When it comes to sun exposure’s impact on cancer risk, there’s a thin line between risk increase and risk reduction. As summer is just around the corner, now is a good time to talk to your patients about risk vs. reward when it comes to vitamin D.

While most people know that sun exposure increases the risk of skin cancers like melanoma, people may be less aware that sun exposure may help reduce the risk of other cancers, like breast cancer or colorectal cancer.

This discovery was first brought to light (no pun intended) 60 years ago and is still under research. Today, researchers continue to evaluate to what extent sunlight can reduce these risks and what mechanisms are responsible for the reductions.

In recent weeks, here’s how some understandings have changed.

Sunlight may reduce cancer risk, but more research is needed

The sun’s anti-cancer properties have historically been attributed to vitamin D, which people can absorb through sunlight. Studies have shown that people with higher blood levels of vitamin D have lower risks of cancer development.

Garland CF, Garland FC, Gorham ED, et al. The role of Vitamin D in Cancer Prevention. American Journal of Public Health. 2006;96(2):252-261. doi:10.2105/ajph.2004.045260

According to Sean Marchese, MS, RN, a registered nurse at The Mesothelioma Center with a background in oncology clinical trials, some of this risk reduction may be due to vitamin D’s ability to help maintain healthy cell growth and division, boosting immune system function and promote strong bones and muscles, but now, researchers are suggesting vitamin D isn’t the sun’s only cancer-fighter, nor are they entirely sure it deserves credit for this work.

An analysis looked at studies comparing cancer risk to sunlight exposure and vitamin D levels in people with breast cancer, colorectal cancer, prostate cancer, and non-Hodgkin’s lymphoma (NHL). It found that while high levels of vitamin D correlated to reduced risk of some cancers, this connection was not clear in colorectal cancer or breast cancer.

van der Rhee H, Coebergh JW, de Vries E. Is prevention of cancer by sun exposure more than just the effect of vitamin D? A systematic review of epidemiological studies. European Journal of Cancer. 2013;49(6):1422-1436. doi:10.1016/j.ejca.2012.11.001

In 2018, the National Cancer Institute (NCI) also announced that while correlations have been noted between vitamin D levels and reduced cancer risks, a cause and effect has not been established. NCI then warned patients that vitamin D supplements are not proven to reduce cancer risks.

National Cancer Institute. Vitamin D Supplements Don’t Reduce Cancer Incidence, Trial Shows. December 13, 2018.

The researchers also suggested that other sun-based mechanisms, like sunlight’s aid in modulation of the immune system modulation, circadian rhythm modulation, and folic acid degradation, could be partially responsible for reducing cancer risks, particularly for prostate cancer.

According to Marchese, “research has also suggested that sunlight exposure may reduce the risk of breast, colon, and prostate cancers by producing melatonin [which is] a hormone that responds to darkness and helps regulate the body's circadian rhythm.”

Marchese adds that sunlight exposure may likewise help the body fight off cancer cells (a result of boosting the immune system), offer antioxidant properties, and reduce inflammation.

“Chronic inflammation can increase cancer risk; sunlight exposure may help reduce inflammation,” he adds.

With sunlight’s risks and benefits toward cancer development, it may be helpful to talk to patients about how to safely spend time in the sun. Some tips suggested by Marchese include advising patients to:

• Be mindful of the time of day they are in the sun. It can be smart to avoid intense sun exposure from 10 a.m. to 4 p.m., as that is when the sun is the strongest, he says.

• Use sunscreen with a high SPF. This can protect the skin from UV rays and reduce the risks of some skin cancers while allowing the person to continue to spend time outdoors. 

• Wear a hat or long sleeves for extra skin-based sun protection.

• Take breaks in the shade. For people who aren’t ready to go back inside, taking breaks in the shade can give them a quick rest from risky sun rays.

• Schedule regular skin cancer screenings to detect any sun-based concerns.

• Discuss personal or family history risks of all cancers with a doctor.

• Discuss their time in the sun to help assess if it is helping or hurting them.