Why shorter days call for a dose of vitamin D

It is officially winter in our household because I have pulled out the vitamin D supplements. There are good reasons to take vitamin D in the winter, even though we should get all of our other nutrients from whole foods.

Our bodies naturally derive vitamin D from two main sources: sunlight and food. In the winter, there is no way most people get enough sunlight on bare skin. The sun is low, the days are short, long sleeves and gloves prevail and almost everyone in the winter spends a majority of the day inside.

Also, most people do not eat enough of the foods that are naturally high in vitamin D: fatty fish such as mackerel, herring and salmon, and cod liver oil. Although many brands of milk and orange juice are fortified with vitamin D, it is still almost impossible to derive enough of it solely from food.

Why is vitamin D important?

Vitamin D is not actually a vitamin but rather a group of hormones. They help the body absorb nutrients such as calcium, iron, magnesium and zinc. This is why vitamin D is added to calcium-rich milk. Studies show that only 10 to 15 percent of calcium in food is absorbed without vitamin D.

Vitamin D can help boost your immune system. If you would like strong, healthy bones, vitamin D is king. If you don’t want your heart to putter out at an early age or your mind to deteriorate, look to vitamin D. It is also shown to prevent cancer by regulating cellular growth.

The current recommended daily allowance for individuals ages 1 to 70 is 600 IU, or international units, but more recent research at the Boston University School of Medicine recommends up to 2,000 IU. Other studies recommend even higher levels for optimal health. The confusion around the ideal daily allowance prompted a 2010 large-scale study at a Boston affiliate of Harvard University to investigate whether vitamin D can help prevent cancer, heart disease, stroke and other chronic conditions in more than 25,000 American men and women. Stay tuned — the study is expected to wrap up later this year.

Because one glass of milk provides just 100 IU of vitamin D, a piece of salmon offers 360 IU and an egg yolk under 50 IU, even the lowest recommendation of 600 IU a day is hard for most children to attain without regular sun exposure.

Seidenberg is co-founder of Nourish Schools, a Washington, D.C.-based nutrition education company, and co-author of “Super Food Cards,” a collection of healthful recipes and advice. Consult your doctor as to whether a supplement is right for your child. Individual needs differ based on how much time people spend outside, where they live, their skin color, the foods they eat and their use of sunscreen.