According to my calendar, winter began just a few days ago. But as far as my ability to make vitamin D is concerned, winter actually began in October and will last until the middle of March. If I run short of vitamin D before March, I have three options for getting more vitamin D: take a tropical vacation, go to a tanning salon, or take vitamin D pills.
Vitamin D isn’t really a vitamin. It’s a hormone that is made when the ultraviolet light from sunlight hits your skin. Some of the sun’s ultraviolet light gets filtered out by the atmosphere, especially by the ozone layer. Where I live, the sunlight is at such a low angle from October through March that practically all of the ultraviolet light gets filtered out. Thus, we have a tanning index of zero even if there isn’t a cloud in the sky.
A light-skinned person in Boston can get enough vitamin D from getting only a few minutes’ worth of sun exposure on his or her face, arms, and hands at midday two to three times a week during the spring, summer, and fall. A person of African ancestry might need ten times as much sun exposure to make the same amount of vitamin D.
Natural summer sunshine is the best way to get vitamin D. Sunshine may have other important effects on the body besides producing vitamin D. Of course, too much sun exposure can cause skin damage and increase the risk of skin cancer.
Sunlamps or a tanning bed can also help restore normal vitamin D levels in the wintertime, especially in people who have an intestinal disease that makes it hard for them to absorb fat-soluble vitamins from their food. Tanning beds should be used cautiously because the ultraviolet light they produce is so intense.
You can also buy vitamin D supplements, but one nutrition expert warns that vitamin pills should be used as a last resort. Although low vitamin D levels have been associated with various diseases, such as multiple sclerosis, treatment with vitamin D supplements has not necessarily been shown to be useful in treating those conditions.
The only reliable natural food source of vitamin D is cold-water fish, such as cod-liver oil. However, fish contains too much protein and fat and is often contaminated by environmental toxins. Plus, our fisheries are in a state of collapse.
There are two forms of vitamin D. Vitamin D3 is made from cholesterol, which is found only in animals, never in plants. Vitamin D2 is made from a similar sterol that is found in fungi, such as mushrooms and yeast. You could get some vitamin D from eating mushrooms that have been exposed to ultraviolet light.
Cow’s milk is not naturally a reliable source of vitamin D. The vitamin D in cow’s milk is a supplement that has been added at the dairy. It comes from yeast that has been exposed to ultraviolet light.