Check the NHS guidance and it says 10 micrograms a day. Check the supplement shelf and most bottles are selling 4,000 IU — that's 100 micrograms, ten times more. Neither figure is wrong. They're answering different questions, and knowing which one applies to you is the actual point of this article.
Why the NHS number is lower than you'd expect
Between October and early March, UK sunlight isn't strong enough for your skin to make vitamin D, and getting enough from diet alone is genuinely difficult. The NHS recommends everyone — including pregnant and breastfeeding women — consider a daily supplement of 10 micrograms (400 IU) through autumn and winter as a baseline, deficiency-prevention figure. It's not a target for optimal levels; it's the minimum most people need to avoid falling short.
Certain groups are advised to take that amount year-round rather than just in winter: people who are housebound, who cover most of their skin outdoors, or who have darker skin, since melanin reduces how efficiently skin produces vitamin D from sunlight.
Why supplements are often dosed so much higher
4,000 IU (100 micrograms) isn't a random marketing number — it happens to sit exactly at the EFSA-established tolerable upper intake level for adults, the highest daily amount considered safe for sustained use without a doctor's supervision. Products dosed at this level are aimed at people who want a more meaningful correction, often because a blood test has shown genuinely low levels, rather than people just topping up a diet that's already doing reasonably well.
The gap between 400 IU and 4,000 IU isn't a sign that one figure is wrong. It's the difference between “enough to prevent deficiency in most people” and “the upper end of what's considered safe for daily supplementation.” If you don't know which camp you're in, a blood test is genuinely the most useful next step — vitamin D deficiency doesn't have obvious symptoms until it's fairly advanced, and guessing in either direction has a cost.
What vitamin D is actually authorised to do
Vitamin D has a solid list of EFSA-authorised claims: it contributes to the normal function of the immune system, supports normal absorption and utilisation of calcium and phosphorus, contributes to the maintenance of normal bones and teeth, and — specifically — may reduce the risk of falling in adults aged 60 and over, a claim tied to its role in muscle function. Combined with calcium, it's also linked to reduced bone mineral loss in post-menopausal women.
That's a genuinely strong claims list as these things go, which is probably part of why vitamin D has become the default “just take this” recommendation. The claims are real; they're just specific, and don't extend to vaguer promises like mood or energy that get attached to it informally.
Why K2 keeps showing up alongside D3
Vitamin D increases how much calcium your body absorbs from food. Vitamin K2 has a separate, complementary role: it activates proteins that help direct that calcium toward bone rather than leaving it circulating in soft tissue. The two aren't interchangeable and don't have a combined EFSA claim between them, but the mechanistic logic for pairing them is sound enough that it's become a standard formulation choice rather than a marketing gimmick.
Ours delivers 4,000 IU of D3 alongside 150mcg of K2 across two tablets a day — at the EFSA upper limit for D3 specifically, so worth being mindful of if you're also getting vitamin D from a multivitamin or fortified foods, rather than stacking sources without checking the total.
The standard, sized properly
Becoming the standard here means giving you the actual reference points rather than just a bigger number and a sunshine metaphor. Ten micrograms keeps most people out of deficiency. A hundred micrograms is the safe ceiling for people correcting a real shortfall. Where you sit between those two is worth an actual blood test, not a guess — especially going into another UK winter.
Frequently asked questions
Can you take too much vitamin D3?
Yes, though it takes sustained high intake over time — EFSA sets the safe upper limit at 100 micrograms (4,000 IU) a day for adults. The risk mainly comes from stacking multiple high-dose sources at once (a multivitamin plus a standalone D3 supplement, for instance) rather than a single product taken as directed.
Do you need vitamin D3 all year round in the UK?
Most people only need it during autumn and winter, when UK sunlight isn't strong enough to trigger production in skin. People who are housebound, cover most of their skin outdoors, or have darker skin are generally advised to take it year-round, per NHS guidance.
Why is vitamin D3 often paired with K2?
D3 increases calcium absorption; K2 helps direct that calcium toward bone rather than soft tissue. They address different, complementary steps in the same process, which is why they're commonly formulated together.
How do you know if you're actually deficient in vitamin D?
Symptoms are often vague or absent until deficiency is fairly advanced, so a blood test (25-hydroxyvitamin D) through your GP is the reliable way to know, rather than guessing from how you feel. We've touched on related seasonal wellbeing patterns in What Happens During Menopause, if hormonal changes are also part of the picture for you.
At Oxford Origins, your wellbeing is our priority, and although we pride ourselves on our expertise in nutrition and supplementation, it's important to acknowledge the individuality of each person. Articles published by Oxford Origins are not intended to treat, diagnose, cure or prevent any disease, or replace the advice of your GP. We always recommend consulting with a healthcare provider if you encounter any health concerns, and we'll always be here to support you so you're never alone on your journey.
