Vitamin K

Vitamin K is needed to make several of the proteins the body uses to control blood clotting. Without enough, blood cannot clot properly and the body bruises easily and bleeds for longer after injury. The clotting role is the main one, and it is the reason newborn babies in the UK are routinely offered vitamin K at birth (their stores are low and clotting can fail in the first weeks of life).

Vitamin K also plays a role in bone metabolism. It activates osteocalcin, a protein involved in laying calcium into bone. Long-term low vitamin K status is associated with somewhat lower bone density and slightly higher fracture risk in observational studies; the evidence on supplementation for bone is still developing and is not part of current NHS routine advice.

Two main forms reach the body. K1 (phylloquinone) is in dark green leafy vegetables; kale, spinach, broccoli, Brussels sprouts, watercress, and herbs like parsley are dense sources. K1 is the dominant form in the UK diet. K2 (menaquinone) is in fermented foods (natto is famously high; certain aged cheeses contain modest amounts) and in some animal foods (liver, egg yolks). Some K2 is also made by gut bacteria, though the contribution to overall status is small. The body can convert some K1 to K2 internally.

Anticoagulants and vitamin K, the safety story: warfarin and several related anticoagulants work by interfering with vitamin K's role in clotting. The blood test that monitors warfarin (the INR) is sensitive to changes in dietary vitamin K. NHS standing advice for anyone on warfarin: keep your vitamin K-rich foods steady week to week, rather than swinging between a kale-heavy week and a no-greens week. Follow the advice from your anticoagulant clinic. This advice applies to people on warfarin and similar coumarin anticoagulants; the newer direct oral anticoagulants (DOACs) such as apixaban, rivaroxaban, dabigatran, and edoxaban work differently and are not affected by dietary vitamin K. If you are unsure which anticoagulant you take, ask your pharmacist or clinic.

Vitamin K1 is concentrated in dark green leafy vegetables. Values per 100g come from USDA SR Legacy and McCance and Widdowson 7th edition; the site's food entries currently store macros only.

Practical: a single portion of cooked dark greens easily covers a day's vitamin K, often several times over. The body excretes the excess. The numbers look extreme but vitamin K is one of the vitamins where dietary excess from food is not a concern (except as a consideration for people taking warfarin, see above).

K2 sources: natto (a Japanese fermented soya food) is the densest natural K2 source, around 1,000 micrograms per 100g. It is available in larger UK Asian supermarkets but is not a mainstream UK food. Smaller amounts of K2 are in some aged cheeses (Gouda, certain hard cheeses), egg yolks, liver, and chicken. For people not eating any of these, gut bacteria make some K2 and the body converts some K1 to K2, but the K2 contribution to overall status is modest.

The UK position on vitamin K is a "safe intake" tied to body weight, set by SACN (1991). NHS guidance reproduces this as "approximately 1 microgram a day for each kilogram of your body weight".

SACN did not set a strict RNI for vitamin K, partly because gut bacteria contribute some, and because frank deficiency in healthy adults eating a normal mixed diet is uncommon. The body-weight-based safe intake is the value the NHS reproduces for everyday guidance.

Vitamin K deficiency is rare in healthy UK adults eating any green vegetables. It is much more relevant in newborn babies and in adults with malabsorption.

Newborn babies (NHS practice)

Newborns are born with low vitamin K stores, and breast milk contains modest amounts. To prevent vitamin K deficiency bleeding (VKDB), which can be serious or fatal, NHS hospitals routinely offer all newborn babies a vitamin K dose at birth. Parents can choose between an injection (one dose, very effective) or oral doses (three doses over several weeks). This is standard practice and parents are involved in the choice.

Adult signs of deficiency

• Easy bruising
• Heavy or unusual bleeding from minor wounds
• Nosebleeds
• Bleeding gums when brushing
• Heavier than usual menstrual periods
• Blood in urine or stool

Who is at higher risk in the UK

• People with cystic fibrosis, coeliac disease, Crohn's, or other fat-malabsorption conditions. Vitamin K is fat-soluble; absorption falls when fat absorption is impaired.
• People with cholestatic liver disease.
• People on long-term broad-spectrum antibiotics, which can disrupt the gut bacteria that make some K2.
• People with severely restricted diets over long periods.
• Newborn babies before the vitamin K dose (the reason it is given).

If unusual bruising or bleeding develops, see a GP. Vitamin K-related causes are uncommon but treatable.

Vitamin K from food is not a realistic risk for healthy adults; the body excretes the excess. The exception is the specific situation of people taking warfarin and similar anticoagulants (see below).

NHS supplement guidance: taking 1mg (1,000 micrograms) or less a day of vitamin K from supplements is unlikely to cause harm.

Vitamin K and warfarin or similar anticoagulants

Warfarin and the related coumarin anticoagulants work by blocking vitamin K's role in clotting. The blood test that monitors warfarin (the INR) is sensitive to vitamin K intake. NHS advice for people on warfarin: keep your vitamin K-rich foods steady week to week, rather than swinging between a kale-heavy week and a no-greens week. Follow the advice from your anticoagulant clinic.

This is a caution to keep intake consistent, not to avoid vitamin K-rich foods. Cutting out greens entirely is not the goal; sudden week-to-week changes are what destabilise warfarin control.

The newer direct oral anticoagulants (DOACs), including apixaban, rivaroxaban, dabigatran, and edoxaban, work by a different mechanism and are not affected by dietary vitamin K. If you are unsure which medicine you take, ask your pharmacist or clinic.

Do not start a vitamin K supplement while on warfarin without speaking to your anticoagulant clinic.

Vitamin K supplementation is not routinely needed for healthy UK adults.

Newborn babies

All newborns in the UK are offered vitamin K shortly after birth to prevent vitamin K deficiency bleeding. The choice of injection or oral doses is discussed with parents. This is the standing NHS approach.

When adult supplementation is indicated

• Cystic fibrosis and other malabsorption conditions, as part of routine care.
• Confirmed deficiency on blood test.
• Long-term broad-spectrum antibiotic use, on personalised advice.

People on warfarin

Do not start a vitamin K supplement without speaking to your anticoagulant clinic. Sudden changes in vitamin K intake, including from supplements, change how warfarin works.

If supplementing for any reason

Choose a product that delivers a moderate dose (around 70 to 100 micrograms is a typical maintenance level). Stay at or below 1mg per day total from supplements unless directed by a clinician.

• Fat-soluble vitamins together: Vitamin A, Vitamin D, Vitamin E.
• Calcium and bone health pair with vitamin K: Calcium.
• Best food sources: Kale, Spinach, Broccoli, Brussels sprouts, Watercress, Cabbage.

• NHS. Vitamins and minerals: Vitamin K. nhs.uk/conditions/vitamins-and-minerals/vitamin-k. Adult safe intake, food sources, supplement upper.
• NHS. Warfarin and food. Standing guidance to keep vitamin K-rich food intake steady when on warfarin.
• NHS. Vitamin K for newborn babies. Routine practice in UK hospitals.
• SACN. Dietary Reference Values for Food Energy and Nutrients for the United Kingdom. Department of Health Report 41 (1991). Source of the safe-intake value.
• Public Health England. McCance and Widdowson's The Composition of Foods, 7th summary edition (2015).

This page is reference information for UK shoppers. It is not medical advice. People on warfarin and similar anticoagulants should follow personalised advice from their anticoagulant clinic and not change their vitamin K intake based on web information alone.