Vitamin K is essential for blood clotting, enabling the liver to produce functional clotting factors that prevent excessive bleeding. This fat-soluble vitamin undergoes a process called gamma-carboxylation, which activates proteins including factors II, VII, IX, and X—key components of the coagulation cascade. Without adequate vitamin K, these clotting factors remain inactive, increasing bleeding risk. Whilst deficiency is uncommon in healthy adults consuming a balanced diet, certain medical conditions, medications such as warfarin, and malabsorption disorders can compromise vitamin K status. Understanding how vitamin K supports haemostasis helps individuals maintain optimal clotting function and recognise when medical assessment may be needed.

How Vitamin K Supports Blood Clotting

Vitamin K plays an essential role in the body's haemostatic system, acting as a crucial cofactor in the synthesis of several clotting factors. This fat-soluble vitamin enables the production of functional clotting proteins in the liver, including factors II (prothrombin), VII, IX, and X, as well as anticoagulant proteins C and S. Without adequate vitamin K, these proteins remain in an inactive form, unable to participate effectively in the coagulation cascade.

The mechanism by which vitamin K supports clotting involves a process called gamma-carboxylation. Vitamin K-dependent carboxylase enzymes add carboxyl groups to glutamic acid residues on clotting factor proteins, converting them into gamma-carboxyglutamic acid (Gla). This modification allows the clotting factors to bind calcium ions and attach to phospholipid surfaces on platelets and damaged blood vessels, which is essential for the formation of stable blood clots. During this process, vitamin K is oxidised and must be recycled by the enzyme vitamin K epoxide reductase to maintain adequate levels.

The body requires relatively small amounts of vitamin K for normal clotting function, with the NHS reference nutrient intake set at approximately 1 microgram per kilogram of body weight daily. However, certain medications, particularly warfarin and other vitamin K antagonists, work by inhibiting the vitamin K epoxide reductase enzyme, thereby reducing clotting factor production. This pharmacological principle underlies the use of warfarin as an anticoagulant therapy. Patients taking vitamin K antagonists require careful monitoring of their International Normalised Ratio (INR) to balance the risk of bleeding against thromboembolic events, and dietary vitamin K intake must be kept consistent to maintain stable anticoagulation. It's important to note that direct oral anticoagulants (DOACs) such as apixaban, rivaroxaban, edoxaban and dabigatran are not vitamin K-dependent and do not require INR monitoring.

Dietary Sources of Vitamin K for Healthy Clotting

Vitamin K exists in two main forms in the diet: phylloquinone (vitamin K1), found predominantly in green leafy vegetables, and menaquinones (vitamin K2), present in fermented foods and animal products. Understanding these dietary sources helps individuals maintain adequate vitamin K status for optimal blood clotting function.

Primary sources of vitamin K1 include:

• Dark green leafy vegetables – kale, spinach, spring greens, and cabbage are exceptionally rich sources, with a single 80g portion of cooked kale providing approximately 400-700 micrograms (several times the daily requirement)

• Brassica vegetables – broccoli and Brussels sprouts contain substantial amounts, while cauliflower provides more modest quantities

• Salad vegetables – lettuce, rocket, and watercress contribute meaningful quantities

• Herbs – parsley, coriander, and basil are concentrated sources when used fresh

• Vegetable oils – rapeseed oil and soybean oil contain moderate amounts

Vitamin K2 sources are more varied and include fermented foods such as natto (fermented soybeans), certain cheeses (particularly aged varieties like Gouda and Brie), and animal products including egg yolks, chicken, and liver. The gut microbiome also produces small amounts of vitamin K2, though the contribution to overall vitamin K status remains uncertain.

For most individuals following a balanced diet including regular portions of vegetables, vitamin K deficiency is uncommon. The NHS Eatwell Guide recommendations naturally provide adequate vitamin K through the suggested five portions of fruit and vegetables daily. However, absorption of vitamin K requires dietary fat, as it is a fat-soluble vitamin. Individuals with fat malabsorption conditions or those following extremely low-fat diets may require additional consideration. Cooking methods generally preserve vitamin K content well, though prolonged boiling may cause some losses.

Patients taking warfarin should maintain consistent rather than restricted vitamin K intake, as fluctuations can destabilise INR control. It's important to note that multivitamins or 'greens' supplements containing vitamin K can also affect INR in people taking vitamin K antagonists, and any significant dietary changes should be discussed with your anticoagulation clinic.

When to Seek Medical Advice About Vitamin K Levels

Most healthy individuals do not require routine monitoring of vitamin K levels, as deficiency is uncommon in those consuming a varied diet. However, certain circumstances warrant medical assessment and potential investigation of vitamin K status and clotting function.

Seek emergency medical attention (call 999 or go to A&E) if you experience:

• Severe or uncontrolled bleeding from any site

• Signs of intracranial bleeding (sudden severe headache, confusion, weakness on one side, difficulty speaking)

• Significant blood loss causing dizziness, rapid heartbeat, or fainting

Contact your GP or NHS 111 promptly if you notice:

• Unexplained bruising that appears without trauma or is disproportionate to minor injuries

• Prolonged bleeding from minor cuts that fails to stop within 10–15 minutes despite pressure

• Spontaneous nosebleeds (epistaxis) that are frequent, prolonged, or difficult to control

• Blood in urine (haematuria) or stools (melaena or fresh rectal bleeding)

• Unusually heavy menstrual bleeding (menorrhagia) representing a change from normal pattern

• Bleeding gums during routine tooth brushing or eating

These symptoms may indicate impaired clotting function, which could relate to vitamin K deficiency or other haematological conditions requiring investigation. Your GP will typically arrange blood tests including a full blood count and coagulation screen (prothrombin time/INR and activated partial thromboplastin time) to assess clotting function. If results suggest vitamin K deficiency, further investigations may explore underlying causes such as malabsorption disorders.

Specific groups should discuss vitamin K status proactively with their healthcare provider:

• Individuals with chronic liver disease, as the liver synthesises clotting factors

• Those with malabsorption conditions including coeliac disease, Crohn's disease, ulcerative colitis, or chronic pancreatitis

• Patients taking long-term antibiotics, which may disrupt gut bacteria producing vitamin K2

• Newborn infants, who routinely receive vitamin K prophylaxis to prevent vitamin K deficiency bleeding

• Anyone prescribed warfarin or other vitamin K antagonists requiring regular INR monitoring (note that direct oral anticoagulants such as apixaban, rivaroxaban, edoxaban and dabigatran are not affected by vitamin K and do not require INR monitoring)

NICE guidance emphasises that patients on warfarin should report any significant dietary changes or new medications to their anticoagulation clinic, as these may affect vitamin K metabolism and INR stability. Never supplement with vitamin K without medical supervision if taking anticoagulants, as this can dangerously reduce the medication's effectiveness.

Vitamin K Deficiency and Bleeding Risks

Vitamin K deficiency results in impaired synthesis of functional clotting factors, leading to a coagulopathy characterised by prolonged prothrombin time and increased bleeding risk. Whilst uncommon in healthy adults consuming adequate diets, certain populations face elevated risk of deficiency and its associated complications.

Causes of vitamin K deficiency include:

• Inadequate dietary intake – rare in adults but may occur with severely restricted diets or eating disorders

• Malabsorption syndromes – conditions affecting fat absorption (coeliac disease, inflammatory bowel disease, cystic fibrosis, short bowel syndrome) impair vitamin K uptake

• Biliary obstruction – reduced bile flow limits fat-soluble vitamin absorption

• Chronic liver disease – impairs both vitamin K storage and clotting factor synthesis

• Prolonged antibiotic therapy – particularly broad-spectrum antibiotics that disrupt vitamin K-producing gut bacteria

• Medication interactions – certain drugs including bile acid sequestrants (colestyramine, colesevelam), orlistat, and some anticonvulsants may interfere with vitamin K absorption or metabolism

The clinical manifestations of vitamin K deficiency range from mild bruising to life-threatening haemorrhage. Early signs include easy bruising, petechiae (small red spots on skin), and prolonged bleeding from minor injuries. Severe deficiency typically causes mucocutaneous, gastrointestinal and genitourinary bleeding. In extreme cases, bleeding into muscles or intracranial haemorrhage may occur. Joint bleeding (haemarthrosis) is less common in vitamin K deficiency but can occur in severe cases or over-anticoagulation.

Vitamin K deficiency bleeding (VKDB) represents a particular concern in newborns, who have limited vitamin K stores at birth, sterile intestines lacking vitamin K-producing bacteria, and low vitamin K content in breast milk. The UK follows NICE guidance (NG194) recommending prophylactic vitamin K administration to all newborns, typically as a single intramuscular injection or oral doses, to prevent early, classical, and late VKDB.

Treatment of vitamin K deficiency depends on severity and underlying cause. Mild deficiency may respond to increased dietary intake or oral vitamin K supplementation. Moderate to severe deficiency, particularly with active bleeding, requires intravenous phytomenadione (vitamin K1), with typical doses of 5–10mg for adults. The MHRA advises that intravenous vitamin K should be administered slowly to minimise the rare risk of anaphylactoid reactions. Intramuscular administration should generally be avoided in patients with coagulopathy due to the risk of haematoma formation, unless specifically advised. Response to treatment is generally rapid, with improvements in coagulation parameters within 6–24 hours, though normalisation of clotting factor levels may take several days. Addressing underlying causes, such as treating malabsorption disorders or adjusting medications, remains essential for preventing recurrence.

If you experience any suspected side effects from vitamin K medicines, report them via the MHRA Yellow Card scheme (yellowcard.mhra.gov.uk or the Yellow Card app).

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