The plant form of vitamin K, concentrated in green leaves where it participates in photosynthesis. It is the dominant form in the diet and the one measured in food composition tables.
Vitamin K1 has one function nobody disputes — it is the cofactor that allows the liver to make functional clotting factors — and a second, more speculative role in bone and vascular biology through the carboxylation of osteocalcin and matrix Gla protein. Dietary deficiency in adults is rare; the clinically important situations are newborns, who are routinely given a vitamin K injection because they are born deficient, and people on warfarin, whose dose is titrated against their vitamin K intake. For everyone else, the case for a supplement over a plate of greens is weak.
Sold as
Tablet, Softgel in oil, Drops, Injectable phytonadione (medical use), Component of multivitamins
Also known as
Phylloquinone, Phytonadione, Plant vitamin K
Regulation: No Tolerable Upper Intake Level has been established for vitamin K — the IOM found insufficient data to set one, which is not the same as a demonstration that any amount is safe. Injectable phytonadione is a prescription medicine used for newborns and to counteract over-anticoagulation.
What the research used
The amounts, forms and durations published trials of Vitamin K1 (phylloquinone) actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Adequate Intake, adults
90–120 µg
Moderate
90 µg/day for women and 120 µg/day for men. This is an Adequate Intake rather than an RDA, meaning it reflects observed healthy intakes rather than a measured requirement.
Studied for Bone trials
1000–5000 µg
Limited
Phylloquinone tablet, daily · 2–4 years
The largest trial at 5 mg/day found no effect on bone mineral density, with a reduction in clinical fractures only as a secondary finding that the authors treated cautiously. These doses are 10–50 times the Adequate Intake.
Studied for Stabilising anticoagulation
100–200 µg
Limited
Daily supplement alongside warfarin · 6 months
Small trials in people with unstable INR report steadier control with a fixed low daily intake. This is done under anticoagulation monitoring, never independently.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
Each timing below was actually compared against an alternative in a trial, which is unusual.
🍽️With food
Moderate
Absorption of phylloquinone is poor from raw greens and improves markedly with dietary fat, which absorption studies have quantified. Supplemental K1 in an oil base is better absorbed than a dry tablet taken fasted.
Timing was tested. A trial compared this window against another and reported a difference.
The evidence in depth
Best supported: Limited
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
Vitamin K carboxylates osteocalcin, and low vitamin K status tracks with fracture risk in observational data. The randomised evidence is much weaker: the largest trial of high-dose K1 found no change in bone mineral density over four years.
What may be going on: Vitamin K is the cofactor for gamma-carboxylation of osteocalcin, which is required for the protein to bind calcium in bone matrix.
Limited evidence. Early human work, or mostly lab and animal research.
Randomised controlled trial · Osteoporosis international : a journal established as result of cooperation between the European Foundation for Osteoporosis and the National Osteoporosis Foundation of the USA · 2013 · PMID 23525894
Observational data link higher vitamin K intake with less arterial calcification, but randomised trials of K1 supplementation have not shown a change in vascular calcification or cardiovascular events.
Not enough evidence. Not enough human research to say anything useful yet.
Randomised controlled trial · Journal of lipid research · 2002 · PMID 12032162
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Interaction
Directly antagonises warfarin — consistency matters more than avoidance
Warfarin works by blocking vitamin K recycling, so vitamin K intake and warfarin dose are titrated against each other. The common advice to "avoid vitamin K" is wrong and can be dangerous: swinging between high and low intake destabilises the INR in both directions. What matters is keeping intake steady day to day and telling the anticoagulation clinic before starting or stopping any vitamin K supplement.
Worth knowing
Newborn deficiency is a medical situation, not a supplement one
Babies are born with very low vitamin K stores and breast milk supplies little, which is why vitamin K is given by injection at birth. Declining it raises the risk of vitamin K deficiency bleeding, including intracranial haemorrhage; oral regimens exist but are less reliable. This is a decision for a clinician, not a supplement shelf.
Interaction
Absorption reduced by several drug classes
Interacts withCholestyramine, orlistat, mineral oil, long-course broad-spectrum antibiotics
Bile-acid sequestrants, orlistat and mineral oil reduce vitamin K absorption, and prolonged broad-spectrum antibiotics reduce bacterial production in the gut. Fat-malabsorption states — coeliac disease, cystic fibrosis, cholestasis — are the usual causes of genuine deficiency in adults.
Worth knowing
No upper limit has been set
The absence of a Tolerable Upper Intake Level reflects insufficient data, not established safety at any dose. Trials have used milligram amounts without notable toxicity, but long-term safety at those levels is unquantified.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.