Vitamin K2 (MK-4 / MK-7)

What it is

Vitamin K comes in two main families: K1 (phylloquinone) from leafy greens, and K2 (menaquinones) made by bacteria and found in fermented foods such as natto, and in small amounts in some cheeses, egg yolks and meat. MK-4 and MK-7 are the two forms you will most often see as supplements.

People take K2 mainly for bone and artery health, often paired with vitamin D3 and calcium. The idea is that vitamin D helps you absorb calcium, and K2 helps put it in the right place.

It is not essential for most healthy people eating a varied diet to supplement, and the research is more mixed than the marketing suggests.

How it works

Vitamin K is needed to activate certain proteins, including osteocalcin (which binds calcium in bone) and matrix Gla protein (which helps stop calcium building up in blood vessel walls). Without enough K, those proteins stay inactive.

What the evidence says

• Strong Activating bone proteins (a lab marker)
  Supplements reliably reduce undercarboxylated osteocalcin, but that is a marker, not proof of stronger bones.
• Limited Bone density and fracture risk
  Some trials, mostly using MK-4 at high doses or in postmenopausal women, hint at benefit; results are inconsistent.
• Limited Arterial stiffness and calcification
  Small trials show modest changes in some markers, but data on heart attacks or strokes is lacking.
• Strong Preventing vitamin K deficiency bleeding
  This applies to K1 given to newborns, not K2 supplements for adults.

Evidence ratings are our plain-English summary of how much good human research exists, not a promise of results. How we rate evidence.

Dosage and timing

• There is no official UK recommended intake for K2 specifically; for total vitamin K, the UK figure is around 1 microgram per kg of body weight per day, mostly met by diet. Check current guidance.
• Typical MK-7 supplement doses are 90 to 200 micrograms a day; MK-4 studies have used 45 milligrams, which is a very different scale.
• Take with a meal containing fat for better absorption.
• If you take warfarin, do not start or stop without talking to your anticoagulation clinic.

Forms and what to look for

• MK-7 has a longer half-life, so one daily dose keeps levels stable; it is usually made by fermentation of natto.
• MK-4 is cleared fast and is usually given in multiple large daily doses in studies; low-dose MK-4 supplements may do little.
• Look for all-trans MK-7, which is the active form.
• K1 and K2 are different; a product labelled just "vitamin K" may be K1.

What to expect, and when

• Lab markers of K status can shift within weeks.
• Any effect on bone density would take a year or more to see in trials.
• You are unlikely to feel anything.

Side effects

• Generally well tolerated at normal doses.
• Occasional stomach upset, nausea or loose stools.
• Rare skin reactions or rashes.
• Natto-derived products can carry a fishy or fermented smell or taste.

Interactions

• Warfarin: vitamin K directly opposes warfarin, so any change in intake can change your INR. Do not supplement without clinical supervision.
• Other anticoagulants (apixaban, rivaroxaban, edoxaban) work differently, but check with your pharmacist.
• Orlistat and bile acid binders can lower absorption.
• Long courses of antibiotics can reduce gut-made K, which is mostly a concern in poor diets.

Who should avoid it

• Anyone on warfarin unless supervised.
• People with clotting disorders or a history of blood clots should check with their doctor first.
• Pregnant or breastfeeding women should follow NHS advice and avoid high-dose supplements unless prescribed.
• People with liver disease should speak to a clinician.

Risks and safety

• No upper limit has been set for K2 because of low toxicity, but that is not the same as proven safety at all doses.
• The main real-world risk is interaction with anticoagulants.
• It is not banned in sport.
• Some products have low or unverified doses; label accuracy varies.

Buying well: quality and testing

• Check for the form (MK-7, all-trans) and the dose in micrograms per serving.
• Third-party testing (NSF, USP, Informed Sport) is a plus.
• Avoid products with vague blends and no doses.
• Combined D3+K2 products are fine if the D3 dose suits you.

Bottom line

K2 is a reasonable, low-risk add-on for people interested in bone health who do not take warfarin, but it is not a proven way to protect your bones or arteries. Food sources, enough vitamin D, calcium and strength training matter more.