Vitamin K2 and Vitamin D3: How They Work Together

Vitamin D3 and vitamin K2 turn up together on so many labels that shoppers reasonably assume the pairing is marketing. It is not. The two vitamins do different jobs on the same mineral, and the reason they share a bottle is a question of where calcium ends up once your body has absorbed it.

Below: what each vitamin actually does, why they are formulated as a pair, what a supplement sold in Great Britain is legally allowed to say about them, and the two questions people ask most — when to take them, and what the side effects are.

TL;DR

• D3 absorbs, K2 directs. Vitamin D increases calcium absorption; vitamin K activates osteocalcin and matrix Gla protein, the proteins that handle calcium once it is in circulation^[6].
• Both are authorised for bones. In Great Britain, "contributes to the maintenance of normal bones" is on the register for vitamin D, vitamin K and calcium alike^[4]. Nothing beyond the register may be claimed.
• Deficiency is a real UK problem. 18% of adults aged 19 to 64 were vitamin D deficient in 2019–2023, rising to 31% in the January-to-March window^[3].
• Take them with food that contains fat. Both are fat-soluble. Timing of day matters far less than whether there is a meal involved^[8].
• If you take warfarin, ask first. Vitamin K interacts with warfarin, and the NHS advises getting advice before starting any vitamin K supplement^[9].

What vitamin D3 actually does

Vitamin D3 — cholecalciferol — is the form your skin makes from summer sunlight and the form used in most supplements. Its central job is unglamorous: it makes calcium and phosphorus available. Without enough of it, a good chunk of the calcium in your diet passes straight through you.

That is why the authorised Great Britain wording is phrased the way it is. Vitamin D "contributes to normal absorption/utilisation of calcium and phosphorus", "contributes to normal blood calcium levels", and "contributes to the maintenance of normal bones", "normal muscle function" and "the normal function of the immune system"^[4]. Those are the sentences a label is allowed to print. Anything stronger — anything about treating, preventing or curing — is not permitted on a food supplement, and for good reason.

The UK problem is supply. Between roughly October and March, sunlight at British latitudes is too weak for the skin to make meaningful vitamin D, which is why the NHS advises that everyone consider a 10-microgram daily supplement through autumn and winter, and why some groups — people who are housebound, who cover their skin, or who have darker skin — are advised to supplement year round^[1]. The Scientific Advisory Committee on Nutrition set the 10-microgram reference intake in its 2016 review, on musculoskeletal grounds^[5].

How short of vitamin D are people in the UK, really?

The National Diet and Nutrition Survey measures it directly, in blood. Deficiency here means serum 25-hydroxyvitamin D below 25 nmol/L — the threshold below which musculoskeletal health is at risk. Few vitamin D foods exist and the portions are small, so diet alone rarely closes the gap. These are the 2019–2023 figures.

The seasonal swing is the part that gets lost in the averages. Measured in January to March alone, deficiency reaches 31% of adults aged 19 to 64 and 36% of 11-to-18-year-olds^[3]. Roughly a third of British teenagers, in other words, spend late winter below the threshold. If you want the fuller picture of what low status looks like and who is most affected, we cover it in vitamin D deficiency in the UK.

What vitamin K2 actually does

Vitamin K comes in two families: K1 (phylloquinone), which you get from leafy greens, and K2 (menaquinones), which comes from fermented foods and gut bacteria. K2 is subdivided further by the length of its side chain — MK-4 and MK-7 are the two you see on labels.

Vitamin K's role is to switch proteins on. It acts as a cofactor for an enzyme that adds a carboxyl group to certain proteins, and until that happens those proteins cannot bind calcium. Two of them matter here: osteocalcin, made by bone-building cells, and matrix Gla protein, made in cartilage and vascular tissue^[6]. Uncarboxylated, they are inert. Carboxylated, they can do their job.

MK-7 is the form used in most modern supplements — including ours — because of pharmacokinetics rather than folklore. In a head-to-head comparison with synthetic K1, natto-derived MK-7 showed markedly better absorption and a far longer half-life, meaning a single daily dose keeps blood levels steady rather than spiking and clearing^[7]. Reviews of the bone literature reach the same conclusion about MK-7's staying power^[6].

A necessary caveat, and it is the one most supplement pages skip: the wider research on vitamin K-dependent proteins is not the same thing as an authorised claim. Great Britain permits exactly two health claims for vitamin K — normal blood clotting, and maintenance of normal bones^[4]. Everything else is an active area of study, and we are not going to dress a research literature up as a benefit.

Why D3 and K2 are sold together

Put the two mechanisms side by side and the logic is obvious. Vitamin D increases how much calcium you take in. Vitamin K activates the proteins that determine what happens to it. One raises supply; the other handles allocation. Neither substitutes for the other, and the NHS itself notes there is "some evidence vitamin K may help keep bones healthy" alongside its established clotting role^[2].

What a label is legally allowed to say

In Great Britain, a food supplement may only make health claims that appear on the GB nutrition and health claims register^[4]. Not "similar to", not "supported by studies" — on the register, in the authorised wording. Here is what that leaves for the three nutrients in a typical D3+K2 product.

The best time to take vitamin D3 and K2

Both vitamins are fat-soluble, so the thing that moves the needle is fat, not the clock. In a study of patients whose vitamin D levels had stalled, simply switching the dose to the largest meal of the day raised serum 25-hydroxyvitamin D by around 50% over two to three months — same dose, same product, different moment^[8].

So: take it with a meal that contains some fat. Breakfast with eggs, lunch with olive oil, dinner — whichever one you will not forget. Morning versus evening has no meaningful evidence behind it for this pair. Consistency beats optimisation; a supplement taken four days a week is doing less than a worse one taken seven.

Vitamin D3 and K2 side effects

At the intakes sold in the UK, both vitamins have a good safety record — but they are not equivalent in risk, and the asymmetry matters.

Vitamin D has a firm ceiling. Excess vitamin D over time raises blood calcium (hypercalcaemia), which is why the NHS is explicit: do not take more than 100 micrograms (4,000 IU) a day as an adult; no more than 50 micrograms for children aged 1 to 10; no more than 25 micrograms for babies under 12 months^[1]. The European Food Safety Authority reached the same 100-microgram adult limit and reaffirmed it in its 2023 review^[10]. Short-term symptoms of overdoing it include nausea, poor appetite, thirst and needing to urinate frequently.

Vitamin K2 has no established upper limit, and no toxicity has been demonstrated at supplemental intakes. The NHS positions adult requirements at about 1 microgram per kilogram of body weight per day — roughly 70 micrograms for a 70 kg adult — and notes that taking vitamin K supplements at that sort of level "is unlikely to cause any harm"^[2]. The real vitamin K caution is not toxicity. It is interaction.

Medicines to check before you start

Warfarin works by blocking vitamin K. Changing your vitamin K intake — up or down, from food or from a supplement — can shift your INR. NHS guidance is not to avoid vitamin K but to keep intake consistent, and to get advice before taking supplements containing it^[9]. If you are on warfarin, that conversation with your anticoagulant clinic or pharmacist comes first; a supplement can usually be accommodated, but only when your dose is adjusted around it.

People taking other medicines — statins, calcium-channel blockers, thiazide diuretics, or anything for kidney conditions or high blood calcium — should ask a pharmacist rather than a search engine. That is not evasion. Pharmacists do this for free, they can see your full medicines list, and we cannot.

Where Vitgem fits

Our Vitamin D3 K2 Gummies deliver 4,000 IU (100 µg) of vitamin D3, 200 µg of vitamin K2 as MK-7, and 600 mg of calcium in one orange gummy a day. Every amount is printed per serving rather than folded into a blend, and the K2 is MK-7 for the absorption and half-life reasons set out above^[7].

One honest note about that number. 4,000 IU is the NHS upper limit for adults, not a mid-range dose^[1]. That is deliberate for a high-strength winter supplement, and it means two things: do not take more than one gummy, and if you are already taking vitamin D in a multivitamin or a prescription, count that in before you add ours. If you would rather start lower, the 10-microgram NHS baseline is available almost everywhere, and it is the right choice for plenty of people. You can see how the rest of the range is dosed on the gummy vitamins page, and if the format itself is the thing you are unsure about, we tackle it head-on in do gummy vitamins actually work.

Frequently asked questions

Should vitamin D3 and K2 be taken together?

They work on the same mineral from two different angles, which is why they are commonly formulated as a pair: vitamin D contributes to normal absorption and utilisation of calcium, and vitamin K contributes to the maintenance of normal bones. Taking them in one product is convenient rather than compulsory — there is no requirement to take them in the same swallow.

What is vitamin K2 for?

Vitamin K is a cofactor for the enzyme that activates certain calcium-binding proteins, including osteocalcin in bone. In Great Britain the authorised claims for vitamin K cover normal blood clotting and the maintenance of normal bones; other areas remain research, not permitted claims.

Are there downsides to vitamin K2?

No upper intake level has been established for vitamin K, and supplemental intakes have not been shown to cause toxicity. The genuine caution is the warfarin interaction: vitamin K opposes how warfarin works, so anyone taking it should speak to their prescriber or pharmacist before starting a K2 supplement.

What is the best time of day to take vitamin D3 and K2?

With the largest meal of the day that contains some fat. Research on patients whose levels had plateaued found that moving the dose to the main meal raised blood levels of 25-hydroxyvitamin D by about half, with no change in dose. Morning or evening makes little difference by comparison.

What are the side effects of vitamin D3 and K2?

Within recommended intakes, side effects are uncommon. Excess vitamin D over time can raise blood calcium, producing nausea, thirst, poor appetite and frequent urination — the reason for the 100-microgram adult daily limit. Vitamin K2 has no established upper limit, but it interacts with anticoagulant medicines.

Can I take vitamin D3 and K2 with other supplements?

Usually, yes — but add up your vitamin D across everything you take, including multivitamins and prescriptions, and keep the adult total at or under 100 micrograms (4,000 IU) a day. If you take prescription medicines, check with a pharmacist first.

How much vitamin D do I actually need in the UK?

The NHS recommends 10 micrograms a day for adults and children over one, and advises everyone to consider a supplement during autumn and winter because UK sunlight is too weak in those months for the skin to produce enough. Some groups are advised to supplement all year.

This article is general information about food supplements, not medical advice. Food supplements are not a substitute for a varied diet and a healthy lifestyle. If you are pregnant, breastfeeding, taking prescription medicines or managing a health condition, speak to your GP or pharmacist. How we research and reference these pages is set out in our editorial policy.