Elyssa - About.com User
The amount of exposure also depends on the time of the year. In the northern hemisphere, the UVB is more intense during the summer months and less intense during the winter months. In fact, if you live north of the 42-degrees latitude, you will have a difficult time getting enough vitamin D from the sun from November through February. If you live north of a line drawn on a map from the northern border of California to Boston, Massachusetts, you will probably need additional vitamin D from the foods you eat during the winter.
The intensity of UVB rays is also reduced by clouds, pollution and UVB will not travel through glass, so sitting next to a window will not give you enough sunlight to make vitamin D.
Vitamin D is found in oily fish like tuna and salmon as well as in fortified foods like milk and breakfast cereals. You can also take vitamin D supplements.
Vitamin D is necessary for absorption and utilization of calcium, so you need adequate amounts of vitamin D for healthy bones. A deficiency of vitamin D in children will cause rickets in children and osteomalacia in adults. Research studies also suggest that getting enough vitamin D may help to prevent high blood pressure, multiple sclerosis and some forms of cancer.
Source:
National Institutes of Health Office of Dietary Supplements. "Dietary Supplement Fact Sheet: Vitamin D." Updated May 2008.
