Vitamin D Is Not a Vitamin — And Low Levels May Be Costing You More Than You Think

The designation of vitamin D as a "vitamin" is a historical accident. It was classified alongside vitamins in the early 20th century because deficiency caused rickets and could be corrected through diet. The dietary sources are minor, the primary source is cutaneous synthesis, and the molecule functions as a steroid hormone — not a vitamin in the biochemical sense.

That distinction changes how deficiency is understood and treated.

The Biology

Vitamin D3 (cholecalciferol) is synthesized in the skin from 7-dehydrocholesterol upon UVB radiation exposure. It is then hydroxylated in the liver to 25(OH)D — the measured circulating form — and in the kidneys to 1,25(OH)2D (calcitriol), the biologically active form.

Calcitriol binds to the vitamin D receptor (VDR), expressed in over 40 tissue types including:

• Immune cells (T cells, B cells, dendritic cells, macrophages)
• Muscle fibers
• Brain neurons
• Cardiac tissue
• Testicular Leydig cells (testosterone production) [1]

That receptor distribution explains why low vitamin D produces effects across such diverse systems.

The Deficiency Problem

Latitude and season: UVB radiation sufficient for vitamin D synthesis is unavailable at latitudes above approximately 35–40°N for 3–6 months per year. Stockholm (59°N), London (52°N), and New York (41°N) all have extended periods of inadequate UVB for cutaneous synthesis.

Skin pigmentation: Melanin reduces UVB absorption. People with darker skin types require substantially longer sun exposure for equivalent synthesis — a significant contributor to higher deficiency rates in populations of African and South Asian descent living in northern climates.

Dietary intake: Very few foods provide meaningful vitamin D without fortification. Fatty fish (salmon, sardines, mackerel), egg yolks, and fortified dairy are the primary dietary sources — rarely enough to cover the deficit left by inadequate sun exposure.

> 📌 A 2011 meta-analysis in BMJ covering 26 prospective cohort studies found that vitamin D deficiency was associated with significantly increased risk of cardiovascular disease, cancer mortality, diabetes, and all-cause mortality — with dose-response relationships suggesting the protective effect extended up to 25(OH)D levels of 75–100 nmol/L (30–40 ng/mL), well above the rickets-prevention threshold of 50 nmol/L.[1]

Optimal Levels and Supplementation

Target: 75–100 nmol/L (30–40 ng/mL) for 25(OH)D — above the conventional deficiency threshold of 50 nmol/L.

Typical supplementation dose for adults in northern climates: 2000–4000 IU/day D3 (not D2 — up to 70% less potent) with vitamin K2 (100–200 mcg MK-7 form). K2 directs calcium to bone rather than vascular tissue.

Test, don't guess: 25(OH)D is routinely measurable and inexpensive. Dosing should be adjusted to hit target levels — not based on a universal dose.

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