Vitamin D

Not really a vitamin

Vitamin D is synthesised in skin from 7-dehydrocholesterol under UVB exposure, hydroxylated in the liver to 25-hydroxyvitamin D (the storage form measured in blood tests), and hydroxylated again in the kidney to calcitriol, the active hormone. Calcitriol binds to the vitamin D receptor, a nuclear receptor found in most tissues in the body — including Leydig cells in the testes, the pituitary, and sperm cells.

That receptor distribution is the mechanistic basis for the interest in vitamin D beyond bone health, and it is why the compound behaves more like a steroid hormone than a dietary vitamin.

Why deficiency is the default in northern Europe

Above roughly 37° latitude — which includes essentially all of Europe north of Sicily — the sun's angle from October to March is too low for UVB to produce meaningful vitamin D in skin, regardless of how long you are outside. Add indoor work, sunscreen use, darker skin pigmentation (which requires substantially longer exposure for the same synthesis) and older age (which reduces skin synthesis capacity), and the result is a population where winter insufficiency is normal rather than exceptional.

Dietary sources are limited: oily fish, egg yolk, liver and fortified foods. Reaching sufficiency through food alone is difficult without regular oily fish intake.

The testosterone evidence

A frequently cited randomised trial published in Hormone and Metabolic Research (Pilz et al., 2011) gave 3,332 IU daily of vitamin D or placebo to overweight men undergoing a weight reduction programme for one year. Total testosterone rose significantly in the vitamin D group and did not change in the placebo group. Participants were vitamin D insufficient at baseline.

Observational studies consistently show a positive association between 25(OH)D and testosterone levels. Interventional trials in men who are already vitamin D replete generally show no effect — the same deficiency-correction pattern seen with zinc.

Outside hormonal endpoints, the evidence for correcting deficiency is strong for bone health and reasonably supported for reducing respiratory infection risk, particularly in those with low baseline levels.

Practical dosing

Test rather than guess. A serum 25(OH)D test is inexpensive and available through most laboratories, often without a doctor's referral. Below 30 nmol/L is deficiency, 30–50 nmol/L is insufficiency, 50–125 nmol/L is the generally accepted adequate range.

Use D3. Cholecalciferol raises and maintains serum 25(OH)D more effectively than ergocalciferol (D2). Vegan D3 derived from lichen is available.

Take it with fat. It is fat-soluble; absorption from a fat-free meal is substantially lower.

Consider K2. Vitamin D increases calcium absorption; vitamin K2 (menaquinone-7) directs calcium into bone rather than soft tissue. The combination is common practice and mechanistically sensible, though outcome evidence for the pairing is still developing.

Respect the ceiling. EFSA's tolerable upper intake is 4,000 IU per day for adults. Toxicity — hypercalcaemia, kidney stones, vascular calcification — is real but requires sustained intake far above this, typically 10,000 IU+ daily for months.