Vitamin D is a fat-soluble vitamin necessary for human health. There are very few foods with naturally occurring vitamin D, so most dietary sources are fortified. Most Americans get their dietary vitamin D from fortified milk, orange juice, or cereal, or by eating eggs laid by hens who have been fed foods with vitamin D.
The body also manufactures vitamin D when bare skin is exposed to sunlight. Though even short periods of direct exposure to sunlight may be sufficient to maintain minimum levels, many people do not get enough sunlight, and need dietary sources or vitamin supplements. The same steps taken to reduce the risks associated with sunlight exposure -- sunblock and protective clothing -- also drastically reduce the amount of vitamin D produced in the body.
Some groups are at particular risk of low vitamin D levels: older adults, infants who are breastfeeding, people who are obese, people with darker skin, and anyone who suffers from a condition which reduces their ability to absorb dietary fat, such as Crohn's or celiac disease may need to consider supplements.
Vitamin D and Osteoporosis
According to the National Institutes of Health, recent studies indicate that both calcium and vitamin D supplements may help to prevent the fractures, falls, and bone loss associated with osteoporosis. However, since most supplements contain both vitamin D and calcium, it is difficult to determine the effects of each individual nutrient. Other correlations also suggest that osteoporosis responds to a variety of factors, including both diet and exercise.
A lack of vitamin D does cause other serious bone problems, such as Rickets -- a childhood disease in which the bones soften and lead to fractures and deformities -- and osteomalacia -- the same softening of the bones in adults.
Vitamin D and Other Health Issues
There is some, tenuous evidence that adequate levels of vitamin D may help to prevent a wide variety of other conditions, including type I and type II diabetes, multiple sclerosis, and hypertension, and may be correlated with lower overall mortality rates. Since it is nearly impossibly to overdose on vitamin D except through the careless use of dietary supplements, taking steps to ensure adequate vitamin D (5 mg or 200 IU per day for children and most adults, and 10-15 mg or 400-600IU per day for older adults) is a good idea.
Vitamin D and Depression
Though further research is needed, it is possible that vitamin D deficiencies may contribute to mild depression. Indeed, reduced vitamin D production may be one factor in Seasonal Affective Disorder (SAD): a mood disorder that affect sufferers only during the winter months.
Because sunlight on exposed skin is one means by which the human body manufactures vitamin D, a correlation between low levels and a mildly depressed mood is perhaps not surprising. It may be that the lack of sunlight in winter is the underlying cause of both, though at least one study does indicate that increasing vitamin D through supplements may help those suffering from SAD; further research is necessary to determine if vitamin D can help with other forms of depression. Those suffering from mood disorders can certainly have their vitamin D levels checked as a precaution.
