Vitamin D
A fat-soluble vitamin (in fact a pro-hormone) made by the skin under the effect of sunlight, essential for calcium absorption, immunity and many physiological functions.
Vitamin D exists in two forms: D2 (ergocalciferol, plant-based) and D3 (cholecalciferol, animal/skin synthesis). The body makes it from cholesterol when the skin is exposed to UVB.
An original angle: technically, vitamin D is not a vitamin but a pro-hormone: the body can make it, and it acts on more than 200 genes through receptors found in almost every tissue. This explains its effects well beyond bones: immunity, mood, muscle function, regulation of inflammation.
Deficiency is massive and underestimated: up to 80% of the population in winter in northern countries. Causes: little sun exposure (indoor work, latitude, sunscreen), dark skin (less synthesis), age (reduced synthesis), obesity (vitamin D gets trapped in fat).
A geographical paradox: in Rotterdam or further north, skin synthesis is almost nil from October to March because the sun’s angle is too low for UVB. Winter supplementation is often justified at these latitudes.
Vitamin D works in synergy with vitamin K2 (which directs calcium to bones rather than arteries) and magnesium (a cofactor for its activation). Taking D alone in high doses without K2 and magnesium can be counter-productive.