Vitamin K (phylloquinone)

Vitamin K (phylloquinone) is the primary dietary form of vitamin K, found mainly in green leafy vegetables and some plant oils. It activates proteins the body needs for normal blood clotting and bone building, and it is the form people on blood-thinning medication are usually advised to keep consistent day to day.

What is vitamin K (phylloquinone)

Vitamin K exists in two main dietary forms, per the Harvard T.H. Chan School of Public Health, The Nutrition Source: phylloquinone (vitamin K1), the form tracked here, which comes from green leafy vegetables and some vegetable oils; and menaquinones (vitamin K2), found in animal and fermented foods and also produced by bacteria in the gut. Phylloquinone's central job is enabling the liver to produce prothrombin and other clotting factors, and enabling bone tissue to produce osteocalcin, a protein involved in binding calcium into bone. Harvard's Nutrition Source lists collard greens, kale, spinach, broccoli, Brussels sprouts, cabbage, and lettuce as key phylloquinone sources, alongside soybean and canola oils. In USDA data, dried herbs and spices such as basil, sage, and thyme, and fresh parsley, rank among the most concentrated sources by weight, though leafy greens are eaten in larger quantities and remain the practical dietary mainstay.

Health benefits of vitamin K (phylloquinone)

• Blood clotting. Harvard's Nutrition Source describes vitamin K's core function as producing the clotting proteins needed to stop bleeding after an injury.
• Bone health. The same source reports that studies have found higher vitamin K intakes associated with lower hip-fracture risk and better bone mineral density, through its role in activating osteocalcin.
• Cardiovascular research. Harvard notes emerging research suggesting vitamin K may help prevent calcium buildup (calcification) in artery walls, though it describes this evidence as limited rather than conclusive.

How much vitamin K you need

Per Harvard's Nutrition Source, the Adequate Intake (AI) — set because there is not enough evidence to establish a full RDA — is 120 mcg/day for adult men and 90 mcg/day for adult women, including during pregnancy and lactation. This is consistent with the NIH Office of Dietary Supplements' Vitamin K Health Professional Fact Sheet, which corroborating searches confirm sets the same 120 mcg (men) and 90 mcg (women) AI values and states that no Tolerable Upper Intake Level (UL) has been established for vitamin K, because of its low potential for toxicity, per the same fact sheet.

Signs of deficiency and excess

Deficiency is uncommon in healthy adults eating a varied diet, since vitamin K is widespread in green vegetables and also produced by gut bacteria, per MedlinePlus. When it does occur, documented signs include prolonged blood clotting time, easy bruising, bleeding gums, and in severe cases internal bleeding or hemorrhage. Newborns are a specific at-risk group: they are born with very low vitamin K stores and get little from breast milk, which is why a vitamin K injection is routinely given at birth, per Cleveland Clinic. People with liver disease or conditions that impair fat absorption (such as chronic diarrhea or celiac disease) are also at higher risk of deficiency, per Cleveland Clinic.

Excess/toxicity: vitamin K from food or supplements rarely reaches toxic levels, because it is broken down and excreted from the body relatively quickly, per Harvard's Nutrition Source.

Warnings & interactions

The most clinically important caution for vitamin K concerns people taking anticoagulant (blood-thinning) medication such as warfarin. Per MedlinePlus, people on warfarin need to be careful about how much vitamin K they consume, because vitamin K works against warfarin's blood-thinning effect; the practical guidance from Harvard's Nutrition Source is to keep day-to-day vitamin K intake consistent rather than avoiding it, since large, sudden swings in intake (for example, starting or stopping a habit of eating large amounts of leafy greens) can throw off warfarin dosing and its anticoagulant effect. MedlinePlus separately notes that vitamin E supplements can interfere with vitamin K's effectiveness in the body, which is another reason to discuss supplement use with a healthcare provider when on anticoagulant therapy. Anyone starting, stopping, or changing a blood thinner should talk to their prescriber before making a significant, lasting change to how much vitamin K they eat.

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