Also known as: Phylloquinone, Phytonadione, K1, Plant vitamin K
VitaminsStrong evidence
The form of vitamin K found in green vegetables, used primarily by the liver for blood clotting. Rarely supplemented alone, and the counterpart to K2 rather than a substitute for it.
Typical dose
75.0–120.0 mcg per day
With a fat-containing meal
What it is
Vitamin K1, or phylloquinone, is the form found in plants, concentrated in green leafy vegetables. Kale, spinach, broccoli and spring greens are the richest sources, and it is what gives the vitamin its name, from the German Koagulation.
It accounts for the large majority of vitamin K in a typical diet, and shortfall is uncommon in anyone eating vegetables.
K1 and K2 are not interchangeable. K1 is taken up preferentially by the liver and used mainly for clotting factors. K2 circulates more widely and reaches bone and vascular tissue. That difference in distribution is why K2 is the one paired with vitamin D3 and why K1 is rarely sold on its own.
How it works
Vitamin K acts as a cofactor for the enzyme that carboxylates specific proteins, switching them from inactive to active. Without it those proteins remain uncarboxylated and cannot do their job.
K1's principal targets are the clotting factors produced in the liver, which is why it is the form that matters most for normal blood clotting.
The body can convert some K1 to K2, though the efficiency of that conversion in humans is limited and debated. It is not efficient enough to treat K1 intake as a substitute for K2 if bone and vascular effects are what you are after.
What the evidence shows
Vitamin K1's role in normal blood clotting is firmly established and beyond dispute. Adequate intake is straightforwardly important.
Shortfall from diet is uncommon in adults eating vegetables, and supplementation in people with adequate intake has no established benefit.
Research on bone and vascular outcomes has generally favoured K2 over K1, consistent with the difference in how the two distribute around the body. If those outcomes are your interest, K2 is the better-supported choice.
Dosing
Adequate intake is around 1 microgram per kilogram of bodyweight daily, roughly 70 to 90 micrograms for most adults. A portion of cooked greens supplies several times that.
K1 absorption from vegetables is improved considerably by cooking and by eating them with fat, since it is fat-soluble and bound within plant cell structures.
There is rarely a good reason to supplement K1 specifically. If you eat green vegetables you are covered, and if you want vitamin K for bone or vascular reasons, K2 is the form to buy.
At a glance
Common forms
Phylloquinone, Within multivitamins, Combined K1 and K2 products, Green vegetables
Half-life / duration
Short plasma half-life of a few hours; body stores are limited
Timing
With a fat-containing meal
Competition status
Permitted — not on the WADA Prohibited List
Category
Vitamins
Safety and side effects
Vitamin K1 from food and supplements has a good safety record, and no tolerable upper intake level has been established due to low toxicity.
The one thing to check is the same as for K2: if you take anything that affects blood clotting, speak to your prescriber before starting a vitamin K supplement or making a large change to your intake of green vegetables.
Consistency of intake matters more than quantity in that situation, which is a point best discussed with whoever manages your prescription.
Who should avoid it
Check with your prescriber first if you take anything affecting blood clotting, before starting a supplement or significantly changing your green vegetable intake. Speak to your doctor before use if you take regular medication.
This entry is provided for information only. Food supplements should not be used as a substitute for a varied and balanced diet and a healthy lifestyle. Consult a qualified healthcare professional before use if you are pregnant, breastfeeding, taking medication or under medical supervision.
