As the days get shorter, we tend to spend more time indoors. Festive though they are, candles, holiday lights, and fireplaces can’t make up for the sunlight we lack this time of year. And the sun’s cheerful light and warmth aren’t the only things we miss through the winter: our bodies need sunlight to help produce vitamin D.
Vitamin D is critical for the healthy growth and maintenance of our bones throughout life. In addition, many of our cells contain vitamin D receptors. Low levels of vitamin D in the blood appear to be linked to hypertension and heart disease, some cancers, impaired immune function, and inflammation.
Vitamin D is not present in many foods, so our needs must be met by supplementation (vitamins) and from the form of vitamin D our skin makes in the presence of UV radiation. Yes, the same UV radiation we try to avoid in summer by slathering ourselves in sunscreen. But in winter, the sun is too weak north of latitude 37°—roughly a horizontal line between San Francisco and Richmond—to help our skin produce the vitamin. This means that for up to six months a year, most of the people living in the United States cannot manufacture vitamin D in our bodies.
Numerous studies have found that we’re getting insufficient vitamin D during the winter months. According to the studies, young children and the elderly are particularly at risk.
In 2010, the Institute of Medicine raised the amount of vitamin D recommended in its Dietary Reference Intakes. Adults, up to age 70, went from from 400 international units (IU) to 600 IU daily. For those older than 70, the recommendation doubled, from 400 IU per day to 800 IU.
In 2008, the American Academy of Pediatrics (AAP) increased its recommended daily intake of vitamin D in infants, children, and adolescents to 400 IU—only to have the IOM raised it in 2010 to 600 IU.
More D, Please
So how can you increase your vitamin D intake, especially during the darkest winter months, from November to February?
Fish: the only good food source is fatty fish, such as salmon or mackerel. A serving of salmon (about 400 IU) or mackerel (about 350 IU) is a good food source of vitamin D as well as essential fatty acids. Cod liver oil, taken as a supplement, is also particularly high in vitamin D (1,360 IU in one tablespoon).
Fortified Food: vitamin D can also be consumed in fortified foods and in vitamin form. There are two types of vitamin D—D2 and D3. Vitamin D2 is synthesized from plant sources. Vitamin D3 is from animal sources. D3 is the more potent and absorbable form of the vitamin, but likely not suitable for vegans. Milk was first fortified with vitamin D in the 1930s to prevent rickets, a bone deformity in children with low vitamin D levels. Today, a glass of fortified milk (about 100 IU) provides nearly 30 percent of the daily recommended amount, but unfortunately most other dairy products are not similarly fortified. Many cereals are, however, but make sure to check that they are fortified with vitamin D3.
Supplements & Vitamins: Look for supplements and fortified foods containing Vitamin D3. Cod liver oil can be a good choice, but make sure that the oil has been purified to remove toxins.
To read more on vitamin D, see previous FruitGuys Almanac stories, “Vitamin D – What’s All the Fuss” and “Vitamin D: How Much More? New Study Raises Dosage Questions.”
Always discuss diet changes, including vitamin D intake, with your healthcare specialist.
Rebecca Taggart is a San Francisco-based writer, teacher, and yoga instructor.