Nearly 1 in 3 UK adults are vitamin D deficient by the end of winter. Here’s what to do before it starts.

Let's start by getting the number right, because almost nobody does.

You have probably read somewhere that half of UK adults are vitamin D deficient. It's a great stat. It's also wrong, and we'd rather tell you that than repeat it because it sells more D3.

Here is what the actual data says. The National Diet and Nutrition Survey (the government's rolling survey of what the country eats and what's in its blood) published its 2019 to 2023 results in June 2025. Among UK adults aged 19 to 64, 18% were vitamin D deficient across the year as a whole. Not half. Roughly one in five.

But now look at what happens when you split that figure by season. Between January and March, it climbs to 31%.

That's the number worth knowing. Not because it's scarier, but because it's the one that tells you something useful: this is not a permanent national affliction. It's a seasonal one. It arrives, predictably, every year, in the same months. And a thing that arrives predictably is a thing you can get ahead of.

Which is why we're writing about it in September and not in February.

Vitamin D is the odd one out in the nutrition world, because it's the only one you're supposed to manufacture yourself, using a raw material that is free, and that Britain is famously short of.

The mechanism is genuinely lovely. Your skin holds a cholesterol derivative called 7-dehydrocholesterol. UVB light, a specific, short-wavelength slice of sunlight that hits it and snaps a ring in the molecule open. That becomes previtamin D3, which rearranges itself into vitamin D3, which your liver and then your kidneys convert into the active hormone your body actually uses. You are, functionally, a solar panel with opinions.

The catch is UVB. Unlike UVA, which reaches us more or less year-round, UVB comes in at a steep enough angle to be filtered out by the atmosphere when the sun sits low in the sky. And the UK sits between roughly 50 and 60 degrees north, level with Newfoundland, north of most of the US mainland, north of the entire population of Canada.

The practical result: from about October to March, sunlight in the UK contains too little UVB for your skin to make meaningful vitamin D at all. Not "less". Effectively none. You can stand in bright, cold, beautiful February sunshine all afternoon and make almost nothing. The light is there. The wavelength isn't.

So through the winter you're not topping up. You're drawing down on whatever you banked over the summer, plus whatever you eat. That's the entire explanation for why the deficiency figure nearly doubles by March. Nothing has gone wrong with the nation's health. The angle of the sun changed.

The obvious follow-up: fine, so I'll load up in summer and coast.

Partly. Vitamin D is fat-soluble, so unlike the water-soluble vitamins you flush out daily, it does get stored in body fat and liver and released gradually. That store is real, and it's why the deficiency figures are lowest in late summer.

But it doesn't last six months. The 2026 review in Frontiers in Endocrinology by Christie and colleagues pulled together UK Biobank data covering adults aged 40 to 69, and the seasonal swing in it is stark: measured at a threshold of 50 nmol/L, 38 to 46% were below it in summer and autumn, rising to 68 to 71% in winter and spring.

Two honest notes on that figure, because we said we'd get the numbers right. That 50 nmol/L threshold is not the UK's definition of deficiency. The official line is 25 nmol/L, which is where the 18% and 31% come from. And that cohort is 40 to 69 year olds, not the whole population. So don't read "70% of Britain is deficient in winter", because that isn't what it says. Read it for the shape instead: whatever line you draw, roughly twice as many people fall below it by the end of winter as at the end of summer. The store drains. It just doesn't drain to zero for everyone.

Vitamin D status varies more between people than almost any nutrient we sell, and it's worth knowing where you sit rather than assuming you're average.

The same review found deficiency below 25 nmol/L in 12% of White participants, 35% of Black participants and 54% of Asian participants. The reason is straightforward and not remotely a flaw: melanin is a highly effective natural sunscreen, which is exactly what you want at the latitudes where it evolved, and a genuine handicap at 53 degrees north in October. More melanin means more UVB exposure needed for the same vitamin D yield. In a country that barely supplies enough UVB for anyone, that difference matters a lot.

Other things that shorten your summer store: being indoors during the middle of the day, when UVB is at its brief best; covering up, whether for sun safety, skin sensitivity or modesty; being over 65, since skin gets less efficient at the conversion with age; and having a higher body fat percentage, because vitamin D distributes into fat tissue and less of it stays circulating.

And the one that always gets a rueful laugh, being sensible about sunburn. Sunscreen blocks UVB, because blocking UVB is precisely its job. We are not going to tell you to skip it. We'd just rather you knew that being good about sun safety and being good about vitamin D pull in opposite directions, and that supplementing is how you get to do both.

Worth saying plainly, because it's easy to assume supplement advice only ever comes from supplement companies.

The UK government recommends that everyone consider a daily supplement containing 10 micrograms (400iu) of vitamin D during autumn and winter and that people in the higher-risk groups above consider it year-round. That's not our line. That's the NHS position, based on SACN's 2016 review, and it's about as unglamorous and uncontroversial as public health advice gets.

10 micrograms is a floor, not a target — it's the amount modelled to keep most of the population above that 25 nmol/L bone-health line. Plenty of people choose more. What matters is that "should I take vitamin D in a British winter" was settled some time ago, by people with no supplements to sell.

That's worth a sentence of context, because it's the reason this company exists at all. Vitamin D comes in two forms: D2, from yeast and fungi, and D3, the form your own skin makes and the one your body uses most efficiently. For decades every D3 supplement on earth was made from lanolin — the grease from sheep's wool. Which meant that for a vegan, the choice was D2, or nothing, or a compromise.

Our founders are scientists, and they were not vegans, and they looked at that and thought it was a rubbish state of affairs. So they went and found D3 in lichen and built Vitashine — the world's first vegan-certified D3. Same molecule your skin makes from sunshine. No sheep involved.

Our hero dose for autumn is Vitamin D3 2500iu. It comfortably clears the government's 400iu floor with room to spare, and it's one small tablet a day through the dark months.

he rest of the range exists because people are different, not because bigger is better:

• 1000iu — a steady, modest daily dose, and a sensible starting point.
• 1000iu Spray — for anyone who finds tablets difficult, and for children who'd rather not be negotiated with every morning.
• 4000iu — for people who've been advised they need more, or who know they're in one of the higher-risk groups.
• 4000iu + K2 — vitamin D increases calcium absorption; vitamin K2 is involved in directing calcium into bone. A pairing for anyone whose focus is bone health.

Pick the one that suits your situation, not the biggest number on the shelf. 4000iu a day is the upper level EFSA considers safe for adults over the long term — it's a ceiling, not a goal, and there's no prize for climbing towards it.

You can't build a vitamin D store in December. The UVB isn't there, so the only inputs left are diet and a supplement.

Starting in September doesn't mean panicking early. It means the store you built over the summer gets topped up rather than replaced, so it never runs down to the point where you'd notice. It's the difference between maintaining something and rescuing it, and maintaining is considerably less effort.

The 31% figure isn't a warning about the country's health. It's a calendar. Winter is on the way, the sun's going to stop doing this particular job in a few weeks, and now is a good time to pick up the slack.

Office for Health Improvement and Disparities. (2025). National Diet and Nutrition Survey 2019 to 2023. GOV.UK. https://www.gov.uk/government/statistics/national-diet-and-nutrition-survey-2019-to-2023

Christie, S., Uday, S., Hill, T. R., Rhodes, J. M., Sempos, C. T., Ruxton, C. H. S., & Hewison, M. (2026). Vitamin D in the UK: an urgent call to redefine the threshold for deficiency. Frontiers in Endocrinology, 17. https://doi.org/10.3389/fendo.2026.1870408