Vitamin D, and the supplement question generally

Vitamin D is technically a hormone rather than a vitamin, and it occupies an unusual position: genuinely deficient in a large share of the population and consistently disappointing in trials of supplementation for the things it was expected to fix.

What it does

It regulates calcium and phosphate absorption and is essential for bone mineralisation.

Severe deficiency causes rickets in children and osteomalacia in adults, both of which are unambiguous and both of which still occur.

Receptors are present in many tissues, which generated a large literature hypothesising roles in immunity, cardiovascular health, cancer and mood.

Why deficiency is common

It is synthesised in skin exposed to ultraviolet B radiation, which at higher latitudes is insufficient for several months of the year regardless of time outdoors.

Darker skin requires substantially longer exposure to produce the same amount.

Covering clothing, indoor working, sunscreen use, older age and obesity all reduce available vitamin D.

Dietary sources are limited — oily fish, egg yolk, fortified products — and rarely sufficient alone.

Which is why public health guidance in many countries recommends supplementation for the whole population during winter months, and year-round for specific groups.

What the trials found

The instructive part.

Large randomised trials of vitamin D supplementation in generally healthy, largely replete populations have not shown the benefits that observational studies suggested for cardiovascular disease, cancer incidence or overall mortality.

Evidence for fracture prevention is mixed and appears strongest in older adults in institutional settings, particularly combined with calcium.

Respiratory infection findings have been mixed, with some meta-analyses suggesting modest benefit concentrated in those who were deficient.

The pattern is consistent: correcting deficiency helps, and adding more to someone who is replete does not.

Which is the general principle for nutrients and one that the supplement market is structured to obscure.

Why observational studies mislead here

Worth understanding because it recurs.

Low vitamin D is associated with being unwell, being indoors, being inactive and being obese.

Which means low levels may be a marker of poor health rather than a cause of it, and reverse causation is difficult to exclude.

Mendelian randomisation studies, which use genetic variants to reduce confounding, have generally not supported causal effects for most of the proposed non-skeletal outcomes.

Dosing and safety

Practical points.

Routine daily supplementation at the doses recommended in public health guidance is safe.

Very high doses are not better and toxicity is possible, causing hypercalcaemia with nausea, confusion, kidney stones and kidney damage.

Cases of toxicity from high-dose supplements bought online are documented and are rising.

Large intermittent bolus doses have performed worse than daily dosing in some trials, including for falls.

And routine testing of vitamin D levels is not recommended in the general population by most guidance, since it does not change the advice.

The supplements worth taking

A short list where there is a genuine case.

Folic acid before and in early pregnancy, which prevents neural tube defects and has strong evidence.

Vitamin D as above, at population level in relevant latitudes and for at-risk groups.

Vitamin B12 for people on vegan diets, where deficiency is otherwise likely.

Iron where deficiency is documented, and not otherwise, since iron overload causes harm.

Specific supplements for documented deficiencies or malabsorption.

And certain formulations for specific conditions, such as those used in age-related macular degeneration.

The ones that are not

Multivitamins in people eating adequately, where large trials have not shown mortality or cardiovascular benefit.

Antioxidant supplements, where some trials have found harm, including increased lung cancer in smokers taking beta-carotene.

Most supplements marketed for energy, immunity, hair, gut, hormones and sleep, where evidence ranges from thin to absent.

And any supplement taken because a direct-to-consumer test flagged an unvalidated "optimal" range.

The regulation point

Which explains much of the above.

Supplements are regulated as foods rather than medicines in most jurisdictions, which means they do not require proof of efficacy before sale.

Content testing has repeatedly found products containing more, less or different ingredients than labelled, including undeclared pharmaceuticals in some categories.

Interactions are real: St John's wort with numerous medications, high-dose vitamin E and fish oils with anticoagulants, and biotin with laboratory assays among them.

Which makes disclosing supplement use to a clinician genuinely important rather than a formality.

General information only, not medical advice. Consult a qualified clinician before taking supplements, particularly at high doses or alongside medication.