Vitamin K2 does something no other nutrient does as specifically: it activates proteins that direct calcium into bones and teeth, and away from soft tissues like arteries. Most people have heard of vitamin D helping calcium absorb, and calcium building bones β but without enough K2, calcium absorbed with the help of vitamin D can end up deposited in artery walls rather than bone. This is why K2 is increasingly described as the essential third piece of the bone-and-heart nutrient puzzle, alongside calcium and vitamin D.
Key Takeaways
- Vitamin K2 (menaquinone) activates osteocalcin (which mineralises bone) and matrix Gla protein (MGP), which prevents calcium from depositing in artery walls.
- Multiple randomised controlled trials have found that MK-7 (the most studied K2 form) reduces bone loss at the spine and wrist, and reduces arterial stiffness.
- K2 is different from K1 (phylloquinone, found in leafy greens) β K1 is used mainly for blood clotting in the liver, while K2 works throughout the body in bone and blood vessels.
- The NIH Adequate Intake for vitamin K is 90 to 120 mcg per day (including both K1 and K2), but most people have inadequate K2 status even when total vitamin K intake appears sufficient.
- People taking warfarin must consult their doctor before taking any vitamin K supplement.
Vitamin K1 vs K2: Why They Are Not the Same
Vitamin K is a family of fat-soluble vitamins, not a single compound. Vitamin K1 (phylloquinone) is found in leafy green vegetables and is used primarily in the liver to activate clotting factors β the coagulation cascade that stops bleeding. This is what warfarin (a blood thinner) targets by blocking K1.
Vitamin K2 (menaquinones) is found in animal foods and fermented foods and works throughout the body, especially in bone and blood vessels. The two most studied forms are MK-4 (found in animal products, shorter half-life) and MK-7 (found in natto and produced by gut bacteria, longer half-life, requires smaller doses). They activate different vitamin K-dependent proteins from K1, which is why K2 status can be low even in people who eat plenty of leafy greens.
Vitamin K2 and Bone Health
K2 activates osteocalcin, the most abundant non-collagen protein in bone. Carboxylated (activated) osteocalcin binds calcium and incorporates it into the bone mineral matrix. Without K2, osteocalcin remains undercarboxylated and cannot bind calcium effectively, leaving bone mineralisation incomplete.
Multiple randomised controlled trials, particularly using MK-7 at 180 to 360 mcg per day, have found reductions in bone loss at the lumbar spine and forearm and reductions in fracture risk. A large clinical study found that MK-7 supplementation for three years significantly reduced bone mineral loss in postmenopausal women. Evidence is strongest for postmenopausal women and older adults where bone turnover is accelerated. For more on the calcium and vitamin D side of bone health, see our guide on vitamin D3 and K2 benefits and our guide on calcium and magnesium supplement benefits.
Vitamin K2 and Heart Health
Matrix Gla protein (MGP) is produced by cells in artery walls. In its activated state (requiring K2), MGP is one of the most potent inhibitors of arterial calcification known. Undercarboxylated MGP is associated with increased vascular calcification, which stiffens arteries and raises cardiovascular risk.
A critical review published in the American Journal of Clinical Nutrition noted that evidence linking K2 intake to cardiovascular outcomes is growing but still limited and sometimes controversial. The most consistent finding is that higher menaquinone intake is associated with reduced coronary calcification and lower coronary heart disease risk in observational studies, and that a randomised trial found MK-7 supplementation reduced arterial stiffness in healthy postmenopausal women over three years. The review cautioned that large definitive trials are still needed before clinical recommendations can be firm.
Best Food Sources of Vitamin K2
- Natto: fermented soybeans, the richest dietary source β around 1,000 mcg of MK-7 per 100 g. A small serving covers multiple days’ worth.
- Hard cheeses: gouda and edam are among the highest, providing around 75 mcg per 100 g as MK-8 and MK-9.
- Soft cheeses: brie and camembert, around 56 mcg per 100 g.
- Egg yolks: around 32 mcg per 100 g as MK-4.
- Chicken and beef liver: good sources of MK-4.
- Butter and ghee from grass-fed animals: modest amounts of MK-4.
Fermented foods and animal products are the practical sources. Gut bacteria also produce some MK forms, which connects to the relationship between probiotics and vitamin status covered in our guide on probiotics and vitamins.
Who Is Most at Risk of Low K2 Status?
- People who eat little fermented food or animal products
- Adults over 40 (one study found adults over 40 had the largest K2 deficiency)
- Postmenopausal women, where bone turnover is accelerated
- People on long-term antibiotics, which can reduce gut bacteria that produce K2
- People with fat malabsorption conditions (K2 is fat-soluble)
K2 Supplement Forms and Dosing
MK-7 is the preferred supplement form because of its long half-life (around 72 hours) β a single daily dose maintains stable blood levels. Most trials use 90 to 360 mcg of MK-7 per day. MK-4 has a much shorter half-life and requires multiple daily doses. The NIH AI for total vitamin K is 90 mcg per day for women and 120 mcg for men, but this covers K1 and K2 together, and researchers have argued that the K2-specific requirement for bone and vascular protein activation is higher than what the AI accounts for.
K2 is fat-soluble β take it with a meal containing some fat for optimal absorption.
Who Should Be Cautious?
Anyone taking warfarin or other vitamin K antagonist anticoagulants must consult their doctor before taking K2 supplements. Even though K2 affects different proteins than K1, it can still influence the INR (the clotting measure used to monitor warfarin therapy). Do not self-supplement with K2 if you are on warfarin.
Frequently Asked Questions
Should I take vitamin K2 with vitamin D3?
Many nutritional researchers and clinicians recommend combining them because vitamin D increases calcium absorption and K2 ensures the calcium is directed correctly. Most over-50 and bone-health supplements now include both. Evidence that D3 alone without K2 increases arterial calcification risk is mixed, but the biological rationale for combining them is strong.
What is the difference between MK-4 and MK-7?
MK-4 is found in animal products, has a short half-life (around 6 hours) and requires multiple daily doses to maintain blood levels. MK-7 is found primarily in natto, has a 72-hour half-life and is effective with a single daily dose. Most supplement research uses MK-7 at 90 to 180 mcg per day.
Can vitamin K2 cause side effects?
At food and supplemental amounts, K2 is well tolerated and has no known upper limit set by the NIH (unlike most other nutrients). The primary concern is for people on warfarin, where vitamin K in any form can alter drug dosing. Otherwise, side effects are rare.
Final Thoughts
Vitamin K2 fills a specific and important gap in the calcium story: it activates the proteins that put calcium in bones and keep it out of arteries. The evidence base for bone health, particularly in postmenopausal women, is the most developed and comes from multiple randomised trials. The cardiovascular evidence is promising but still growing. The most practical starting point is food β adding fermented cheeses, egg yolks and, if you like it, natto to a regular diet. For people with confirmed bone loss or high cardiovascular risk, MK-7 supplementation at 90 to 180 mcg per day alongside vitamin D3 is a well-supported strategy β provided you are not on warfarin, in which case a doctor’s input is essential.
This article is for educational purposes only and is not a substitute for professional medical advice, diagnosis or treatment.
