Dr. Kearns is a Medical Resident and Dr. Tanguricha is Associate Professor, the Division of Endocrinology, Metabolism and Lipids of the Department of Medicine at Emory University School of Medicine in Atlanta, Georgia.

Vitamin D was the fourth vitamin discovered, preceded by vitamins A, B and C. Though initially controversial, considered a miracle drug by some and a fad by others, it has received increasing attention. In the nearly 100 years since its discovery, beyond its importance in bone health, scientists have identified many more potential uses for vitamin D — in cardiovascular disease, pregnancy, infection and cancer.

Though some vitamin D2 and D3 can be acquired through diet, sun exposure remains the main source of the body’s vitamin D.

Nearly half of the healthy population of developed countries is deficient in vitamin D , skin age, color and sun exposure can all contribute to a lack of vitamin D synthesis. So can certain medications, obesity, and diet.

Research has focused on treating and preventing vitamin D deficiency through increased sun exposure, food fortification and taking vitamin D supplements.

The results of this research have, however, been variable, making it difficult to draw definite conclusions about the many potential roles of vitamin D in the body. But here is an outline of what we know — and what we still have to learn.

What We Mean When We Talk about Vitamin D

There are two major forms of vitamin D in humans: vitamin D3 and vitamin D2 . Though some vitamin D2 and D3 can be acquired through diet, the synthesis of vitamin D3 in the skin with sunlight exposure remains the main source of the body’s vitamin D . The liver converts the D2 and D3 into an active form, which circulates in the body for two-to-three weeks .

Nearly all young, healthy adults who spend less than 10 hours a week outdoors are likely vitamin D deficient or insufficient.

Vitamin D status is determined by measuring how much of this active form is in the blood. Most scientists believe the level should be greater than 30 ng/mL since this is ideal for calcium absorption and bone health . Nearly half of the healthy population , and even more of many diseased populations, are vitamin D deficient or at risk for deficiency .

Sources of Vitamin D Deficiency

Too Little Sun Exposure
The major cause of vitamin D deficiency is the inadequate synthesis of vitamin D in the skin. This is usually due to reduced sun exposure, whether from spending little time outdoors or using too much sun protection, is one reason for sub-optimal vitamin D synthesis . The lack of time in the sun affects both older patients in nursing homes or hospitals and younger people who work indoors. Nearly all young, healthy adults who spend less than 10 hours a week outdoors are likely vitamin D deficient or insufficient..

Because vitamin D is fat soluble, people who are overweight need more vitamin D to maintain adequate levels in their blood.

Seasonal variations in sun exposure also contribute to vitamin D insufficiency ); during winter, changes in the angle of the Earth to the sun cause less UVB radiation to reach the Earth’s surface and contribute to the well-documented decline in vitamin D status following winter months .

Aging Skin and Skin Pigment/Color
Characteristics of the skin, such as aging and skin pigmentation, can also reduce vitamin D synthesis and lead to insufficiency . People with darker skin also experience reduced skin vitamin D synthesis ( and are more likely to have vitamin D insufficiency compared to lighter-skinned individuals ( ). Between 50% and 75% of non-Hispanic blacks were vitamin D deficient in the winter, while only 8-33% of non-Hispanic whites were vitamin D deficient .

Breast milk is another culprit. The low level of vitamin D in breast milk (less than 25-78 IU/L depending on the mother’s vitamin D status) makes infants susceptible to vitamin D deficiency. The American Academy of Pediatrics (AAP) recommends that all infants receive 400 IU of vitamin D a day until they are drinking more about four cups of vitamin D-fortified formula or whole milk each day .

Vitamin D deficiency often continues into childhood. About 1 in 10 children have vitamin D deficiencies. Particularly at risk for deficiency are teen girls who are nonwhite and obese

Medical Conditions and Obesity Contribute to Vitamin D Deficiency
Certain diseases can also cause vitamin D deficiency. For example, since vitamin D is absorbed with dietary fat , diseases with poor fat absorption (such as celiac disease, short bowel syndrome, liver disease, cystic fibrosis [CF] and inflammatory bowel disease) can lead to vitamin D deficiency .

Vitamin D seems to boost innate immunity by increasing white blood cells' ability to kill bacteria.

Vitamin D deficiency is also common among those with damage to the organs that activate vitamin D, such as patients with end stage liver or chronic kidney disease (CKD) . Kidney disease causing loss of protein can also result in vitamin D deficiency .

Finally, obesity (a body mass index greater than 30) can contribute to vitamin D deficiency. Vitamin D is fat soluble, so people who are obese and deficient require more supplementation of vitamin D to achieve an equivalent increase, and need a higher vitamin D intake generally to maintain adequate levels of D in the blood. . Certain medications, such as anti-convulsants like phenobarbital and glucocorticoids, accelerate the breakdown of vitamin D, making it less available to the body.

Why You Need Sufficient Vitamin D and May Want to Take A Supplement

Bone Health
Vitamin D is important for good bone health because your ability to absorb calcium depends largely on vitamin D concentration . With low levels (below 20 ng/mL) of vitamin D only 10-15% of dietary calcium is absorbed .

Treating vitamin D deficiency can reduce your risk of hip and non-vertebral fractures , but it is possible to have too much of a good thing, however. High doses of vitamin D (300,000-600,000 IU of vitamin D2 and D3) have been found to increase fracture risk in seniors . Follow the directions from your physician or those on the label of any supplement you take.

Because vitamin D is fat soluble, People who are obese require more vitamin D to achieve an equivalent increase so higher vitamin D intake is required to maintain adequate levels of vitamin D.

Cancer and Infectious Disease
Being deficient in vitamin increases the risk of cancer in adults . More conclusive clinical trials are underway to establish the role of vitamin D in treating and preventing cancer, and so far, the results are mixed and vary with the type of cancer being studied. .

Vitamin D may also be helpful in reducing the effects of infectious disease, in particular respiratory illness. It seems to boost innate immunity by increasing white blood cells' ability to kill bacteria and reducing inflammation .

Respiratory and Heart Problems
As mentioned above, studies have consistently linked vitamin D deficiency to respiratory tract infections. Taking vitamin D supplements appears to improve cardiovascular disease (CVD) markers, including glucose intolerance , insulin sensitivity and high blood pressure .

Overall Mortality
Vitamin D supplementation also appears to increase life expectancy, but the results have been mixed. . Concentrations between 20-36 ng/mL were associated with the lowest risk of mortality and morbidity. But at the other end of the spectrum, having concentrations greater than 36ng/mL caused the risk of mortality to rise. .

Fetal Development
Vitamin D deficiency may be particularly risky when a woman is pregnant, and is one reason why women should take multivitamin supplements during pregnancy. Women who are vitamin D deficient at 18 weeks of pregnancy are likely to have fetuses showing impaired lung development, neurocognitive difficulties, increased risk of eating disorders, lower peak bone mass . Vitamin D status is also important to mothers during pregnancy, as low levels of vitamin D are associated with higher rates of gestational diabetes and pre-eclampsia .

In our second and final installment, we will cover how you can be sure you are getting enough vitamin D and what to do if you aren't.