Vitamin D is a hormone, not a vitamin

Cholecalciferol is made in skin under UVB, hydroxylated in the liver, then again in the kidney, and the result — calcitriol — binds a nuclear receptor in muscle, immune cells, prostate, brain. That is not the behaviour of a vitamin you sprinkle on cereal. It is the behaviour of a hormone.

Office men, northern latitudes, darker skin, winter: the overlap is where deficiency lives. Surveys across Europe and North America keep finding large shares of adult men below 50 nmol/L. Symptoms are quiet — flat energy, more colds, a training block that will not land.

The receptor for vitamin D sits in nearly every tissue. A ‘vitamin’ that widespread is doing more than bone.

Sun, then sense

Ten to twenty minutes of midday sun on arms and face can be enough in summer at mid-latitudes — and is useless through glass, and useless in a London January. Sunscreen on the face is still non-negotiable; the trade is not ‘burn or be deficient’. It is ‘some uncovered skin, some of the year, and a blood test if you live in a coat’.

Fatty fish, egg yolks, and fortified foods help. Supplementation is a doctor’s conversation, not a shopping-cart reflex. Too much is a real toxicity. The interesting fact is the middle: most men have never measured, and guess from mood.

Journal notes are for curiosity, not diagnosis. Speak to a clinician before changing medicine, supplements, or training.