Vitamin K2: What It Is, MK-7, Best Foods, D3 and K2, and Warfarin (UK)

What is vitamin K2?

Vitamin K2 is the name for a group of compounds called menaquinones. They are shortened to MK-4 up to MK-13, depending on the length of their side chain. Vitamin K1 (phylloquinone) comes from plants, while K2 is mostly made by bacteria, including those used to ferment foods and some in your gut. Both are vitamin K and do the same basic job.

There is no separate UK or EFSA reference amount for K2. EFSA said in 2017 that the evidence on menaquinones was too limited, so its adult figure of 70 micrograms covers K1 only.

Vitamin K1 vs K2: what’s the difference?

Vitamin K1 and K2 differ in where they come from and how long they stay in your blood. K1 comes from green leafy vegetables and is cleared within hours. K2 comes from bacteria, fermented foods and animal foods, and its MK-7 form lasts for days. Both help make the proteins your body needs for blood clotting.

Most of the vitamin K in a UK diet is K1. If you want to know where vitamin D fits alongside it, see our guide to vitamin D vs D3.

What is vitamin K2 MK-7?

Vitamin K2 MK-7 (menaquinone-7) is the long-chain form of vitamin K2 found in natto and used in most K2 supplements. It is absorbed well and stays in the blood for days rather than hours, so levels build up steadily with daily use. That long life is also why it can interfere with warfarin at low doses.

What the studies found:

• MK-7 vs K1: in healthy volunteers, both were absorbed well, but MK-7 lasted much longer and built up to 7 to 8 times higher levels with daily intake. The researchers warned that 50 micrograms a day or more of MK-7 may interfere with anticoagulant treatment.
• MK-7 vs MK-4: in healthy Japanese women given 420 micrograms of each, MK-7 was detectable in the blood for up to 48 hours, while MK-4 couldn’t be detected at all at that nutritional dose.

MK-4 is used at very high doses (45mg a day) as a prescription osteoporosis medicine in Japan, which is a medical treatment rather than a supplement amount.

Which foods contain vitamin K2?

Natto, a Japanese dish of fermented soya beans, is by far the richest food source of vitamin K2, mainly as MK-7. Smaller amounts come from hard and soft cheeses, curd cheese, egg yolks, butter, chicken, other meat and liver. Leafy greens are rich in vitamin K1 rather than K2.

• Natto: the standout source. A Dutch food analysis found blood K2 levels after a natto meal were about 10 times higher than K1 levels after a large portion of spinach.
• Cheese: fermented with bacteria, so it contains longer-chain menaquinones; amounts vary by type and ripening.
• Egg yolk, butter, chicken and liver: mainly MK-4, in small amounts.
• Leafy greens: broccoli, spinach, kale and cabbage give you K1, which the NHS lists as the main source of vitamin K.

Fermented foods are popular for other reasons too; see our guide to signs of poor gut health and fermented foods. If you are cutting back on dairy or meat, vitamin K1 from greens still covers your basic need.

How much vitamin K2 do you need?

There is no official daily amount for vitamin K2 on its own. The NHS says adults need about 1 microgram of vitamin K (K1 and K2 together) per kilogram of body weight a day, so 65 micrograms at 65kg or 75 micrograms at 75kg, and that a varied diet should provide it.

• Food first: the vitamin K your body doesn’t need straight away is stored in the liver, so you don’t need it every day.
• Supplements: K2 products commonly contain 50 to 200 micrograms of MK-7. The NHS says taking 1mg (1,000 micrograms) or less of vitamin K supplements a day is unlikely to cause harm.
• High doses: the NHS says there isn’t enough evidence to know what high daily doses of vitamin K supplements might do.
• Add it up: multivitamins and D3 and K2 combinations may already contain vitamin K, so count every product. See are multivitamins worth taking? and can you overdose on vitamins?

What does vitamin K2 do?

These are the authorised health claims for vitamin K in Great Britain, which cover K1 and K2:

• Vitamin K contributes to normal blood clotting.
• Vitamin K contributes to the maintenance of normal bones.

Vitamin K works by activating proteins, including osteocalcin in bone and matrix Gla protein in blood vessel walls. That is why K2 is studied for bones and arteries. The research so far is mixed:

Our honest summary: vitamin K matters for clotting and bones, and you get it from food. Whether extra K2 from a supplement improves bones or arteries in healthy people isn’t settled. For other nutrients that matter to bones, see calcium supplements, magnesium and bone health and magnesium and calcium together.

Should you take vitamin D3 with K2?

For most people, taking vitamin D3 with K2 is optional, not necessary. The NHS advises 10 micrograms (400 IU) of vitamin D a day in autumn and winter and doesn’t advise adding K2. Vitamin D3 and K2 are paired because both are involved in handling calcium, but trials of the combination are limited and mixed.

• The theory: vitamin D contributes to normal absorption and utilisation of calcium, and vitamin K activates the proteins that bind calcium in bone.
• The evidence: a 2024 review of 31 studies of vitamin D with vitamin K in women after menopause found some positive results for bone and heart markers, but the doses, forms and trial lengths varied widely.
• If you choose a D3 + K2 product: check the D3 amount (most adults shouldn’t go above 100 micrograms, 4,000 IU, a day from all sources), keep vitamin K under 1mg a day, and don’t take it at all if you’re on warfarin.

For timing and dose, see the best time to take vitamin D, too much vitamin D and vitamin D in winter. For a product comparison, see the best vitamin D supplement UK guide.

Vitamin K2 , warfarin and other blood thinners

Vitamin K reverses the effect of warfarin, so a K2 supplement can make warfarin work less well and change your INR blood test result. The NHS says not to take vitamin K supplements while you’re taking warfarin.

• Warfarin (and other vitamin K antagonists such as acenocoumarol): speak to your GP or anticoagulant clinic before taking any product containing vitamin K, including D3 and K2 combinations and multivitamins. In one study, as little as 10 micrograms of MK-7 a day noticeably changed the effect of a similar blood thinner in some healthy volunteers.
• Food on warfarin: the NHS says foods high in vitamin K, such as broccoli, spinach and other green leafy vegetables, can affect warfarin. You can still eat them, but ask your warfarin clinic for advice and talk to your doctor before any big change to your diet.
• Other anticoagulants and antiplatelets (such as apixaban, rivaroxaban, edoxaban, dabigatran or clopidogrel): these work differently, but still check with your GP or pharmacist before adding any supplement.
• Also check first if you have liver or kidney disease, a bleeding or clotting disorder, are due an operation, or are pregnant or breastfeeding.

Starting or stopping a K2 supplement suddenly is the main risk for anyone on warfarin, so never do either without your clinic’s advice. If you take a statin as well, see do statins make you tired?