Vitamin D

Vitamin D is a fat-soluble vitamin that functions as a hormone in the body, regulating calcium absorption and bone health. Unlike most vitamins, it is produced endogenously when skin is exposed to sunlight, making dietary sources secondary for many people. Consumers encounter vitamin D on nutrition labels of fortified foods like milk, plant-based milks, orange juice, and cereals, as well as in supplements. It is also naturally present in fatty fish, egg yolks, and some mushrooms exposed to UV light.

In the real world, vitamin D synthesis depends on geographic latitude, skin pigmentation, time of day, and sunscreen use. People living in northern climates or with darker skin may produce insufficient amounts, especially during winter. Fortification of foods is a public health strategy to prevent deficiencies, but levels vary by country and product. For example, cow's milk in the US is fortified with 100 IU per cup, while many plant-based milks match this. Supplements typically provide 400–2000 IU per dose, though higher amounts require medical guidance.

For someone eating or shopping, vitamin D matters because deficiency can lead to bone disorders like rickets in children and osteomalacia in adults, and is linked to immune dysfunction. However, relying solely on food is challenging since few foods are naturally rich in it. Fortified plant-based milks are a reliable option for vegans, but not all brands fortify equally—check labels. Sun exposure remains the most efficient source, but balancing skin cancer risk is nuanced. Supplementation may be prudent for those with limited sun exposure, but excessive intake can cause toxicity.