What is Vitamin K1 (Phylloquinone)?

Vitamin K1, properly called phylloquinone, is the form of vitamin K that plants make. It sits inside the chloroplasts of green leaves, which is why kale, spinach, collards, parsley and broccoli are by far the richest sources — and why vitamin K intake tracks almost perfectly with how many green vegetables someone eats. It accounts for the great majority of vitamin K in a typical Western diet.

Its job in the body is narrow and specific. Vitamin K is the cofactor for an enzyme that adds a carboxyl group to certain proteins, switching them from an inactive form to an active one. Four clotting factors depend on this step, which is why severe vitamin K deficiency causes bleeding. So do several proteins outside the clotting system: osteocalcin in bone, and matrix Gla protein, which helps stop calcium from depositing in artery walls. Those two proteins are the reason vitamin K keeps appearing in bone and cardiovascular research.

Vitamin K1 is not the same as vitamin K2 (the menaquinones, made by bacteria and found in natto and some cheeses). They share the same core chemistry but differ in how long they stay in the blood and which tissues they reach. Most bone-specific trials have used K2; most dietary intake is K1. Treating the two as interchangeable is one of the commonest errors in supplement marketing.

How does Vitamin K1 (Phylloquinone) work?

Vitamin K1 is absorbed from the small intestine with dietary fat, packaged into chylomicrons and taken up mostly by the liver. There it acts as the cofactor for gamma-glutamyl carboxylase, an enzyme that converts glutamate residues into gamma-carboxyglutamate (Gla) on a small set of proteins. Carboxylated clotting factors II, VII, IX and X can then bind calcium and participate in coagulation; carboxylated osteocalcin binds calcium in bone matrix; carboxylated matrix Gla protein inhibits calcification of arteries and soft tissue. Each round of carboxylation oxidises vitamin K, which is then recycled by vitamin K epoxide reductase — the enzyme warfarin blocks. Because the liver takes up K1 preferentially and clears it quickly, plasma levels rise and fall within hours of a meal, and extrahepatic tissues such as bone see less of it than they do of the longer-lived K2 forms.

What forms does Vitamin K1 (Phylloquinone) come in?

Food remains the sensible primary source: a single serving of cooked kale or spinach supplies several times the daily adequate intake. Supplements usually contain 50–500 mcg of synthetic phylloquinone; oil-based softgels are absorbed better than dry tablets, and any form is absorbed better with a meal containing fat. Injectable phytonadione is a prescription hospital product used for newborn prophylaxis and for reversing over-anticoagulation — the intravenous route carries a small risk of anaphylactoid reaction and belongs entirely in clinical hands. Topical vitamin K1 creams are sold for bruising and post-procedure skin redness; the evidence behind them is thin. Multivitamins vary widely, and many 'vitamin K' products contain MK-7 (K2) rather than K1, so check the label if it matters to you.

Where it comes from:

Henrik Dam identified a fat-soluble anti-haemorrhagic factor in chicks in 1929 and named it Koagulationsvitamin — hence K. Edward Doisy's group isolated and synthesised it from alfalfa in 1939, and the two shared the 1943 Nobel Prize. Because vitamin K crosses the placenta poorly and breast milk contains little, newborns are born with low stores; routine intramuscular vitamin K at birth was introduced in the 1960s and has made haemorrhagic disease of the newborn — now called vitamin K deficiency bleeding — rare wherever it is given. From the 1970s onward, the discovery of Gla proteins in bone and blood vessels opened the second chapter of vitamin K research, which is still unfinished.

What does Vitamin K1 (Phylloquinone) help with?

• Vitamin K deficiency bleeding in newborns: This is the strongest use of vitamin K1 by a wide margin. Babies are born with very low vitamin K stores and breast milk supplies little, so a small minority develop serious bleeding, including into the brain, in the first weeks of life. A single intramuscular dose of phytonadione at birth has made this condition rare in every country where it is standard practice. This is given by a clinician, not something to manage at home, and oral regimens are less reliable than the injection, particularly for late-onset bleeding.
• Vitamin K deficiency from fat malabsorption: Vitamin K needs dietary fat and bile to be absorbed. People with cystic fibrosis, coeliac disease, cholestatic liver disease, short bowel, chronic pancreatitis or a history of bariatric surgery can become genuinely deficient, showing up as easy bruising, nosebleeds or a prolonged prothrombin time. Supplemental K1 corrects the biochemical deficiency reliably in these groups, and it is routinely included in fat-soluble vitamin regimens in cystic fibrosis care. Dosing here should be set by the treating team, since it depends on how badly absorption is impaired.
• Reversal of excessive anticoagulation (medical use): Phytonadione is the standard antidote when the INR is dangerously high in someone taking warfarin, or when bleeding occurs. It works by restoring the vitamin K supply that warfarin blocks, and it is effective — but this is a prescription intervention with a real balance of risks, since overcorrecting leaves the person unprotected against clotting. Nobody on warfarin should take vitamin K supplements to adjust their own INR. The relevant everyday advice is the opposite: keep your green vegetable intake steady rather than swinging up and down.
• Bone health and osteoporosis: Vitamin K activates osteocalcin, and low intake is associated with lower bone density and more hip fractures in observational studies. Supplement trials have been much less encouraging. The largest controlled trial of high-dose K1 in postmenopausal women found no protection of bone mineral density over several years, though fewer fractures were recorded as a secondary, underpowered finding. Most positive fracture data come from Japanese studies of vitamin K2 (MK-4) at pharmacological doses, not K1. Honest summary: adequate dietary K1 is part of good bone nutrition; supplemental K1 is not a demonstrated osteoporosis treatment.
• Arterial and vascular calcification: Matrix Gla protein is one of the body's main brakes on calcium deposition in artery walls, and it needs vitamin K to work. Low vitamin K status and high levels of the uncarboxylated protein track with more coronary calcium in population studies. Supplement trials have been mixed and mostly modest: some have shown slowed progression of coronary calcium in older adults who took the supplement consistently, while a trial of high-dose K1 in aortic valve calcification did not show meaningful benefit. It remains a plausible, actively studied idea rather than an established treatment.
• Chronic kidney disease: People on dialysis frequently have poor vitamin K status and heavy vascular calcification, which makes them an obvious group to study. Supplementation reliably improves the biochemical markers — it lowers uncarboxylated matrix Gla protein — but the trials that have looked at whether calcification, arterial stiffness or cardiovascular events actually improve have so far not shown clear benefit. Most of this work has used K2 (MK-7) rather than K1. If you have kidney disease, discuss any vitamin K supplement with your nephrology team first.
• Bruising and post-procedure skin discolouration (topical): Vitamin K1 creams are widely sold for bruising, spider veins and the discolouration that follows laser treatment or injectables. The rationale is that vitamin K helps clear extravasated blood, and a handful of small dermatology studies suggest topical K1 may slightly speed the fading of bruises after cosmetic procedures. The studies are small, short and often industry-linked, and results are inconsistent. It is low-risk to try, but expect a minor cosmetic effect at best and do not use it in place of medical assessment for unexplained bruising.
• Heavy or prolonged bleeding related to low vitamin K status: Where bleeding or easy bruising genuinely stems from low vitamin K — malabsorption, prolonged broad-spectrum antibiotics with poor intake, severe liver disease — restoring vitamin K corrects the clotting factor deficit, usually within a day or two. This is a specific situation confirmed by blood tests, not a general remedy for heavy periods or frequent nosebleeds, which have many other causes. Unexplained or worsening bleeding always needs a diagnosis before any supplement, because taking vitamin K can mask the pattern that would have identified the problem.
• Insulin sensitivity and glucose metabolism: Osteocalcin, one of the vitamin K-dependent proteins, has signalling roles in glucose and energy metabolism, and observational data link higher vitamin K intake with lower risk of type 2 diabetes. A few small supplementation trials, mainly in men and using K1 at doses around 500 mcg daily, have reported modest improvements in insulin sensitivity, while others found nothing. This is early, inconsistent research on a biologically interesting idea. It is not a reason to take vitamin K1 for blood sugar control, and it should not replace established diabetes care.

What can Vitamin K1 (Phylloquinone) help prevent?

• Vitamin K deficiency bleeding in newborns: routine phytonadione at birth has made this rare wherever the practice is followed
• Vitamin K deficiency and its associated bleeding in people with fat malabsorption or long-term parenteral nutrition
• Low vitamin K status in people with very low vegetable intake or after prolonged broad-spectrum antibiotic use combined with poor eating

Which foods are rich in Vitamin K1 (Phylloquinone)?

Vitamin K1 is one of the easiest nutrients to get from food, because it is concentrated in green leaves. The richest sources are kale, collard greens, turnip greens, spinach, Swiss chard, parsley, mustard greens and beet greens — a single cooked serving of any of these supplies several times the daily adequate intake. Broccoli, Brussels sprouts, cabbage, asparagus, romaine lettuce and green beans are solid moderate sources. Plant oils matter too: soybean, canola and olive oil each contribute usefully, and they also improve absorption from the greens they are cooked or dressed with. Kiwifruit, avocado, blueberries and green peas add smaller amounts. Absorption from raw vegetables is poor — cooking and adding a fat roughly doubles it, so buttered greens or an oil-dressed salad beat plain leaves. Note that fermented soy (natto) and some aged cheeses are rich in vitamin K2, a related but different form. If you take warfarin, do not avoid these foods; instead keep your intake roughly the same from week to week, which is what allows your dose to be set correctly.

What are the side effects of Vitamin K1 (Phylloquinone)?

• Oral vitamin K1 is very well tolerated; no toxic upper level has been established for the natural forms
• Occasional mild stomach upset with high-dose supplements
• Rare rash or itching with oral supplements
• Injectable vitamin K given intravenously can cause a serious anaphylactoid reaction — this is why hospitals prefer the intramuscular route
• Injection site pain or a temporary firm lump after intramuscular dosing
• Transient flushing or altered taste after injection
• Not a side effect but the main hazard: unannounced changes in vitamin K intake destabilise warfarin control and can cause clotting or bleeding

Good to know:

For a supplement, vitamin K1 has an unusually clean safety record. Unlike vitamins A and D, it does not accumulate to toxic levels, and large oral doses have not been associated with excessive clotting in people with normal clotting systems. The important cautions are about interactions and about clinical routes of administration, not about the vitamin itself.

How much Vitamin K1 (Phylloquinone) should you take?

• Recommended daily: Adequate intake for adults is 120 mcg/day for men and 90 mcg/day for women, easily met by one generous serving of cooked leafy greens. Over-the-counter supplements typically supply 50–500 mcg/day of phylloquinone. Research doses have ranged from 500 mcg up to 5 mg/day, but the higher end has not shown clear benefit and should not be self-prescribed. Newborn prophylaxis (1 mg intramuscularly at birth) and warfarin reversal are clinician-administered and are not home doses. In malabsorption states, dosing must be set by the treating team.
• Best time of day: No meaningful circadian preference — take it with whichever meal is easiest to remember. Consistency matters more than timing, especially if you are on an anticoagulant.
• With or without food: Always with food containing some fat. Absorption of phylloquinone is poor without it; a little olive oil or butter on your greens genuinely increases how much you take up.
• Time to effect: Clotting factor activity responds within 6–24 hours. Markers of osteocalcin and matrix Gla protein carboxylation shift over 2–6 weeks. Any structural effect on bone or arteries, if it happens at all, would take a year or more.
• How long to take it: Vitamin K1 is an essential nutrient with no established toxicity from oral intake, so ongoing dietary intake is the goal rather than a course of treatment. If you are supplementing for a specific reason — malabsorption, a documented low level — review it with your doctor every 6–12 months. If you are taking it hoping to protect bone or arteries, be aware you are acting ahead of the evidence.