Vitamin D is often referred to as “the sunshine vitamin” because the human body produces it when the skin is exposed to solar radiation. This fat-soluble compound is a family of nutrients, primarily Vitamin D2 (ergocalciferol) and Vitamin D3 (cholecalciferol). Vitamin D3 is the form synthesized by the skin and is also found in animal-sourced foods.
How Sunlight Triggers Vitamin D Production
The production of Vitamin D involves a precise photochemical reaction in the skin layers. This process begins when the skin is exposed to ultraviolet B (UVB) radiation. Inside the epidermal cells, a precursor molecule called 7-dehydrocholesterol (7-DHC), a form of cholesterol, acts as the starting material.
UVB rays strike the 7-DHC, converting it into an intermediate compound known as pre-vitamin D3. This pre-vitamin D3 rapidly transforms into Vitamin D3 (cholecalciferol). Once formed, Vitamin D3 enters the bloodstream and is transported first to the liver and then to the kidneys for further processing.
In the liver, Vitamin D3 is converted into 25-hydroxyvitamin D (calcidiol), the main circulating form measured in blood tests. The kidneys perform the final conversion, turning calcidiol into the biologically active hormone, 1,25-dihydroxyvitamin D (calcitriol). This multi-step process regulates the amount of active vitamin D, preventing a toxic overdose from excessive sun exposure.
Critical Roles in Human Health
Once activated, calcitriol functions like a hormone, mediating several physiological processes. Its primary function is regulating calcium and phosphate levels in the bloodstream. Vitamin D enhances the absorption of dietary calcium in the small intestine, ensuring sufficient mineral is available for the body.
Vitamin D is necessary for the proper mineralization, growth, and remodeling of bone tissue by maintaining optimal concentrations of calcium and phosphate. Inadequate levels can cause bones to become soft and brittle. The vitamin also plays a role in modulating the immune system.
Many immune defense cells, including T-cells and macrophages, possess vitamin D receptors, indicating a direct influence on their function. The vitamin helps regulate inflammation and supports the body’s response to infections.
Dietary Sources and Alternatives
Reliance on solar exposure alone for Vitamin D is often inadequate, despite the sun being the primary natural source. Factors like latitude, season, time of day, and skin pigmentation limit UVB exposure, especially for people living in higher latitudes during winter months. Therefore, food and supplements are important alternative sources.
Few foods naturally contain high amounts of the vitamin. The richest natural sources of Vitamin D3 include the flesh of fatty fish, such as salmon, tuna, and mackerel. Other natural sources include cod liver oil, egg yolks, and beef liver.
To combat widespread inadequacy, many common foods are fortified, including milk, cereals, orange juice, and plant-based milk alternatives. When diet and sun exposure are insufficient, supplements offer a reliable alternative to maintain healthy levels. Supplements are available as Vitamin D2 or D3, though D3 is often considered more effective at raising blood levels. Since Vitamin D is fat-soluble, it is best absorbed when consumed with a meal containing fat.