Exposing skin to sunlight is a convenient way to meet our vitamin D needs, which is crucial for bone health. That might make you wonder: Does wearing sunscreen interfere with vitamin D levels and potentially cause vitamin D deficiency?
The short answer is yes. But before you toss your SPF 30, dermatologists say the relationship between sunscreen and vitamin D is much more nuanced than that.
According to Dr. Henry W. Lim, chair emeritus of the department of dermatology at the Henry Ford Health System and former president of the American Academy of Dermatology, if sunscreen is supplied in a thick layer it "can effectively block sunlight and cause a lack of vitamin D synthesis in skin," Lim said. A thick layer is defined as roughly equivalent to about 1 ounce, or the size of one golf ball, for the full body, he said.
"However, in the real world ... most people apply less than this amount," Lim added. In other words, "the 'in use' SPF is actually lower than the labeled SPF."
So, you may not have to worry about vitamin D deficiency if you're not putting enough sunscreen on in the first place. In one Australian study, adequate vitamin D levels were maintained over the course of a summer whether individuals used a broad spectrum sunscreen or a placebo cream. Researchers attributed the results in part to "the lack of total skin cover at all times."
Those who do apply sunscreen generously, and religiously wear hats and SPF-protected clothing when exposed to sunlight -- something Lim recommends in order to reduce the risk of skin cancer -- will have a higher probability of vitamin D insufficiency, Lim explained.
"For the vast majority of the public, there should be no concern with vitamin D levels, but for those who practice rigorous protection, namely, wearing photoprotective clothing, hats, staying in the shade when outdoors, and applying sunscreen -- they do have a higher risk," Lim said.
Vitamin D deficiency versus skin cancer risk
Vitamin D is synthesized in skin when it's exposed to UVB wavelengths from the sun.
So if 15% to 20% of one's body surface is exposed to sunlight, unprotected, for 15 to 20 minutes, two to three times per week, one could achieve adequate vitamin D levels. But even with low levels of sunlight, cumulative sun damage can occur over the long-term, Lim explained.
"When the skin tans, there is DNA damage, and so with repeated low-dose exposure, the DNA damage could accumulate and potentially increase the risk of skin cancer and photoaging," Lim said. Squamous cell and basal cell carcinoma are the most common skin cancers in humans, and are correlated with the degree of one's sun exposure, while melanoma, the deadliest form of skin cancer is often the result of more intermittent, intense sun exposure, Lim explained.
"Patients need to use sun protection to minimize their risk for skin cancer, and yes it does impede the body's ability to make vitamin D. I tell patients if we found out cigarette smoking increased vitamin D levels, we wouldn't tell patients to smoke," said Dr. Patricia Farris, a dermatologist and fellow of the American Academy of Dermatology. "In other words, you can't recommend exposing patients to a known carcinogen (UV light) just to raise their vitamin D levels."
How to have your D and eat it too
To reduce the risk of skin cancer and skin aging, dermatologists recommend practicing sensible photoprotection. That means wearing wide-brimmed hats, sunglasses, and sun protective clothing. Photoprotective clothing, which is labeled "UPF," is tightly weaved, and so "the amount of UV radiation that would penetrate is quite low," Lim said.
A broad spectrum sunscreen with an SPF of 30 or higher, which protects against harmful UVA and UVB rays, should be generously applied to all exposed areas of the body when one is outside for a prolonged period of time, in order to minimize the risk of sunburn and skin cancer, Lim explained. (A sunscreen's SPF, or sun protection factor, is a relative measure that indicates the degree of sunburn protection that a sunscreen provides. For example, when using a SPF 30 sunscreen, it would take 30 times the UV dose to produce redness compared to skin without sunscreen.)
While individuals with darker skin have a lower risk of skin cancer from sunlight, they should still follow these guidelines, according to Lim. The American Academy of Dermatology is in the process of developing photoprotection guidelines for individuals of different skin types, according to Lim.
As far as getting enough vitamin D, Lim recommends taking a daily multivitamin, which typically contains an amount of vitamin D that would provide adequate blood levels of the vitamin. Optimizing vitamin D levels are important for bone health, and can help to minimize the thinning of bone and risk of fractures. The National Institute of Medicine's Recommended Dietary Allowance for vitamin D, which is based on a person receiving minimal or no sun exposure, is:
"To achieve adequate vitamin D levels, it is best to take a multivitamin each day rather than relying on sun exposure. The former is simple and inexpensive, and at the same time prevents the development of skin cancer and skin aging," Lim said.
Vitamin D supplements can also help, and are very effective for those who have low vitamin D levels, Farris added.
In addition to vitamin D supplements, the American Academy of Dermatology recommends getting vitamin D from a healthy diet that includes foods naturally rich in vitamin D, such as salmon, tuna, sardines, cod liver oil, and egg yolks, or foods and beverages fortified with vitamin D, such as cereals, yogurt, milk and orange juice.