Vitamin K2 is one of the most underappreciated nutrients in bone health — and one of the most misunderstood. Most people know that calcium builds bones and vitamin D helps absorb it. Far fewer know that without adequate vitamin K2, the calcium you absorb may not reach your skeleton at all. Instead, it can deposit in blood vessel walls, contributing to cardiovascular risk while leaving bones weaker. This guide explains exactly what vitamin K2 does, how it works alongside vitamin D3, who is most at risk of deficiency, and what the clinical evidence says about supplementation.
- Vitamin K2 activates osteocalcin — the protein that locks calcium into bone matrix — without it, calcium cannot be properly incorporated into bone
- Vitamin K2 and vitamin D3 work as a team: D3 increases calcium absorption, K2 directs where that calcium goes
- The best dietary source of vitamin K2 is natto (fermented soya); smaller amounts are found in hard cheeses, egg yolks, and chicken liver
- Most people in the UK consume very little vitamin K2 through diet; supplementation with MK-7 (100–200 mcg daily) is clinically supported for bone health
- If you have osteoporosis or osteopenia, vitamin K2 alone is not a treatment — it should complement a properly assessed, consultant-led plan including a DEXA scan
What Is Vitamin K2 and How Is It Different From Vitamin K1?
Vitamin K is a fat-soluble vitamin that exists in two main forms: K1 (phylloquinone) and K2 (menaquinone). Most people are familiar with K1, which is found abundantly in leafy green vegetables and plays a primary role in blood clotting. Vitamin K2 is a distinct compound with a very different function — and a very different distribution in the body.
While K1 is rapidly cleared from the bloodstream and taken up mainly by the liver, K2 has a much longer half-life and circulates to peripheral tissues including bone, blood vessels, and the kidneys. This pharmacokinetic difference is critical: K2 is the form that reaches the skeletal system in meaningful concentrations and activates the proteins responsible for bone mineralisation.
Within vitamin K2, there are several subtypes (MK-4 through MK-13), with MK-7 being the form most studied for bone health due to its longer half-life of approximately 72 hours, compared to just a few hours for MK-4. This makes MK-7 more practical as a supplement, as a single daily dose maintains stable blood levels.
How Vitamin K2 Directs Calcium Into Bone
The central mechanism by which vitamin K2 supports bone is through the activation of osteocalcin — a protein produced by osteoblasts (the cells responsible for building bone). In its inactive form, osteocalcin cannot bind calcium. Vitamin K2 carboxylates osteocalcin, converting it into its active form, which then attracts calcium ions and integrates them into the hydroxyapatite mineral matrix of bone.
Without sufficient vitamin K2, osteocalcin remains undercarboxylated — circulating but functionally inert. Studies have shown that elevated levels of undercarboxylated osteocalcin are independently associated with increased fracture risk, even when bone mineral density appears normal on a DEXA scan [1].
Vitamin K2 also activates matrix Gla protein (MGP), which inhibits calcium from depositing in soft tissues and arterial walls. This dual action — promoting bone mineralisation while preventing vascular calcification — is why K2 is increasingly studied in the context of both bone and cardiovascular health.
Vitamin D3 and K2: Why They Must Work Together
Vitamin D3 and vitamin K2 are frequently discussed in isolation, but they are functionally interdependent. Vitamin D3 increases intestinal absorption of calcium, raising circulating calcium levels in the bloodstream. This is essential — without it, bone mineralisation cannot occur regardless of dietary calcium intake.
However, raising calcium availability without ensuring it reaches bone creates a problem. Unguided calcium can accumulate in arterial walls, contributing to atherosclerosis and arterial stiffness. Vitamin K2 is the directing agent: it activates the proteins that pull calcium into bone and the proteins that prevent it from depositing in vessels.
Research published in the Journal of Bone and Mineral Research has demonstrated that combined supplementation with D3 and K2 produces greater improvements in bone mineral density than either nutrient alone [2]. In clinical practice, we consider them together — not as alternatives.
Dietary Sources of Vitamin K2
Vitamin K2 is not widely distributed in the modern Western diet, which largely explains why deficiency is so prevalent in the UK. The richest dietary sources are:
- Natto — a Japanese fermented soya product and by far the richest source, providing up to 1,000 mcg per 100g. Not commonly eaten in the UK.
- Hard cheeses — particularly Gouda and Edam, which contain MK-8 and MK-9 from bacterial fermentation during ageing. Approximately 75 mcg per 100g.
- Soft cheeses and Brie — lower amounts but still a meaningful source.
- Egg yolks — particularly from pasture-raised hens; approximately 32 mcg per 100g.
- Chicken liver and dark poultry meat — modest but consistent amounts of MK-4.
- Butter and cream — from grass-fed animals; small but present.
For context, the estimated adequate intake of vitamin K2 from diet alone in the UK is rarely achieved without deliberate dietary focus or supplementation. This is particularly relevant for those following plant-based diets, as most K2 sources are animal-derived (natto being the exception).
Should You Supplement With Vitamin K2?
For individuals who are not regularly consuming fermented foods or significant quantities of aged cheese, supplementation with vitamin K2 as MK-7 is a reasonable and well-tolerated approach. The clinically studied dose range is typically 90–200 mcg of MK-7 per day.
Important considerations:
- Warfarin interaction: Vitamin K2 should not be taken by individuals on warfarin (or other vitamin K antagonist anticoagulants) without medical supervision, as it can reduce the drug’s effectiveness.
- Fat-soluble absorption: As a fat-soluble vitamin, K2 is best absorbed when taken with a meal containing dietary fat.
- Combined supplements: Many bone health supplements combine D3 with K2-MK7. This is a practical approach, though dosage and quality vary significantly between products.
- Not a substitute for assessment: Supplementing with K2 is a supportive measure, not a replacement for a clinical bone health assessment, DEXA scan, and — where indicated — prescription medication.
We regularly see patients who are taking calcium supplements and vitamin D but have never heard of vitamin K2. In some cases — particularly post-menopausal women and older men — their DEXA scans show ongoing bone loss despite what appears to be an adequate supplementation programme. When we look more closely, K2 is often missing from the picture entirely. I would not go so far as to say K2 deficiency explains all of this, but the osteocalcin carboxylation pathway is real and clinically relevant. Our approach at LOC is to review the full nutritional picture alongside bone density data — not to treat supplements in isolation. If you are concerned about your bone health, the starting point should always be a proper assessment, not a supplement purchase.
Who Is Most at Risk of Vitamin K2 Insufficiency?
Certain populations are at higher risk of insufficient vitamin K2 status:
- Post-menopausal women — oestrogen plays a role in osteocalcin regulation; declining levels after menopause increase the relevance of K2 adequacy
- People on long-term antibiotic therapy — gut bacteria contribute to MK production; prolonged antibiotic use can deplete this source
- Those with fat malabsorption — conditions such as coeliac disease, Crohn’s disease, and cholestatic liver disease impair absorption of all fat-soluble vitamins including K2
- Individuals following plant-based or vegan diets — unless consuming natto regularly, dietary K2 intake is negligible
- Older adults generally — both dietary intake and gut production of menaquinones tend to decline with age
Frequently Asked Questions
Can I get enough vitamin K2 from diet alone without supplements?
It is possible if you regularly eat natto or large quantities of aged hard cheese, but for most people in the UK this is not realistic. The typical British diet provides very little MK-7 or MK-8. For those with bone health concerns, supplementation with MK-7 at 100–200 mcg daily is a practical and evidence-supported option — provided you are not on anticoagulant therapy.
Does vitamin K2 actually improve bone density?
Clinical trials show that MK-7 supplementation over 12–36 months can produce modest but statistically significant improvements in bone mineral density at the lumbar spine. A well-cited Dutch study (the MenaCal.7 trial) found that post-menopausal women taking MK-7 had significantly less bone loss at the lumbar spine and femoral neck than the placebo group over three years [2].
If I already take vitamin D, do I need to add K2?
Many clinicians now recommend taking them together. High-dose vitamin D3 supplementation without K2 may increase the production of osteocalcin without ensuring it is fully carboxylated — meaning the protein is made but not fully activated. The combination is generally considered safer and more effective than D3 alone for bone-specific outcomes.
Do I need a blood test to check my vitamin K2 levels?
Standard vitamin K blood tests are not a reliable indicator of K2 status. A more specific test measuring the ratio of undercarboxylated to carboxylated osteocalcin can provide a functional assessment of K2 adequacy, but this is not routinely available in NHS settings. At London Osteoporosis Clinic, we assess bone health through DEXA scanning and a full clinical review rather than relying on a single blood marker.
Related reading: Who Should Have a DEXA Scan? · Can Osteoporosis Be Reversed? · Osteoporosis Treatments at LOC
[1] Vergnaud P, et al. Undercarboxylated osteocalcin measured with a specific immunoassay predicts hip fracture in elderly women. Journal of Clinical Endocrinology & Metabolism. 1997. PubMed
[2] Knapen MHJ, et al. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporosis International. 2013. PubMed
Medically Reviewed by Dr. Taher Mahmud — Consultant Rheumatologist & Founder, London Osteoporosis Clinic. FRCP, MB BChir, MA (Cantab). Dr. Mahmud specialises in osteoporosis, metabolic bone disease, and inflammatory arthritis. He leads the clinical team at LOC’s centre at HCA The Shard, London.
This article is for informational purposes only and does not constitute medical advice. If you are concerned about your bone health, please consult a qualified clinician. Book a consultation at London Osteoporosis Clinic.
