An estimated one billion people worldwide have vitamin D deficiency or insufficiency. Individuals at high risk for vitamin D deficiency, include those living far from the equator, those with medical conditions (such as obesity, liver disease, celiac and renal disease), the elderly, and those with darker skin.
Findings from the National Health and Nutrition Examination Survey, which included more than 15,000 adults, indicated that individuals with darker skin have lower vitamin D levels. Dark-skinned individuals have high levels of melanin which impairs absorption of vitamin D, which is made when skin is exposed to ultraviolet radiation found in natural sunlight.
Regardless of cause, deficiency of vitamin D has significant medical and psychological consequences. Every tissue in the body has vitamin D receptors, including the brain, heart, muscles, and immune system, which means vitamin D is needed at every level for the body to function.
Vitamin D is also the only vitamin that is a hormone. After it is consumed in the diet or absorbed (synthesized) in the skin, vitamin D is then transported to the liver and kidneys where it is converted to its active hormone form. Vitamin D as a hormone assists with the absorption of calcium, helping to build strong bones, teeth and muscles.
In addition to its well-known role in calcium absorption, vitamin D activates genes that regulate the immune system and release neurotransmitters (e.g., dopamine, serotonin) that affect brain function and development. Researchers have found vitamin D receptors on a handful of cells located in regions in the brain-the same regions that are linked with depression.