Editor’s Note: Dr. Holly Andersen is dual board certified in Internal Medicine and Cardiology, is a Fellow of the American College of Cardiology and the American College of Sports Medicine and is an assistant professor of medicine at the Weill Cornell Medical Center. Dr. Andersen has been selected as one of America’s "Best Doctors" every year by Castle Connolly since 2001, and in 2008 was named by the Consumers’ Research Council of America as one of "America’s Top Cardiologists."

Vitamin D deficiency is an increasingly prevalent, largely unrecognized problem. A recent study estimates that vitamin D deficiency affects up to half of adults and apparently healthy children in the United States. The role of vitamin D in our body appears far greater than previously thought. In the past, we only focused on its relationship to our bones, but now we are finding that even mild deficiency can cause muscle weakness, impaired balance and depression, and low levels have been linked to heart disease, stroke and an impaired ability to fight the common cold and the flu.

Our diet has very little vitamin D. Only cod-liver oil, oily fish and fortified dairy products contain more than trace amounts, and many commercial brands of milk have less vitamin D than they say; some have zero! So our main source of vitamin D comes from sunlight, which stimulates our skin to produce it. Older skin and darker skin produce vitamin D less well, and vitamin D gets diluted by subcutaneous fat, so obese people are also at increased risk. People who live in northern latitudes and areas of increased air pollution also have a higher risk. But let’s face it, most of us work indoors and, thanks to our dermatologists, we all wear sunblock when we are outdoors.

Clearly, vitamin D supplementation helps prevent age-related bone loss. Vitamin D is essential for calcium absorption and is needed for supplemental calcium to be used efficiently and appropriately – so if you’re taking calcium with insufficient vitamin D, it may not be going where you want it to, but don’t worry — you’re probably not absorbing it in the first place. Vitamin D supplementation has also been shown to reduce your fracture risk by reducing “body sway,” a measurement of balance and/or muscle strength, which reduces your risk of falling. Generalized body aches and pains are also thought to be related to low vitamin D; studies have shown improvement in chronic back pain with supplementation. Even more exciting, vitamin D has been shown to improve athletic performance – the most convincing evidence for this comes from German and Russian literature in the ’70s– remember who won every Olympics then?

Vitamin D is also linked to heart health. Several large studies have shown that people with low vitamin D levels were twice as likely to have a heart attack, stroke or other heart-related event during follow-up, compared to those with higher levels. The lowest rates of heart disease are in the sun-drenched Mediterranean coast and in southern versus northern Europe. Heart-related deaths are highest in the winter. Other studies have shown that high blood pressure, diabetes and high triglycerides were significantly more common in patients with the lowest levels of vitamin D. Severe vitamin D deficiency is commonly seen in patients with heart failure.

Brain function has also been related to vitamin D levels. A recent study from the Journal of Geriatric Psychology and Neurology showed that, compared with those in the highest quarter for vitamin D levels, those in the lowest were 2.3 times more likely to be cognitively impaired, even after adjusting for age, sex, education and ethnicity. Recently, vitamin D has even been touted to have a protective role against ADA and multiple sclerosis. Vitamin D deficiency may also contribute to depression, particularly seasonal affective disorder (SAD) or winter depression, which occurs when vitamin D levels are at their lowest.