Vitamin D

Vitamin D: Test Before You Megadose

One of the few supplements where a blood test is genuinely informative, and one of the easiest to overdo.

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D3 with food, commonly 1000–2000 IU daily. It is fat-soluble and accumulates, so the upper limit is a real limit — test rather than guess.

Why this one is worth measuring

Most supplements cannot be usefully tested for. Vitamin D can: serum 25-hydroxyvitamin D, written 25(OH)D, is a standard, widely available blood test that reflects your actual status.

That matters because the right dose depends entirely on where you start, and starting points vary enormously with latitude, skin tone, sun exposure, body composition and season. Insufficiency is genuinely common at higher latitudes in winter. It is also not universal, and someone already in range gains nothing from supplementing harder.

D3, with fat, consistently

D3 (cholecalciferol) raises and maintains serum 25(OH)D more effectively than D2 (ergocalciferol). Unless you specifically need the vegan-source option, D3 is the default.

It is fat-soluble, so take it with a meal containing some fat. Taken on an empty stomach, absorption is measurably worse.

Daily and weekly dosing both work, since the vitamin accumulates. Daily is simply easier to remember.

The upper limit is a real limit

The tolerable upper intake level for adults is 4000 IU per day. Because vitamin D is stored rather than excreted, sustained intake well above that can cause hypercalcaemia, with real consequences for kidneys and soft tissue.

Very high-dose products — 10,000 IU and up — are sold freely, and have legitimate uses under medical supervision for documented deficiency. Taking them indefinitely on the assumption that more is better is the specific failure mode this section exists to flag.

Nothing here is medical advice, and a deficiency worth treating is worth treating with your doctor, who can order the test and read it in context.