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United States Department of Agriculture

Agricultural Research Service

Winter's Here - Are You Getting Your Vitamin D?
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Janet R. Hunt

Besides a warm coat, gloves and boots, winter-weight oil for the car, and a clean furnace filter, here’s another possible addition to your winter preparation checklist: consider whether you are getting enough vitamin D. Vitamin D is essential for maintaining healthy levels of body calcium and phosphorous, the primary bone minerals.

Although our bodies can produce vitamin D when exposed to the sun, this ability diminishes with age. The elderly may need to rely on dietary sources of vitamin D, which are limited. Foods that naturally contain vitamin D include some fish liver oils, and the flesh of relatively fatty fish such as salmon or herring. In the early 1900s, vitamin D deficiency was found to result in rickets, a debilitating disease that resulted in bowing of weight-bearing bones in growing children. Health officials in the United States and Canada decided to fortify milk with vitamin D. Small amounts of vitamin D are also added to butter, margarine, and some fortified cereals. The amounts added to foods were chosen to provide adequate but not excessive amounts. Too much vitamin D can lead to calcium deposits in tissues other than bone, and this can be as dangerous as too little Vitamin D.

By contrast, our skin controls the amount of vitamin D it generates from exposure to sunlight so that we don’t make too much. Unfortunately, it can’t prevent the effects of too little sun, a problem of special importance in northern latitudes. Not only do we cover-up to keep warm during the winter months, but the angle of the sun is too low to generate much vitamin D.

Scientists estimate that above latitudes of approximately 40 degrees, the skin can’t synthesize vitamin D for most of the winter months-- especially affecting us here in the Red River Valley of the North. Fortunately, the body can store some of the vitamin D the skin produces during the warmer months for use during the winter, but sometimes this is not enough without additional dietary sources.

For people who get little exposure to sunlight, the National Academy of Sciences suggests dietary intakes of 200 I.U. for those under 50; 400 I.U. for those between 51 and 70; and 600 I.U. for those over 70 years of age. The recommendations increase with age because the skin becomes less efficient at producing vitamin D.

Fortified milk contains 400 I.U. per quart, or 100 I.U. 8 ounce glass. So vitamin D is not a problem for those of us who enjoy drinking milk. If you don’t drink milk, even if you are meeting calcium recommendations with yogurt, cheese, or calcium-fortified juices, you may need to consider another source of vitamin D. A daily multivitamin may be a more pleasant choice than your grandmother’s cod liver oil.

The National Academy of Sciences also now recommends upper limits to help people avoid nutritional excess and toxicity. Adults should not exceed 2,000 I.U. of vitamin D daily from all sources. If you take several forms of nutrient supplements, for example, you should not to exceed this amount from supplements plus milk. Fortunately, you don’t need to worry about getting too much vitamin D from sun exposure when skiing or sledding!

Last Modified: 10/23/2006
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