Vitamin K supplements: clotting, bones and warfarin

It is a group of fat-soluble vitamins. The NHS describes them as needed for blood clotting and for wound healing. MedlinePlus explains the mechanism: the liver needs vitamin K to produce clotting factors II, VII, IX and X, and without it blood would not clot normally. Research also points to a role in bone strength in later life, although this has not yet produced firm treatment recommendations.

You will see two main forms on labels. Vitamin K1 (phylloquinone) comes from plants. Vitamin K2 (menaquinones) is found in some fermented and animal foods and is made by gut bacteria. Both feature in supplements, often alongside calcium and vitamin D.

Spinach, kale, broccoli, cabbage and Brussels sprouts are rich in vitamin K1, and the NHS also lists vegetable oils and cereal grains. Because the body stores some in the liver, it does not need to be eaten daily in exactly the same quantity, but people on warfarin are advised to avoid big swings.

• Aim for a fairly constant weekly pattern of green vegetables if you take warfarin.

• Tell your anticoagulant clinic if your diet changes significantly, for example when starting a new diet or recovering from illness.

• Do not cut green vegetables out to “protect” your INR, as erratic intake is the real problem.

Warfarin works by blocking the recycling of vitamin K, so it directly opposes the nutrient. If you suddenly eat much more vitamin K, the effect of warfarin weakens; if you eat much less, the effect strengthens. That is why anticoagulant clinics ask about diet and supplements at each review, and why a multivitamin or bone-health supplement with vitamin K should be mentioned before it is started.

Newer anticoagulants such as apixaban or rivaroxaban work in a different way and are not affected by vitamin K intake in the same manner. Whether you are on warfarin or another agent, do not stop or alter a prescribed anticoagulant on your own.

MedlinePlus says deficiency is uncommon but may follow poor absorption or prolonged antibiotic use, and shows up as easy bruising and bleeding. People with coeliac disease, cystic fibrosis, bile duct obstruction, severe liver disease or after some bowel surgery are more susceptible. A clotting test such as the prothrombin time is the usual first check.

Bleeding from the gums or nose, blood in urine or stool, or very heavy periods in someone with a malabsorption condition are reasons to seek medical advice. Heavy bleeding or head injury while on a blood thinner needs emergency care through 999 or A&E.

Studies have looked at whether extra vitamin K, particularly K2, protects against fractures. The results so far are not strong enough for UK guidance to recommend it for osteoporosis prevention. Established treatments and adequate calcium and vitamin D remain the basis of care, and anyone on warfarin should not add vitamin K for bones without clinic agreement.

Newborns are born with low stores and are offered vitamin K shortly after birth in the UK. That is a distinct, evidence-backed programme covered on its own page.