Vitamin D: Functions, sources, and supplementation guidelines

Vitamin D, often referred to as the “sun vitamin,” plays a critical role in calcium and phosphate metabolism, bone health, muscle function, and immune system regulation. While the body can produce it through skin exposure to sunlight, dietary intake alone may be insufficient for many individuals.

Natural food sources include fatty fish (such as salmon, trout, tuna, and mackerel), fish liver oils, egg yolks, beef liver, and certain mushrooms. However, because few foods are naturally rich in this nutrient, supplementation is often considered, particularly for high-risk groups. These include individuals with limited sun exposure, those with darker skin pigmentation, people with certain gastrointestinal, liver, or kidney diseases, and those taking specific medications like anticonvulsants.

Determining optimal levels involves measuring 25-hydroxyvitamin D in the blood. While reference ranges vary between organizations like the Robert Koch Institute (RKI) and the US Institute of Medicine (IOM), the RKI suggests an optimal range between 30 and 50 ng/ml. Deficiencies can lead to serious conditions such as rickets in children or osteoporosis in adults. Additionally, research suggests that Vitamin A may influence how Vitamin D affects calcium levels, though more study is needed on the interaction of commercial supplements.