Vitamin D is a fat-soluble vitamin, which means it is absorbed along with dietary fats and can be stored in the body’s fatty tissue. It is essential for optimal health; it keeps bones strong and improves mental health and sleep quality.

Vitamin D helps the body absorb calcium and phosphorus, which are important minerals for bone health. When a person is Vitamin D deficient, they cannot absorb dietary calcium well. Therefore, having adequate levels is essential for dietary calcium and phosphorus absorption. When Vitamin D levels are low and the body isn’t able to properly absorb calcium and phosphorus, there is an increased risk of bone fractures.

Vitamin D is essential for a healthy, well-functioning immune system. Vitamin D modulates the immune system and helps immune cells fight pathogens while preventing overreaction and autoimmunity. It influences T-cells, macrophages, and cytokine production and helps activate antimicrobial proteins. Vitamin D levels correspond to the degree of the immune response against viral infections. Deficiency is linked to increased risk of infections and autoimmune diseases such as lupus, multiple sclerosis (MS) and rheumatoid arthritis (RA). Vitamin D also supports the thymus, a vital organ for T-cell development, preventing premature aging and dysfunction.

Vitamin D deficiency is linked to higher risks of metabolic syndrome, insulin resistance, and heart disease. Low D levels can worsen these conditions, while high D levels can be protective.

According to the National Institutes of Health (NIH), almost 1 in 4 U.S. adults are considered low in Vitamin D. Therefore, regular monitoring of Vitamin D levels (25-hydroxyvitamin D) is important. Symptoms of Vitamin D deficiency can include fatigue, insomnia, bone pain, depression, hair loss, muscle weakness, loss of appetite, frequent viral infections, and pale skin.

Earning its nickname as the sunshine vitamin, people typically get enough Vitamin D from sun exposure. This occurs when sunlight hits the skin, and the skin converts that ultraviolet radiation to Vitamin D. However, certain conditions can result in inadequate absorption and processing of Vitamin D. These include darker skin pigmentation, malnutrition, kidney or liver failure, some prescription medications, and certain types of cancer.

Besides sunlight, Vitamin D can be obtained from a limited amount of foods. These include fatty fish (such as salmon, mackerel, and sardines), fish liver oils, and small amounts in beef liver, cheese, and egg yolks. Mushrooms also contain vitamin D, provided they are exposed to UV light during their growing season. They naturally contain a precursor (ergosterol) which converts to vitamin D2 when exposed to sunlight. 

For people who have limited sun exposure or low dietary intake, Vitamin D supplements can be an important source. They are available in two main forms: D2 and D3. However, studies have shown that Vitamin D3 raises levels more effectively than vitamin D2. Vitamin D3 should be taken with Vitamin K2. D3 helps the body absorb calcium, while K2 directs that calcium into bones and away from blood vessels, preventing calcium buildup in arteries.

Although the Vitamin D3 molecule created in response to sunlight is the same as the molecule found in supplements, there are certain differences in how the body processes, absorbs, and utilizes it. Exposure of the skin to UVB radiation initiates the conversion of cholesterol to pre-Vitamin D3, which is also converted into other forms of vitamin D, including its active form, directly within the skin cells for their own use.

Taking too much supplemental Vitamin D can lead to Vitamin D toxicity. Extremely high Vitamin D levels are harmful and can cause nausea, vomiting, confusion, excessive thirst and kidney stones. and can cause hypercalcemia. Extreme Vitamin D toxicity can damage the kidneys, causing calcification and potentially leading to renal failure due to excessive calcium buildup.

While Vitamin D toxicity is possible to achieve from supplements, it is not possible when generating Vitamin D from sun exposure. Vitamin D3 generated by sunshine exposure is also available to the body 2-3 times longer than that taken orally.

Vitamin D supplements can interact with certain medications, so check with your health care provider before starting one.