Vitamin D: The Dose Nobody Can Give You

Vitamin D is not really a vitamin. It is a prohormone the skin makes from sunlight, and it acts on receptors in almost every tissue in the body. Calling it a vitamin made it sound like something you top up, which is why so much of the advice about it is given as a single number.

Why a single number does not work

The blood level reached from a given dose varies several-fold between people. Body fat sequesters it, so heavier people need more. Skin pigmentation, age, gut absorption and genetics in the receptor and transport proteins all move the number. Two people taking 2000 IU can land anywhere from clearly deficient to comfortably sufficient.

This is the one supplement where testing is genuinely worth the cost. Measure 25-hydroxyvitamin D at the end of winter, when it is at its lowest, adjust, and re-test after three months. Everything else is guessing at a variable that happens to be cheap to measure.

The co-factors that get left out

• Vitamin K2 decides where the calcium goes. Vitamin D raises calcium absorption; K2 directs it into bone rather than arterial wall. Taking high-dose D long-term without it is the one real risk in this area
• Magnesium is required by the enzymes that convert vitamin D to its active form. Low magnesium is a common reason supplementation fails to raise the number
• Fat, because it is fat-soluble. Take it with a meal containing some

A sensible protocol

• Autumn to spring at northern latitudes; summer sun does the job if you actually get any
• Test rather than assume. If you will not test, 2000–4000 IU daily is the range most adults land in
• K2 alongside, in the MK-7 form, roughly 100–200 mcg
• Daily rather than a large weekly or monthly bolus, which produces a less steady level

The strongest evidence for vitamin D is in correcting deficiency, not in taking more once you are sufficient. Most of the disappointing trial results tested supplementation in people who did not need it.

What it does and does not do

Established: bone health, calcium regulation, and a clear role in immune function. Reasonable: respiratory infection risk in people who were deficient to begin with. Overstated: the long list of conditions where low vitamin D is associated with poor outcomes but supplementing has not changed them — inflammation lowers vitamin D, so the low reading is often a marker of illness rather than its cause.