by Sarah Olliges
On a warm sunny Saturday in early November, I was pleased to find harvesting vegetables on a friend’s organic farm. Though originally dressed in warm layers appropriate for a New England fall day, I happily stripped down to my t-shirt to take in some unexpected sun. Of course, being a nutrition student, I was excited for this unexpected opportunity to make some Vitamin D, a rare commodity in the winter.
I had learned that we make vitamin D in our skin when it is exposed to sun, but thought that because we cover up during the long winters in New England, it is hard to get enough. Vitamin D, a fat soluble vitamin, is essential for bone health, immunity and mood. Great- on this fall day I was doing my body a double favor, storing up on both veggies and vitamin D.
So imagine my disappointment that evening, when I got online to determine just how much vitamin D I had made while harvesting my veggies, and discovered that above a certain latitude (such as in Boston) during the winter months (mid-October to mid-March), the sun is at such a low angle in the sky that the atmosphere filters out the rays that help make vitamin D. I had made exactly zero vitamin D that day! I was left wondering, what is so great about vitamin D and how am I going to get enough this winter?
What does vitamin D do?
Vitamin D, also known as choleciferol, is a prohormone produced in the body with the help of UVB rays from the sun. Vitamin D plays a critical role in the absorption of calcium in the small intestine and in the regulation of calcium in the blood. According to a recent article in the Tufts Nutrition News, vitamin D plays other important roles in the body. Though the evidenc is still emerging, in addition to mediating immunity and mood, vitamin D deficiency may be linked to heart disease and cancer.
Where do I get vitamin D?
Sunlight is the best “source” of vitamin D. Our skin uses UVB rays to make vitamin D from cholesterol. The UVB rays from the sun are most available in the middle hours of the day during the summer months. It takes anywhere from 5-60 minutes to get adequate UVB exposure, depending on skin tone (darker takes longer) and the UV Index (the UV index is generally given with the weather report). If the UV index is under three, there are no UVB rays. The higher the UV index gets above three, the less sunlight you need to make enough vitamin D.
It sounds simple enough, but the problem for New England dwellers is that in the northern latitudes, the sun’s angle in the sky prevents UVB rays from penetrating the atmosphere at all in the winter, and the UV Index is perpetually below three. Even at latitudes farther south, because of winter clothing and shorter days, people are still at risk of not getting enough sun exposure to make adequate amounts of vitamin D. With mounting evidence suggesting that vitamin D plays a critical role in many processes in the body (including warding off the flu!) it is advisable to seek out alternative sources during the winter.
Unfortunately, there are not many natural food sources of vitamin D. Animal foods contain small amounts and some foods such as cereals and milk, may be fortified. Fatty fish such as cod or tuna are also sources of vitamin D. Because the sources are few and far between, a supplement may be the best way to obtain vitamin D in the winter.
What about artificial light sources?
Tanning beds use Ultra Violet (UV) rays, but many only use UVA rays- the one responsible for tanning. UVB rays are needed to make vitamin D in the skin. Even if a tanning bed does use UVB rays, one must also beware of the harmful effect of both the sun’s rays and the rays from the tanning bed (UVA and UVB rays promote skin cancer).
There are many full spectrum light bulbs on the market that claim to help improve mood. Some even claim to promote vitamin D production; however, unless bulbs are emitting UVB rays, they do not increase vitamin D production. Though one can buy a UVB producing light, these are generally sold as “grow lights” or “reptile lights.” You may be better off with a supplement in the winter, and it is probably safer than exposing yourself to artificial sources of UVA and UVB light.
How much vitamin D do I need?
There is some controversy over how much vitamin D is needed in order to maintain optimum blood levels. The Adequate Intake (AI) is currently 200 IUs (about 5 micrograms) per day for people aged 50 or younger, and 400 IUs for people above age 50. However, these amounts are currently under review due to recent research, and the American Pediatric Association has already doubled its recommendation for children to 400 IUs.
Additionally, the safe upper limit is currently set at 2000 IUs, but some research suggests that up to 10,000 IUs per day is safe. For more detailed information, see The Possible Adventures of Super D by Helene Ragovine in the Fall 2008 Tufts Nutrition Letter. In the meantime, unless you have a trip to a southern beach planned, make sure to get adequate vitamin D, as it may help ward of the winter blues and the winter flues.