Q: Can I get enough vitamin D from sunlight, even if I live in a place with little sunlight during the winter?
A: Most people can make enough vitamin D from sun exposure during the summer, but for many, synthesis can be inadequate in the winter. A 2016 study led by researchers at the Centers for Disease Control and Prevention estimated that 14 to 18 percent of Americans have inadequate vitamin D, although what constitutes a healthy blood level is subject to debate.
Synthesis of vitamin D, which is critical for bone health, requires exposure to ultraviolet-B rays from the sun. In the summer, when the sun is directly overhead, vitamin D synthesis can be very efficient. For someone with light skin in a temperate climate at midday, “10 minutes a day of exposure to 10 percent of your body’s surface area, such as your arms and face, will give you what you need,” said Dr. Bess Dawson-Hughes, director of the Bone Metabolism Lab at the U.S. Department of Agriculture Nutrition Center at Tufts University. Closer to the equator, even less time is required.
While a little sun exposure can be good for vitamin D status, too much can increase the risk of skin cancer, cautioned Dr. Jean Y. Tang, associate professor of dermatology at Stanford University. “You’ve got to do it in a very safe way, where you get enough sun to make the vitamin D, but not so much sun that you cause erythema, or redness, and a sunburn,” she said.
While using sunscreen, a hat and sun protective clothing can inhibit vitamin D synthesis, “for the typical healthy person, your sun-protective behaviors do not lower your vitamin D levels very much,” she said, because most people just aren’t vigilant enough to block all UVB exposure all the time.
During the winter, it’s much harder to get enough UVB to make vitamin D, because people bundle up in cold weather and the sun stays lower in the sky. Several studies have concluded that inadequate UVB limits vitamin D synthesis for at least a few months during the winter above about 35 degrees latitude — which runs through the southern borders of Tennessee and Nevada; Boston is at 42 degrees latitude.
However, a person’s vitamin D supply doesn’t plummet immediately in the winter. “One thing to keep in mind is that our bodies can store vitamin D in the liver and fat tissues, so there are ample opportunities to make vitamin D in spring, summer and fall,” said Carol Haggans, a registered dietitian and spokeswoman for the National Institutes of Health Office of Dietary Supplements.
Still, studies consistently show that vitamin D status drops during the winter, with levels peaking in September and at their lowest in March.
So for some people for at least part of the year, it may be important to rely on dietary sources of vitamin D or supplements. The Institute of Medicine recommends that healthy adults consume 600 International Units of vitamin D per day, increasing to 800 IU for people older than 70, assuming minimal sun exposure. A 3-ounce serving of salmon provides about 450 IU, and 1 cup of vitamin D-fortified cow’s milk has about 120 IU. Fortified plant-based milks, orange juice, yogurt, margarine and breakfast cereals are also available.
“Some people do have trouble getting enough vitamin D from their diet,” Haggans said. To determine if you need a supplement, and in what dose, she recommends talking with your doctor.