The "vitamin" your skin makes — and why it isn't really one

By definition, a vitamin is a compound your body can't make and must get from food. Vitamin D breaks that rule: given enough sunlight, your skin manufactures it from cholesterol, using UVB rays to kick off the reaction. That's why many scientists call it a hormone (technically a secosteroid) rather than a true vitamin — it's a signaling molecule your body produces itself.

Its headline job is managing calcium. Vitamin D tells your gut to build the transport proteins that pull calcium out of food. Without enough of it, you absorb only a fraction of the calcium you eat — which is why loading up on calcium while low in D does surprisingly little for your bones. Severe, prolonged deficiency causes soft bones: rickets in children, osteomalacia in adults.

The catch: very few foods supply it. Fatty fish (salmon, sardines), egg yolks, and fortified milk or cereal are the main dietary sources, so sunlight and fortification do most of the heavy lifting. Skin tone, latitude, winter, age, and sunscreen all reduce how much your skin makes — which is why deficiency is common even in sunny places, and why routine testing and supplements are a conversation to have with a clinician rather than a guess.