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 USDA Nutrition Center at Tufts University. Closer to the Equator, even less time is required.

That’s a rough estimate, though, and many factors can interfere with vitamin D synthesis. People with darker skin need two to three times the sun exposure to make the same amount of vitamin D, and synthesis also declines with age, Dr. Dawson-Hughes said. Environmental factors like clouds, ozone and air pollution can likewise decrease vitamin D production, she said.