Despite increasing scientific scrutiny of the benefits of vitamin D, there is still mystery and confusion about this essential nutrient. Part of the complexity is due to the fact that vitamin D is not simply a nutrient. It functions as a hormone and affects virtually every cell.

Because the skin manufactures vitamin D only after exposure to sunlight, circulating levels often drop during winter months. That’s why grandmothers used to dose their families with cod-liver oil rich in vitamin D as a winter tonic.

Science is still trying to figure out whether the grandmothers were right. As so often happens, the studies are inconclusive. Some controlled studies show that children given vitamin D supplements have fewer colds and other respiratory infections (Pediatrics, September 2012). A recent study of adults examined whether 1,000 IU daily of vitamin D prevented upper-respiratory-tract infections better than placebo and found it did not (Clinical Infectious Diseases, Nov. 15, 2013). Severe illness was less common among those taking vitamin D, but overall there was no protection against colds and flu-like illnesses.

People who have higher levels of vitamin D circulating in their bloodstreams are less likely to experience other sorts of infections, however. One recent study found that overweight individuals undergoing gastric-bypass surgery were three times more likely to suffer an infection following the surgery if their levels of vitamin D (25-hydroxyvitamin D) were below 30 ng/ml (JAMA Surgery online, Nov. 27, 2013). This is a significant concern, because obese people are much more likely to have low circulating levels of the vitamin (Journal of Epidemiology and Community Health online, Nov. 6, 2013).

An epidemiological study found that Americans with 25-hydroxyvitamin D blood levels below 30 ng/ml had a 56 percent greater likelihood of coming down with pneumonia (PLoS One, Nov. 15, 2013). Low levels of this vitamin also have been linked to inflammation (including arthritis), cardiovascular disease, high cholesterol, diabetes, weight gain, infections, multiple sclerosis, depression or anxiety, cognitive decline and poor physical functioning (The Lancet Diabetes and Endocrinology online, Dec. 6, 2013).

In addition, adequate levels of vitamin D are associated with a reduced risk of falls and broken bones, and with better hand and leg strength (Journal of the American Geriatric Society, November 2013).

We have more details on the health benefits of vitamin D in our Guide to Vitamin D Deficiency. In it, we describe how to interpret tests of 25-hydroxyvitamin D as well as how to increase blood levels with supplements. Anyone who would like a copy, please send $3 in check or money order with a long (No. 10), stamped (66 cents), self-addressed envelope to: Graedons’ People’s Pharmacy, No. D-23, P.O. Box 52027, Durham, NC 27717-2027. It also can be downloaded for $2 from our website:

We will have to wait for better studies to determine the best regimen for supplementing vitamin D for prevention of cognitive decline as well as heart disease. In the meantime, though, it makes sense to prevent winter deficiency of this critical vitamin.