Vitamin K2: What It Is, How It Differs From K1 and What Evidence Shows

Vitamin K2 has moved from obscurity to the front of supplement shelves, usually alongside vitamin DClinical. Some of the interest is justified — the biochemistry is real and specific. Some of it runs ahead of the clinical evidence. And there is one interaction, with warfarin, that genuinely matters.

Key points

• Vitamin K contributes to normal blood clotting and to the maintenance of normal bones. These are the authorised claims and they apply to vitamin K as a nutrient.
• K1 (phylloquinone) comes from green leafy vegetables and dominates UK dietary intake. K2 (menaquinones) comes from fermented foods, some animal foods, and gut bacteria.
• UK guidance suggests approximately 1 microgram per kilogram of body weight per day of vitamin K is adequate; most people meet this from food.
• Evidence on K2 for bone density and arterial health is promising in places but not conclusive; it does not support disease-prevention claims.
• Anyone taking warfarin must not change vitamin K intake, including supplements, without medical advice.

What vitamin K is

Vitamin K is a family of fat-soluble compounds sharing a naphthoquinone ring:

• K1 (phylloquinone) — made by plants, concentrated in green leaves. It is the main dietary form in the UK and is taken up efficiently by the liver.
• K2 (menaquinones, MK-n) — produced by bacteria. MK-4 is found in some animal tissues; longer-chain forms such as MK-7 come from fermentation, most famously from natto. MK-7 has a substantially longer half-life in blood than K1, which is why supplements favour it.
• K3 (menadione) — a synthetic form not used in human supplements.

What it does in the body

Vitamin K is a cofactor for gamma-glutamyl carboxylase, the enzyme that activates a family of "Gla" proteins by adding carboxyl groups. Two groups matter here:

• Clotting factors (II, VII, IX, X) made in the liver. This is why vitamin K is essential for normal blood clotting and why warfarin, a vitamin K antagonist, works.
• Osteocalcin in bone and matrix Gla protein (MGP) in vascular tissue. Osteocalcin binds calcium in the bone matrix; MGP inhibits calcification of arteries.

That second group is the mechanistic basis for the interest in K2 — the hypothesis that adequate vitamin K helps direct calcium into bone rather than soft tissue. The mechanism is sound. Whether supplementation produces clinically meaningful outcomes in well-nourished people is the open question.

Food sources and UK examples

Vitamin K1:

• Kale, spinach, spring greens, cabbage
• Broccoli and Brussels sprouts
• Lettuce, watercress, rocket
• Vegetable oils, especially rapeseed and soybean oil

A single serving of cooked greens can supply several times the daily adequate intake, which is why UK deficiency is rare.

Vitamin K2:

• Natto (fermented soybeans) — exceptionally high in MK-7, though rarely eaten in the UK
• Hard and soft cheeses, especially traditionally fermented types such as Gouda and Brie
• Curd cheese, egg yolk, butter
• Liver, and some meats
• Sauerkraut and other fermented vegetables in smaller amounts

Gut bacteria produce menaquinones, but how much reaches the circulation is uncertain and it is not considered a dependable source.

Who may be at higher risk of low status

• Newborn babies, who have low stores and low intake. UK practice is to offer vitamin K at birth to prevent vitamin K deficiency bleeding — a well-established and important intervention.
• People with fat malabsorption — coeliac disease, Crohn's disease, cystic fibrosis, cholestatic liver disease, pancreatic insufficiency
• People on long-term broad-spectrum antibiotics, which reduce gut flora
• People after bariatric surgery
• People with very low intakes of green vegetables and fermented foods

Clinically evident deficiency in healthy UK adults is uncommon.

What research says about supplements

Bone health. Vitamin K contributes to the maintenance of normal bones, which is an authorised claim. Beyond that, trials of K2 (particularly MK-4 at pharmacological doses in Japan, and MK-7 in European trials) have produced mixed results. Some show improvements in markers such as carboxylated osteocalcin and modest effects on bone mineral density; evidence of fracture reduction in Western populations is not consistent. Systematic reviews generally conclude the evidence is insufficient to recommend K2 as a routine osteoporosis intervention. Osteoporosis treatment should be guided by a clinician following NICE guidance.

Arterial calcification. The MGP mechanism has driven trials in people with kidney disease and in the general population, using markers such as dephosphorylated-uncarboxylated MGP and arterial stiffness. Some trials show favourable changes in these markers; translation into reduced cardiovascular events has not been demonstrated. Claims that K2 "cleans arteries" or "prevents calcification" are not authorised and are not supported.

Combining with vitamin DClinical. The pairing is popular on the theory that high vitamin DClinical increases calcium absorption and that vitamin K then directs it appropriately. It is biologically coherent, and combination products are widely sold, but there is no strong outcome evidence that adding K2 is necessary at normal vitamin D doses. It is a reasonable formulation choice rather than a proven requirement — provided you are not taking warfarin.

MK-4 versus MK-7. MK-7 has a longer half-life and raises circulating levels more with once-daily dosing, which is why most European products use it. This is a pharmacokinetic advantage, not evidence of clinical superiority.

Safety and interactions

• Vitamin K from food and supplements has low toxicity, and no UK safe upper level has been set. The Expert Group on Vitamins and Minerals considered supplemental intakes around 1 mg a day unlikely to cause harm, well above typical supplement doses.
• Warfarin and other coumarin anticoagulants: vitamin K directly opposes their action. Starting, stopping or changing a vitamin K supplement can destabilise INR and cause either bleeding or clotting risk. Do not take vitamin K supplements while on warfarin unless your anticoagulant clinic or GP has advised it. Consistency of dietary vitamin K also matters — the advice is a steady intake of green vegetables, not avoidance.
• Direct oral anticoagulants (apixaban, rivaroxaban, edoxaban, dabigatran) do not act through vitamin K and are not affected in the same way, but tell your prescriber what you take.
• People with kidney disease or on dialysis should seek medical advice before supplementing.
• Vitamin K is fat-soluble; take with a meal containing fat for better absorption.

Choosing a supplement responsibly

• Consider whether you need it. If you eat green vegetables regularly, your K1 intake is likely adequate.
• Check for warfarin before anything else — this is the single most important step.
• MK-7 is the usual form in once-daily products; check the microgram amount, commonly 45–200 µg.
• Combination D3+K2 products are convenient, but note that if you are advised to stop vitamin K you would lose the D as well; separate products give more control.
• Look for the form and source stated clearly — "vitamin K2 as MK-7 (natto-derived)" is more informative than "vitamin K complex".
• Vegan suitability: MK-7 from natto fermentation is typically vegan; MK-4 products may not be.

Frequently asked questions

What is the difference between vitamin K1 and K2?

K1 comes from green leafy vegetables and is the main dietary form; K2 is bacterial in origin, found in fermented foods and some animal products. K2 forms such as MK-7 stay in the blood longer, while K1 is taken up rapidly by the liver for clotting factor production.

Do I need to take K2 with vitamin D?

Not necessarily. The pairing is biologically reasonable, but there is no strong outcome evidence that K2 is required alongside standard vitamin D doses. If you take warfarin, you should not add K2 without medical advice.

Can I take vitamin K2 if I am on warfarin?

Not without advice from your GP or anticoagulant clinic. Vitamin K opposes warfarin and can make your INR unstable.

Does vitamin K2 prevent artery calcification?

Studies show effects on laboratory markers, but there is no demonstrated reduction in cardiovascular events. Claims of preventing calcification are not authorised or supported.

How much vitamin K do I need?

UK guidance suggests around 1 microgram per kilogram of body weight a day, which most people obtain from food.

Is vitamin K2 safe long term?

No toxicity has been established at supplement doses and no safe upper level is set. The main safety issue is the interaction with vitamin K antagonist anticoagulants.

Conclusion

Vitamin K2 is a legitimate nutrient with clear biochemistry and two authorised functions: normal blood clotting and maintenance of normal bones. The wider claims made for it — bone density, arterial health — remain an active research area rather than established fact. If you eat greens and fermented foods, you are probably fine. If you take warfarin, speak to your clinic before touching a K2 supplement.