Sunshine, northern climates, and vitamin D
Sand or snow? People who live far enough north for the white of this scene ever to be snow are likely to be sun-deprived, even if their winter is sunny. Some people need sun to improve their disposition. But there is another reason. The sun is responsible for much of the vitamin D circulating in human tissues. Vitamin D helps brains to function and keeps bones strong. It helps to keep calcium levels normal, which helps to regulate blood pressure and heart rhythms. Winter is a great time to re-evaluate our vitamin D status. Some people’s clinics do vitamin D tests to check blood levels, other people just try to get more D in their diets and hope for the best.
The role of vitamin D is an evolving area of nutrition: scientists are learning more about what vitamin D does, what an adequate blood level is, and how to ensure we are consuming adequate (but not toxic) levels. For example, a study reported in July of 2010 in the Archives of Internal Medicine was able to predict rates of cognitive decline by looking at the blood levels of vitamin D in older adults. Lower levels, more decline. Levels selected to be adequate by that study were 25 nanograms per milliliter, but 30 is the level currently suggested as adequate by many vitamin D researchers who are finding better health at higher blood vitamin D levels. What’s normal depends on your doctor’s interpretations and your clinic protocols. Population studies have suggested that levels below 25 are common, especially after the winter months in northern states. Older adults are particularly vulnerable. A report available from International Osteoporosis Foundation called Three Steps to Unbreakable Bones provides useful information about Vitamin D levels in blood and diet for individuals at various ages.
In central Minnesota where I live, the winter sun is not strong enough to synthesize vitamin D from November to February, even when it is nice enough outside that I can expose bare skin to the sun. In fact, the sun’s rays aren’t strong enough to synthesize vitamin D anywhere north of 42 degrees latitude (approximately the locations of Boston and Chicago) during November, December, January, and February. If I want year-round synthesis, I could move to Puerto Rico.
So should you go to a tanning booth? No, those have been clearly linked to cancer. But fall and winter might be a good time to look at the vitamin D in your diet. Few food sources provide vitamin D; fortified milk is one of them, at the rate of approximately 100 International Units per 8 oz of fluid milk. Yogurt may or may not be fortified. And yes, our bodies do store vitamin D, so we don’t run out in a day or even a week or two if our diet is lacking D. Levels don’t drop overnight.
However, since it’s hard to get enough D in food, and the sun up north is not strong enough at this time of the year, many people rely on a supplement to prevent muscles and bones from weakening and to keep cognitive functioning intact. Currently, adult intakes of 800 to 1000 IUs per day seem to be related to the best health outcomes. This is a somewhat higher level than the RDA, but well below the Tolerable Upper Intake Level, the level considered to be potentially dangerous. The potential for vitamin D toxicity is anything above 4000 IU per day. Higher levels are prescribed for some illnesses; these medicinal doses need to be worked out with a doctor who can also monitor blood calcium levels.
Birds and snowbirds head south when the days get shorter, but if you aren’t one of them, sufficient vitamin D can help you to stay healthy enough to play in the snow.