Chances are you are not getting enough vitamin D.

An estimated 1 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 conditions such as , liver disease, celiac, and renal disease, the elderly, and those with darker skin.

Findings from the National Health and Examination Survey, which included more than 15,000 adults, indicated that individuals with darker skin have lower vitamin D levels. Dark-skinned individuals' high level of melanin 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 . After it is consumed in the diet or absorbed (synthesized) in the skin, vitamin D is 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 that regulate the immune system and release neurotransmitters (e. ., , 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.

(SAD), a featuring depressive symptoms, occurs during the dark times of the year when there is relatively little sunshine, coinciding with the sudden drop in vitamin D levels in the body. Several studies have suggested that the symptoms of SAD may be due to changing levels of vitamin D3, which may affect serotonin levels in the brain.

Due to vitamin D's connection to depression and mood, I test the vitamin D levels, specifically 25-hydroxy-vitamin D, of every new patient. For years, vitamin D blood levels of 20 ng/mL were accepted as normal. Many researchers and clinicians now consider this too low. More recently, the new normal level is anything greater than 30 ng/mL. However, I prefer to see 25-hydroxy-vitamin D levels between 50 and 75 ng/mL. For those who are low, I recommend a supplement that may range from 2,000 IU to 10,000 IU. It's important to note that vitamin D supplementation needs to be monitored by blood testing every few months.

Although vitamin D supplementation may improve mood, vitamin D is only a small, but critical, part of treatment; depression has myriad causes. However, in my experience, vitamin D deficiency impairs and prolongs recovery from depression.

Various studies confirm the link between low vitamin D and mental illness. These studies provide evidence that optimizing vitamin D levels may improve positive psychological well-being:

A study in the Netherlands found that low levels of vitamin D correlated with symptoms of major and minor depression in 169 individuals ages 65 or older. An English study that included 2070 people age 65 and older concluded that vitamin D deficiency is associated with depression in northern countries, although major depression was only seen in individuals with the most severe deficiencies. In one study, adults with vitamin D deficiency who received high doses of the vitamin saw an improvement in their depressive symptoms after two months. A small study with nine women, all of whom were vitamin D deficient or insufficient, found that a daily dose of 5,000 IU of vitamin D significantly improved their depression symptoms.

Although researchers are still unsure of how vitamin D is linked with depression, these findings and many others support the role of vitamin D in the pathophysiology of depression and as a potential treatment for depression.

Groups at higher risk for vitamin D deficiency—the elderly, adolescents, obese individuals, and those with chronic illnesses (e.g., diabetes)—are the same groups that are also reportedly at higher risk for depression.

Vitamin D's effect on mental health extends beyond depression: has also been linked with abnormal levels of vitamin D. John McGrath of the University of Queensland in Australia studied 424 Danish newborns who developed schizophrenia. He concluded that infants born in winter or spring seasons, when birth mothers have decreased levels of vitamin D, are at an increased risk of developing schizophrenia.

Mental health is one of many types of ailments connected to vitamin D deficiency. For more information on vitamin D and its links to mental and physical health, visit the Vitamin D Council Psychiatrist John Cannell, founded the council in 2003 with a keen interest in clinical nutrition and a strong conviction that vitamin D deficiency, a highly preventable yet prevalent condition, contributes to many physical and psychological conditions affecting scores of people.