The best source of natural vitamin D is sunlight. Just 10 to 15 minutes of exposure without sunscreen a couple of times a week usually gives you enough vitamin D. Small amounts of vitamin D is also naturally found in butter, eggs, and fish liver oils. .

Vitamin D is often added to fortified foods, too, such as milk and cereal. Reseachers say a growing body of evidence suggests that vitamin D deficiency increases the risk of heart disease and is linked to other issues such as high blood pressure, obesity and diabetes.  

Researchers are discovering an increasing number of links between the immune, nervous and endocrine systems. Hormones of the endocrine system, such as vitamin D, may help the immune and nervous systems defend the body.

People with limited sun exposure are most likely to be vitamin D deficient.