The supplement aisle is largely a monument to hope over evidence, which makes vitamin D worth singling out: it is the one everyday supplement UK health authorities actually tell the whole population to take, for part of the year, on grounds that come down to astronomy as much as biology.
Vitamin D is made in skin exposed to ultraviolet B light, and at Britain's latitude the winter sun never climbs high enough for the relevant wavelengths to get through. From about October to early March, no amount of standing outside in a British January produces meaningful vitamin D, whatever the sky is doing. Diet contributes something, oily fish, eggs, fortified cereals and spreads, but average intakes fall well short of requirements. Surveys duly find a substantial minority of the population with low levels by late winter, rising further among people with darker skin, which synthesises vitamin D more slowly at these latitudes, and among those who cover skin for cultural reasons, spend little time outdoors or are housebound.
The official response is modest and specific: 10 micrograms, equivalently 400 international units, daily through autumn and winter for everyone over one year old, and year-round for the higher-risk groups above. The dose costs pennies and the tablets are small. Pregnant women and young children have their own long-standing advice, and free supplements exist through the Healthy Start scheme for eligible families, a provision persistently under-claimed.
What the evidence does and does not support
The confident claims stop at bones and muscles. Vitamin D governs calcium absorption, and deficiency causes rickets in children, a Victorian disease that has made a small unwelcome return, and bone softening and muscle weakness in adults, with fracture and fall risks in older people. Correcting deficiency fixes these, which is the core case for the population advice.
Beyond that, enthusiasm has repeatedly outrun results. Observational studies linked low vitamin D to almost everything, cancer, heart disease, depression, infections, but large randomised trials giving supplements to people who were not deficient have mostly found little effect on those outcomes. The pattern suggests low vitamin D often marks poor health, time indoors, obesity, chronic illness, rather than causing it. Mega-dosing, meanwhile, is genuinely harmful: vitamin D is fat-soluble and accumulates, and very high intakes cause calcium overload, with safe upper limits far below what some imported products contain.
The takeaway is pleasantly boring. A small daily dose through the dark months, year-round for those at higher risk, food and sensible summer sun the rest of the time, and scepticism toward any bottle promising transformation. It is the rare corner of nutrition where the official leaflet and the evidence say exactly the same thing.
