Vitamin K, a fat-soluble nutrient best known for blood clotting, plays an increasingly recognised role in bone metabolism. It activates key bone proteins such as osteocalcin, which help incorporate calcium into the skeleton. Whilst the UK reference nutrient intake was established primarily for coagulation, emerging evidence suggests vitamin K status may influence fracture risk and bone quality. Found abundantly in green leafy vegetables and fermented foods, vitamin K works alongside calcium and vitamin D to support skeletal health. Understanding its dietary sources and potential supplementation is relevant for individuals concerned about osteoporosis, though current UK guidance does not routinely recommend vitamin K supplements for bone health.

What Is Vitamin K and Why Does It Matter for Bones?

Vitamin K is a fat-soluble vitamin essential for several physiological processes, most notably blood coagulation and bone metabolism. The term 'vitamin K' encompasses a group of structurally similar compounds, primarily vitamin K1 (phylloquinone) found in green leafy vegetables, and vitamin K2 (menaquinones), which exists in several subtypes produced by bacteria and found in fermented foods and animal products.

Whilst vitamin K's role in blood clotting has been well established for decades, its importance for skeletal health has gained increasing recognition in recent years. Vitamin K acts as a cofactor for the enzyme gamma-glutamyl carboxylase, which is responsible for activating several bone proteins, most notably osteocalcin. Without adequate vitamin K, these proteins remain undercarboxylated and cannot function properly in bone mineralisation.

The UK reference nutrient intake (RNI) for vitamin K is approximately 1 microgram per kilogram of body weight daily. For example, a person weighing 70kg would need about 70 micrograms daily. These recommendations were established primarily to ensure adequate coagulation function, and there is ongoing debate about whether higher intakes may be beneficial for bone health. Some observational studies have suggested associations between low vitamin K status and increased fracture risk, though these associations do not prove causation and the evidence base continues to evolve.

For individuals at risk of osteoporosis or fragility fractures, understanding vitamin K's role alongside calcium and vitamin D is increasingly relevant. The Royal Osteoporosis Society recognises vitamin K as one of several nutrients that may support bone health, though it is not currently part of standard osteoporosis prevention guidelines in the UK.

Dietary Sources of Vitamin K for Bone Health

Achieving adequate vitamin K intake through diet is generally straightforward for most people following a balanced eating pattern. Vitamin K1, which constitutes approximately 90% of dietary vitamin K in Western diets, is abundant in:

• Green leafy vegetables: Kale, spinach, spring greens, broccoli, and Brussels sprouts are particularly rich sources, with a single portion often providing several times the daily requirement

• Vegetable oils: Rapeseed oil and soya bean oil contain moderate amounts

• Herbs: Parsley, coriander, and basil provide concentrated amounts when used fresh

A 100-gram serving of cooked kale can provide over 800 micrograms of vitamin K1—more than ten times the daily reference intake for a 70kg adult, though values vary by variety and cooking method. Even smaller portions of these vegetables contribute meaningfully to daily requirements.

Vitamin K2 is found in different food sources and may have distinct effects on bone metabolism compared to K1, though research is ongoing. Key dietary sources include:

• Fermented foods: Natto (fermented soybeans) is exceptionally rich in the MK-7 form of vitamin K2, though it is not commonly consumed in the UK

• Dairy products: Cheese, particularly hard and soft ripened varieties, contains moderate amounts

• Meat and eggs: Chicken, beef, and egg yolks provide smaller quantities

For optimal bone health, the NHS Eatwell Guide recommendations—which emphasise plenty of vegetables and varied protein sources—naturally support adequate vitamin K intake. Individuals following restrictive diets, particularly those avoiding both green vegetables and animal products, may require particular attention to ensure sufficiency. Vitamin K is relatively heat-stable, though prolonged boiling may cause losses into cooking water, and it is better absorbed when consumed with some dietary fat.

If you are taking warfarin or other vitamin K antagonists, it's important to keep your vitamin K intake consistent from day to day. Sudden changes in vitamin K consumption can affect your anticoagulation control. Speak with your healthcare professional before making significant changes to your diet.

Should You Take Vitamin K Supplements for Your Bones?

The decision to take vitamin K supplements for bone health should be individualised and ideally discussed with a healthcare professional. Currently, NICE guidance does not routinely recommend vitamin K supplementation for osteoporosis prevention or treatment in the general population. The evidence base, whilst promising, remains insufficient to establish vitamin K supplements as a standard intervention for bone health in UK clinical practice.

Several randomised controlled trials have investigated vitamin K supplementation (particularly vitamin K2 as MK-4 or MK-7) and bone outcomes, with mixed results. Some studies have shown improvements in bone mineral density or reductions in undercarboxylated osteocalcin, whilst others have found no significant benefit. A Cochrane review examining vitamin K for preventing fractures concluded that whilst vitamin K2 (specifically MK-4 at high doses used in Japan) may reduce fractures, the evidence quality was limited and further research is needed.

Populations who may be at risk of low vitamin K status include:

• Individuals with malabsorption conditions (coeliac disease, inflammatory bowel disease, cystic fibrosis) may have impaired vitamin K absorption

• People taking long-term antibiotics that disrupt gut bacteria producing vitamin K2

• Individuals with chronic kidney disease or on dialysis, where vitamin K metabolism may be altered

If you fall into one of these groups, discuss with your healthcare provider whether assessment of vitamin K status is appropriate before considering supplements.

Important warning: If you are taking warfarin or other vitamin K antagonists, do not take vitamin K supplements unless specifically advised by your specialist, as this can seriously interfere with anticoagulation control and increase risk of harmful blood clots or bleeding.

If supplementation is considered appropriate, typical doses in research studies range from 45 micrograms to 200 micrograms daily for MK-7, though some Japanese studies used much higher MK-4 doses (45 milligrams daily) that are not standard in UK practice. Over-the-counter supplements in the UK vary widely in strength. There is no established tolerable upper intake level for natural forms of vitamin K (K1 and K2), and toxicity from supplementation has not been reported in healthy individuals. Synthetic vitamin K3 (menadione) is not permitted in UK supplements due to potential toxicity.

Supplements should complement, not replace, a nutrient-rich diet and established bone health interventions such as adequate calcium, vitamin D, weight-bearing exercise, and smoking cessation.

How Vitamin K Supports Bone Strength and Density

Vitamin K contributes to skeletal health through several distinct biological mechanisms, primarily involving the activation of bone matrix proteins. The most extensively studied is osteocalcin, a protein synthesised by osteoblasts (bone-forming cells) that requires vitamin K-dependent carboxylation to function properly. Carboxylated osteocalcin binds calcium ions and hydroxyapatite crystals, facilitating the incorporation of calcium into the bone matrix and promoting bone mineralisation.

When vitamin K status is inadequate, osteocalcin remains undercarboxylated and cannot effectively support bone formation. Undercarboxylated osteocalcin can be measured in blood as a research marker of vitamin K insufficiency, though this test is not routinely available or recommended in NHS clinical practice. Epidemiological studies have demonstrated associations between higher levels of undercarboxylated osteocalcin and increased fracture risk, suggesting that subclinical vitamin K deficiency may compromise bone quality even when coagulation remains normal.

Vitamin K also activates matrix Gla protein (MGP), which inhibits calcification of soft tissues, including blood vessels. Some researchers have proposed that adequate vitamin K status may help maintain appropriate calcium distribution in the body—sometimes referred to as the 'calcium paradox'—though this concept requires further clinical validation.

Emerging research suggests vitamin K may influence bone metabolism through effects on osteoclast activity (cells that break down bone) and inflammatory pathways, though these mechanisms are less well characterised and their clinical significance remains uncertain. Some laboratory studies indicate that vitamin K2, particularly the MK-4 form, may have additional effects on gene expression in bone cells beyond simple protein carboxylation.

It is important to emphasise that bone health is multifactorial. Vitamin K works synergistically with other nutrients—particularly calcium, vitamin D, and magnesium—as well as mechanical loading through physical activity. No single nutrient can compensate for deficiencies in others or for lifestyle factors such as smoking, excessive alcohol consumption, or physical inactivity.

For individuals concerned about bone health or at risk of osteoporosis, a comprehensive approach addressing diet, exercise, and medical risk factors remains the evidence-based standard. If you have concerns about bone health or fracture risk, contact your GP, who can assess your individual situation using tools such as FRAX or QFracture, and arrange appropriate investigations such as DEXA scanning if indicated according to NICE guidelines. After a fragility fracture, referral to a Fracture Liaison Service may be appropriate to reduce future fracture risk.

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