone, and serum calcium.³ Healthy controls were recruited from patients' family members to reduce variations in dietary intake. Among this group, psoriasis patients were younger, more likely to smoke, and had generally higher body mass index than controls. At least 50% of patients had vitamin D deficiency year-round, and that number increased to more than 80% in the winter months.

The implications of low vitamin D in psoriasis patients include risks of osteoporosis, metabolic syndrome, and cardiovascular disease. This is notable because psoriasis itself is associated with concurrent and new-onset obesity, dyslipidemia, hypertension, and cardiovascular disease. Further, patients with psoriasis often have a history of chronic use of potent or superpotent topical steroids over large surface areas of skin, predisposing them to systemic steroid side effects such as osteoporosis, diabetes, obesity, increased lipids, and other features of the metabolic syndrome.

Obese individuals with psoriasis are more likely to have low vitamin D levels. Whether this is due to poor or restrictive diets,