Vitamin D, also called the "sunshine vitamin," has a well-known and critical role in maintenance of healthy bones. Severe deficiencies in vitamin D have long been known to cause defects in bone growth and repair, such as rickets in children and osteomalacia in adults. Recent research has highlighted the effects of vitamin D in tissues other than bones, suggesting a broader role for vitamin D in providing protection against diabetes, hypertension, cancer and some autoimmune diseases.
How It Works
There are two forms of vitamin D: vitamin D-2, or ergocalciferol, which is found in some plant sources; and vitamin D-3, or cholecalciferol, which is made in skin upon exposure to ultraviolet-B rays in sunlight and is also found in fish liver oil and eggs. Both forms are modified in the liver and kidneys to make calcitriol, the most potent form of vitamin D. Vitamin D binds to a protein called vitamin D receptor, or VDR, inside cells in your body. VDR then binds to cellular DNA and cooperates with other proteins to ultimately turn genes on or off. Genes involved in calcium absorption are regulated by vitamin D. In addition, vitamin D also affects genes involved in cell growth cycle; differentiation -- the process by which cells acquire specialized functions; and immune function.
Sources and Recommendations
Exposure to sunlight, diet and supplements are ways to increase vitamin D in the body. Natural dietary sources include fish, fish liver oils and eggs. In addition, foods, such as milk and breakfast cereals, are commonly fortified with vitamin D. The 2010 Dietary Guideline for Americans policy document published by the U.S. Department of Agriculture indicates that the RDA -- or recommended daily allowance -- of vitamin D, assuming minimal sunlight exposure, is 600 international units, or 15 mcg, per day for children and most adults, and 800 IU, or 20 mcg, for adults older than 70. The guidelines also point out that vitamin D intake in excess of 4,000 IU, or 100 mcg, per day has the potential for causing harmful side effects.
Risk Factors
Several sub-populations are at risk for developing vitamin D deficiency. Infants, especially those with darker skin or limited sun exposure, who are fed on breast milk exclusively, without any vitamin D supplementation, are at higher risk for becoming vitamin D deficient. People with naturally low vitamin D synthesis in skin, such as dark-skinned individuals and the elderly, are also likely to become deficient. Certain conditions, such as cystic fibrosis, obesity and Crohn's disease -- an inflammatory bowel disease -- can also impair the absorption or availability of vitamin D.
Rickets and Osteomalacia
Rickets and osteomalacia are two long-known manifestations of severe vitamin D deficiency. Rickets refers to the weakening and softening of bones, along with slowed bone and skeletal growth, due to defects in bone mineralization during development. Severe vitamin D deficiency, due to insufficient dietary intake or lack of exposure to sunlight, can cause rickets in children. Infants exclusively fed on breast milk alone may also develop rickets, in the absence of vitamin D supplementation. Osteomalacia is softening of bone that occurs in adults, in whom bone growth has already ceased. Vitamin D and calcium supplementation are often used in treatment of rickets and osteomalacia.
Other Diseases
Vitamin D provides benefits outside of bone health, including reduced risk of diabetes, certain cancers, infections and death from cardiovascular disease. An Australian study published in "Diabetes Care" in 2011 found that people with lower-than-average serum levels of vitamin D had a 57 percent higher diabetes risk than those with recommended vitamin D levels. In data from the National Health and Nutritional Examination Survey, or NHANES III, by Centers for Disease Control and Prevention, individuals with serum levels of vitamin D lower than 10 nanograms per liter were more likely to have had a recent infection of the upper respiratory tract. This association was stronger in those with asthma and chronic pulmonary obstructive disease. While direct causal links between vitamin-D deficiency and other conditions are not established yet, vitamin D has a critical and broad protective role in human health.
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