Covered by the diet (leafy vegetables).
Necessary for clotting; corrects its deficiency.
An adequate intake is associated with better bone health.
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Vitamin K1, or phylloquinone, is the natural form of vitamin K found predominantly in dark leafy green vegetables (spinach, kale, broccoli, romaine lettuce, parsley) and in vegetable oils (especially soybean, canola, and olive); it was identified in the 1930s by the Danish biochemist Henrik Dam (who received the Nobel Prize in Medicine in 1943 for this discovery) when he observed that chicks fed fat-free diets developed hemorrhages from a clotting defect, and that the antihemorrhagic factor present in vegetables corrected the problem; the K in its name comes precisely from "Koagulationsvitamin" in Danish.
The most classical and best-known biological function of K1 is to serve as a cofactor of hepatic gamma-glutamyl carboxylase, an enzyme that activates, through post-translational carboxylation, clotting factors II (prothrombin), VII, IX, and X, as well as the endogenous anticoagulant proteins C, S, and Z; this hepatic function explains why vitamin K antagonists (warfarin, acenocoumarol) are effective oral anticoagulants and why the administration of vitamin K1 (oral or intravenous) is the immediate antidote to reverse their effect when there is bleeding or a risk of bleeding.
The most solid and indisputable indication is the prevention of hemorrhagic disease of the newborn: neonates are born with low vitamin K levels (the placenta does not transfer maternal K well), and breast milk provides modest amounts; without supplementation, a significant proportion of neonates develop severe bleeding (often intracranial, devastating) in the first weeks or months of life.
The international standard practice is the intramuscular administration of 1 mg of vitamin K1 to all neonates at birth (or orally in several doses if parents refuse the injection), a public health intervention with extremely robust clinical data and 80-100 percent reductions in the incidence of neonatal hemorrhagic disease; this intervention is endorsed by all international pediatric guidelines.
For bone health and cardiovascular prevention, epidemiological data suggest that vitamin K2 (menaquinone, found in fermented and animal-derived foods) may have more relevant effects than K1, possibly because of its broader tissue distribution (K1 accumulates preferentially in the liver, whereas K2 is also distributed to extrahepatic tissues such as bone and vessels); the body partially converts dietary K1 into K2 (specifically MK-4), but the efficiency is variable and modest.
In clinical practice, a diet rich in leafy green vegetables provides very adequate amounts of K1 for normal clotting; for specific bone and cardiovascular health indications, K2 supplements (MK-7) are more efficient than K1 at the doses usually used.
Nutritional needs in adults are 90-120 micrograms daily, easily met with a modest serving of leafy green vegetables (one cup of cooked spinach provides more than 900 micrograms); clinical deficiency in people with varied diets is very rare; specific supplementation with K1 in healthy adults provides no demonstrated benefit and is not usually needed.
The most important precaution is the interaction with anticoagulants such as warfarin and acenocoumarol: vitamin K1 is the direct antagonist of these drugs, and abrupt changes in dietary intake (introducing or removing leafy green vegetables in large amounts, or starting K supplements) can significantly alter the INR and increase the risk of bleeding (if K is lowered) or thrombosis (if K is raised); people taking these anticoagulants should maintain a stable dietary vitamin K intake, not necessarily a low one, and consult their anticoagulation specialist before starting supplements; the direct oral anticoagulants (rivaroxaban, apixaban, edoxaban, dabigatran) do not have this interaction, and vitamin K is unrestricted with them; in neonates, prophylactic supplementation is standard; in pregnancy and breastfeeding, adequate dietary intake is important; oral vitamin K supplements are very well tolerated, with no significant toxicities described at the usual nutritional doses.
Information, not medical advice. Scientific evidence reflects the quality of the studies.