Why winter is a vitamin D problem in the UK
Vitamin D is unusual among vitamins because your body can make it from sunlight, not just food. Ultraviolet B (UVB) rays hit your skin and trigger the synthesis. But this only works when the UVB angle is steep enough, roughly when the sun is above 45 degrees in the sky.
From October to March, Britain's latitude means the sun never climbs high enough for UVB to do its job. It doesn't matter whether it's sunny or overcast, or whether you go outside: the wavelength that triggers vitamin D synthesis simply isn't reaching the surface. Scotland compounds this further than southern England, but the problem is nationwide.
Layer on top of that how most people spend the winter months: more time indoors, more layers of clothing when outside, shorter lunch breaks taken at a desk. The gap between what your body needs and what it's getting widens from October onwards, quietly, without obvious symptoms until it's been running low for a while.
The NHS recommendation: 10µg daily, autumn through winter
The NHS position on this is clear and, usefully, it doesn't require a blood test or a GP appointment to act on. The NHS recommends that everyone aged one year and over consider taking a daily supplement containing 10 micrograms (10µg) of vitamin D during the autumn and winter months. This includes pregnant and breastfeeding women.
That phrasing "consider taking" is the NHS's standard hedge, used the same way across their supplement guidance. In practice, the recommendation is broad: it covers the whole UK population, not just people who feel symptoms.
The 10µg figure is the amount needed to support bone health, muscle function, and normal immune response through the months when sunlight can't contribute. It's the only supplement dose we point to here, because it's the only one with a clear NHS endorsement for the general population.
What kind of supplement, and how to take it
Most vitamin D supplements label their dose in IU (international units) rather than micrograms. On the shelf, 10µg equals 400 IU, so a "400 IU" tablet is the same as a "10µg" one. Both figures refer to vitamin D3 (cholecalciferol), which is the form found in most animal-sourced foods and the form most commonly used in supplements. D2 (ergocalciferol), which comes from fungi, works similarly but is less common.
For adults, a standard tablet or capsule works fine. For young children, liquid drops make the dose easier to give accurately: look for a product designed for infants or young children, as the concentration per drop varies between brands.
Vitamin D is fat-soluble, which means it absorbs better alongside a meal that contains some fat. Taking it with breakfast or lunch works well for most people. The specific timing matters less than the consistency: taking it at the same point in your day makes it easier to build into a routine and harder to forget.
What food does alongside
Food contributes, but can't close the gap alone. Oily fish, salmon, sardines, mackerel, and fortified foods are the main sources. A 140g salmon fillet can clear a day's 10µg, but daily salmon isn't realistic eating. That's precisely why the supplement exists. For the full list with portions and how pairing with iron-rich foods affects absorption, see our dedicated vitamin D foods guide.
Who should consider more than 10µg
The 10µg winter recommendation applies to the general population. The NHS flags several groups who may want to think about a year-round supplement too, rather than only from autumn to spring. These include people who are not often outdoors, people in care homes or similar settings, and people with darker skin, as greater skin pigmentation reduces how efficiently UVB triggers vitamin D synthesis.
If you or someone in your household falls into one of these groups, that's worth raising with a GP. We're not in a position to recommend doses above 10µg ourselves, and neither would you want us to. A GP can arrange a blood test, read the result properly, and advise from there.
What too much looks like
The NHS is explicit on this: taking more than 100µg (4,000 IU) of vitamin D a day over a long period can be harmful. Excess vitamin D raises the level of calcium in the blood, which can damage the kidneys and bones over time.
At the standard 10µg/400 IU level, there is a wide margin. The upper limit is ten times the recommendation, and supplements sold for general use stay well within it. The risk emerges with high-dose products, typically sold in wellness or sports contexts, taken without a clinical reason to do so. If a product offers more than 25µg (1,000 IU) per daily dose and no healthcare professional has recommended it, it's worth pausing before taking it regularly.
A short, honest note
If you're worried about your vitamin D levels specifically, a GP can run a blood test and give you a result you can actually act on. For most people going through a normal British winter, the 10µg daily supplement the NHS recommends covers it.