The Sunshine Vitamin in Winter

During the winter, it’s much harder to get enough UVB to make vitamin D.

Most people can make enough vitamin D from sun exposure during the summer months, but for many, synthesis can be inadequate in the winter. A 2016 study led by researchers at the CDC estimated that up to 20 percent of Americans have inadequate vitamin D levels.

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.  However, 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.

What is vitamin D?

The term “vitamin D” refers to several different forms of this vitamin. Two forms are important in humans: vitamin D-2, which is made by plants, and vitamin D-3, which is made by human skin when exposed to sunlight. Foods may be fortified with vitamin D-2 or D-3.

The major role of vitamin D is to maintain normal blood levels of calcium and phosphorus. Vitamin D helps the body absorb calcium, which forms and maintains strong bones. It is used alone or together with calcium to improve bone health and decrease fractures. Vitamin D may also protect against osteoporosis, high blood pressure, cancer, and other diseases.

Vitamin D is found in many foods, including fish, eggs, fortified milk, and cod liver oil. The sun also adds to the body’s daily production of vitamin D, and as little as 10 minutes of exposure is thought to prevent deficiency.

What are the symptoms of vitamin D deficiency?

The symptoms of vitamin D deficiency are often vague, but can include tiredness and general aches and pains.  Part of the difficulty in detecting D deficiency is that most people won’t have any symptoms at all.  Severe vitamin D deficiency might cause pain in your bones and weakness, or even frequent infections.  If you suspect you might be deficient in D, see your physician, or have a blood test to check your vitamin D levels.

How can you increase your vitamin D levels?

Even though the symptoms of vitamin D deficiency are subtle, there are many ways to prevent or counteract it.

  1. Sunlight encourages the body to make vitamin D. But because of the skin-cancer risk, many people are using sunscreens to block the harmful UV rays. However, a small amount of sun exposure without sunscreen can do the trick. 

”If you’re going to get it from the sun, about 20 to 25 minutes of exposure is helpful,” says Stephen Honig, MD, director of the Osteoporosis Center at the Hospital for Joint Diseases, in New York City.
  2. Fortified Milk can be a good source of vitamin D. Almost all types of cow’s milk in the U.S. are fortified with vitamin D, but ice cream and cheese are not. In general, an 8-ounce glass of milk contains at least 100 IUs of vitamin D, and a 6-ounce serving of yogurt contains 80 IUs, but the amount can be higher (or lower) depending on how much is added. Some soy and rice milks are fortified with the same amount, but check the label since not all contain vitamin D.
  3. Supplements are an excellent choice for maintaining healthy levels of vitamin D. But be careful, not all supplements are the same.  Look for a minimum of 800 IU per serving as well as one that includes the recommended D-3.

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