Vitamin K1 is the clotting vitamin: when a wound stops bleeding, that is because K1 has activated several clotting factors. Nearly all of it comes from dark green vegetables, so people who eat their vegetables rarely run short.
Vitamin K1 (phylloquinone) started out as a part inside plant chloroplasts, helping pass electrons along in photosynthesis, so the darker the leaf, the more K1: kale (see Kale) and spinach (see Spinach) lead, with broccoli (see Broccoli) and lettuce behind. One cup of cooked spinach is far above an adult's daily adequate intake.
"I eat greens every day, so I probably don't lack K1" is basically true: for a healthy person on a varied diet, eating so little vitamin K that clotting tests turn abnormal is almost impossible. The people who do run short are those whose fat-absorption pathway is broken, and newborn babies. A baby is born with almost no vitamin K in reserve, which is why a K1 shot is given after birth; if a baby who did not get the shot shows unexplained bleeding, drowsiness, or repeated vomiting, go to the emergency department immediately.
Numbers · Which greens have the most K1
The greener the leaf, the more K1:
Kale, cooked, 100 g: about 820 µg, the leaderSpinach, cooked, 100 g: about 540 µgBroccoli, cooked, 100 g: about 140 µgLarge-leaf lettuce, 100 g: about 50–100 µgSoybean oil and canola oil: moderate amounts, and one of the main K1 sources in Western diets The adequate intake () set by the US Institute of Medicine (IOM) is 120 µg/day for adult men and 90 µg/day for women. One cup of cooked spinach is far above that.
How K1 and differ in structure: the head is the same (a naphthoquinone core); the difference is the tail, that is, the side chain behind the head. K2 is divided into MK-4 through MK-13 by the number of isoprenoid units on that side chain. The body can turn a small amount of K1 into MK-4, but not enough to cover everything K2 does. For K2's half of the job, in bone and blood vessels, see Vitamin K2.
Myth · Do leafy greens thicken the blood?
"Greens are high in vitamin K, so eating more thickens the blood and invites clots": for almost everyone, this is wrong.
Point by point: in healthy people, K1 intake can move across a range as wide as 50–800 µg/day while prothrombin time (PT / , a test of how quickly blood clots) barely moves; the body uses only what it needs to γ-carboxylate clotting factors, and extra K1 is broken down by the liver and excreted; the US Institute of Medicine set no upper intake limit for vitamin K because no toxicity from high intakes has been found; and epidemiology has not observed any dose-response pattern of more K1, more clots. If anything, observational data point the other way: people who eat more greens have fewer cardiovascular events. That is an association; it may reflect the folate, potassium, fiber, nitrate, and other things in greens together, and it does not show an effect of K1.
The only people who truly need a steady K1 intake are patients on warfarin. Even then the answer is not "no greens" but "about the same amount every day": the doctor sets the warfarin dose against that baseline (covered in the chapter on the two kinds of blood thinners).
People on a direct oral anticoagulant (, such as dabigatran, rivaroxaban, apixaban, or edoxaban) take a drug that does not run through the vitamin K cycle, so they can eat greens as usual.
So for ordinary people, eating more greens carries no risk of thickening the blood. The claim most likely came from diet advice meant for warfarin patients that got carried over to healthy people; the benefits of eating greens are far more real than the imagined risk of thicker blood.
Chapter 2
Absorbed best with some fat
K1 is a fat-soluble vitamin, in the same group as A, D, and E: without some fat alongside, it is poorly absorbed. Chew a raw lettuce leaf on its own, and very little K1 actually reaches your blood.
It travels the fat route. Bile and fat-digesting enzymes from the pancreas first dissolve K1 out of the leaf's membranes and wrap it in tiny oil-and-water spheres (micelles). The gut wall takes up the fat and the K1 together and packs them into chylomicrons, the particles that carry fat, which travel through the lymph into the blood and are unloaded at the liver. The liver is where K1 works, but it keeps almost no stock, so this is a vitamin that has to be topped up every day. That is why cooked greens with some oil are the best deal.
The people who actually run short of K1 are those whose fat route is broken: the pancreas makes too little fat-digesting enzyme, bile cannot get out, or the small intestine itself is damaged. When fat is not absorbed, vitamin K is not absorbed either. Two kinds of drugs work the same way: the weight-loss drug orlistat blocks fat absorption on purpose, and cholesterol-lowering bile-acid resins (such as cholestyramine) bind up bile; both keep vitamin K out along the way. Healthy people are covered by greens with oil; people with an absorption disorder should talk to their doctor about water-dispersible K1 or an injectable form. Babies run short in a different, special way, covered in the chapter on newborns.
Numbers · How well it is absorbed, how to eat it
What the liver stores lasts only a day or two (about 1–2 days): K1 is barely stored, so it has to be topped up from food every day.
Much less K1 is absorbed from vegetables than you might think. According to the US NIH Office of Dietary Supplements (ODS), free K1 (as in a tablet or in oil) is about 80% absorbed, but the K1 in greens is tightly bound to the chloroplasts: the body absorbs only 4%–17% as much K1 from spinach as from the same amount in a tablet. How you eat matters a lot:
Eating vegetables with a little fat raises how much of their K1 is absorbed, though still less than from oilsCooking breaks down cell walls, which should help tooJuicing does not automatically improve absorption; what matters is still whether fat is there to carry it Which conditions block this route: cystic fibrosis and chronic pancreatitis (too little pancreatic fat-digesting enzyme); blocked bile ducts and severe liver disease (bile cannot get out, so the oil-and-water spheres cannot form); Crohn's disease and short bowel syndrome (the small intestine's absorbing surface is damaged).
Chapter 3
Helping clotting proteins grab calcium
K1's job is to let a particular set of proteins grow a hand that grabs calcium.
Liver cells contain a machine called GGCX (γ-glutamyl carboxylase) that looks for glutamate (Glu) residues on these proteins and welds a carboxyl group onto each one, turning it into Gla (γ-carboxyglutamate). K1 is the machine's consumable: each weld uses up one molecule, the spent K1 is inactive, and a second machine, the reductase VKORC1, refurbishes it so it can go round again.
Gla is that hand: it carries a negative charge and clamps calcium ions like a small clip. Only with this hand can a clotting factor pin itself to the cell membranes at a wound and assemble into a clotting complex; factors without Gla drift in the blood and do nothing.
This cycle is exactly what warfarin targets: warfarin holds VKORC1 down, so used K1 cannot be restored to its active form, GGCX runs out of its consumable and stops, and newly made clotting factors are all stuck without their Gla hands (in medicine these are called PIVKA). They cannot grip calcium, so clotting takes longer (the blood test shows a higher ). The reverse also holds: suddenly eating several plates of spinach supplies a flood of active consumable, goes around the blocked step, and cancels the drug's effect. INR drops and the risk of clots rises.
A CLOSER LOOK
Vitamin K supports processing and is then recycled
Reduced vitamin K supports carboxylation and becomes an epoxide. Recycling restores it for the next round of protein processing; the processed protein is distinct from the recycled K.
1GGCX adds a carboxyl group
GGCX converts glutamate residues, Glu, into Gla on vitamin K-dependent proteins; reduced K, carbon dioxide, and oxygen participate in this step.
2Used K is recycled
After carboxylation, K is oxidized to an epoxide. VKOR-mediated recycling first produces quinone K, which is then reduced to the usable form to support protein processing again.
Illustration for understanding; not to scale. Saved figures include explanations and sources.
Mechanism · The five steps of the γ-carboxylation cycle
The reaction takes place in the endoplasmic reticulum of liver cells. The lead enzyme is GGCX (γ-glutamyl carboxylase), and its cofactor is the reduced form of K1 (the hydroquinone, KH₂).
The cycle has 5 steps:
1. KH₂ reacts with O₂ to form a strongly basic intermediate, which pulls a hydrogen off Glu. 2. The Glu residue, now missing that hydrogen, takes on CO₂ and forms a new carbon-carbon bond, becoming Gla (with one extra carboxyl group). 3. In this step KH₂ is oxidized to K1-2,3-epoxide (K1-O) and becomes inactive. 4. VKORC1 (vitamin K epoxide reductase complex subunit 1) reduces K1-O back to KH₂ for the next round. 5. The whole loop closes and the cofactor is reused: estimated from the recycling, one molecule of K1 can activate tens to hundreds of clotting-factor proteins.
Why Gla matters so much: Gla's two carboxyl groups carry negative charges and act like small calcium clamps, gripping Ca²⁺ by electrostatic attraction. Only once clotting factors carry Gla can they use Ca²⁺ to bind to phospholipid membrane surfaces and then assemble into clotting complexes. Factors without Gla wander in the blood and do nothing at all.
The calcium-grabbing hand that K1 fits (Gla) lets a group of clotting proteins line up as a relay that stops bleeding. K1 activates 7 proteins in all. The main ones are four clotting factors: prothrombin (factor II), factor VII, factor IX, and factor X; two more, the anticoagulant proteins C and S, work the brake; and a helper, protein Z, rounds out the set. What they share is that each must use its Gla hands to grip calcium ions and press itself against the cell membranes at a wound before it can do its part in the chain.
Here is the relay during real bleeding. A vessel tears and exposes tissue factor, which first recruits factor VII to strike the spark; the chain then activates factor X and turns prothrombin into thrombin; thrombin weaves the fibrinogen dissolved in blood into a mesh that seals the tear. Every link has to grip calcium through Gla and pin itself to a membrane before it can move, so without K1 these factors just idle in the blood and cannot close the wound.
Serious K1 deficiency is rare. When it does happen, the first sign is small bleeds that will not stop: oozing gums, nosebleeds, bruising at the lightest bump, and clotting tests that run clearly slow. If you notice these, see a doctor to have your clotting checked rather than taking vitamins on your own.
Clinical · What the labs show, how it is replaced
Why factor VII changes first once warfarin starts: it has the shortest life of the seven proteins, only about 4–6 h, so its supply in the blood runs out quickly. In the first days of warfarin taking effect, factor VII is the first to drop, and that is why the lab numbers move earliest.
Which numbers the labs watch: prothrombin time (PT) covers the tissue-factor ignition pathway plus the shared pathway downstream, and it is most sensitive to factors VII, X, and II. Converting PT from different labs onto one common ruler gives the , which is what warfarin dose changes follow.
How a real deficiency is treated: usually 5–10 mg of K1 by intramuscular or intravenous injection; when bleeding is severe, prothrombin complex concentrate (PCC) or a transfusion is added to put the clotting factors back directly. These are decisions for a doctor, not something to do yourself.
Chapter 5
Why newborns get a K1 shot
A single 1 mg injection of vitamin K1 into a muscle within 6 hours of birth is one of the most widespread one-off vitamin interventions in the world. The American Academy of Pediatrics (AAP), the World Health Organization (WHO), and the US Centers for Disease Control and Prevention (CDC) all recommend it.
Why newborns run short: K1 barely crosses the placenta, so babies are born with almost no reserve; the gut has not yet grown the bacteria that make vitamin K; breast milk is low in K1; and the immature liver makes clotting factors slowly. Put together, newborns are the only group who run short even when fed normally.
What happens without it: vitamin K deficiency bleeding (VKDB). The most dangerous form, late-onset VKDB, strikes between two weeks and six months after birth; about half of cases present straight away as bleeding inside the skull, with high rates of death and severe disability. It occurs almost only in babies who are exclusively breastfed and did not get K1 at birth.
Does the shot cause cancer? No. The idea came from a single British study more than thirty years ago, and several later studies failed to reproduce the link.
If a baby who did not get the shot shows unexplained bleeding or bruising, or becomes drowsy, refuses feeds, vomits repeatedly, or has a seizure, go to the emergency department immediately.
Numbers · The three types of VKDB, and their history
Why infants are a high-risk group for vitamin K deficiency: the placenta lets very little K1 through, so reserves at birth are nearly zero; a newborn's gut has no bacteria yet, so there is no made inside the body; breast milk is low in K1 (about 1–4 µg/L, while formula, which is fortified, has about 50 µg/L); and the immature liver has little capacity to make clotting factors to begin with.
What happens without supplementation: vitamin K deficiency bleeding (VKDB) comes in three types:
Type
Timing
Site
Death and disability
Early-onset
< 24 h
Inside the skull, scalp, abdominal organs
Linked to the mother taking anticoagulants or anti-seizure drugs; rare
Classic
1–7 days
Gut, umbilicus, nose, skin
Mostly self-limited; 5–15% die
Late-onset
2 weeks – 6 months
About 50% present as intracranial hemorrhage (ICH)
About 20% die; about 40% of survivors severely disabled
ICH in the table means bleeding inside the skull. Late-onset VKDB occurs almost entirely in babies who are exclusively breastfed and given no K1, and history taught that lesson: before routine injection spread in the 1960s, bleeding disease of the newborn was a recognized cause of death; from 1961 the US made a 1 mg intramuscular K1 shot at birth routine, and the incidence of classic VKDB fell from 0.25–1.7% to almost zero; from the 2000s on, more US parents refused the shot, a cluster of newborn brain bleeds followed, and the AAP restated its recommendation in 2022.
Myth · The cancer rumor, and oral alternatives
Where the "K1 shot causes cancer" rumor came from, and how it was refuted: in 1992, a case-control study by Golding in Bristol, England, linked the K1 injection at birth to childhood leukemia. Several later studies (mostly case-control studies and pooled analyses of them, such as Roman 2002 and Fear 2003) failed to reproduce the link. The consensus today is that the K1 injection is safe, necessary, and does not cause cancer.
Oral alternatives (the Netherlands, Denmark, parts of Germany): for example 2 mg by mouth × 3 doses (at birth, at about 1 week, and at about 4 weeks), or a longer schedule. They work almost as well as the injection, but they need strict follow-up and carry a risk of missed doses; babies with biliary atresia (an absorption problem that is easy to miss) can still develop late-onset VKDB after oral doses. That is exactly why the injection is valuable as the last safety net.
The K1 shot on the first day of life is one of the cheapest and most effective public-health interventions of the past sixty years of newborn medicine.
Chapter 6
Blood thinners and vitamin K
Warfarin works by jamming exactly that K1 cycle: it holds down the reductase VKORC1, so clotting factors II, VII, IX, and X cannot fit their calcium-grabbing hands, and blood clots less readily. The price is that the drug is tied directly to your dinner table. Eating more or fewer greens shifts its effect, so it has to be watched with regular blood tests () and adjusted slowly.
If you or someone in your family takes warfarin, one rule is enough: do not give up greens entirely (that is outdated, wrong advice); eat about the same amount every day, and keep it steady, so the doctor can set the dose against that baseline. A sudden binge or a sudden stop can both push INR out of the safe range. Leave the blood tests and dose changes to the doctor; your job is to stay steady.
The newer direct oral anticoagulants () take a different route that bypasses the vitamin K cycle completely, so greens, natto, and supplements can all be eaten as usual; that is the biggest piece of everyday freedom DOACs bought. A few situations, such as a mechanical heart valve, still require warfarin.
In practice · Blood thinners and vitamin K, drug by drug
Are you or a family member on a blood thinner? One table sorts out how each kind relates to vitamin K:
Drug
Mechanism
Monitoring
K1 in the diet
K2 supplement
Warfarin
Blocks VKORC1
INR every 1–4 weeks
Keep it steady daily
Ask your doctor first
Dabigatran (Pradaxa)
Inhibits thrombin (IIa)
No routine monitoring
No limit
No limit
Apixaban (Eliquis)
Inhibits Xa
No routine monitoring
No limit
No limit
Rivaroxaban (Xarelto)
Inhibits Xa
No routine monitoring
No limit
No limit
Edoxaban (Lixiana)
Inhibits Xa
No routine monitoring
No limit
No limit
Low-molecular-weight heparin (Clexane)
Inhibits Xa and IIa
Occasional anti-Xa test
No limit
No limit
Aspirin
Inhibits platelets
None
No limit
No limit
Why most people now take a : over the past 15 years anticoagulation changed course, and most people moved from warfarin to direct oral anticoagulants (DOACs). Dabigatran (Pradaxa) holds down thrombin directly, while rivaroxaban (Xarelto), apixaban (Eliquis), and edoxaban (Lixiana) hold down factor Xa. The key is that they all bypass the vitamin K cycle: they act quickly and wear off quickly after stopping, they need no repeated tests, and, most conveniently, how many greens you eat and whether you take do not change their effect. In the most important trial, ARISTOTLE (a randomized trial in 18 201 people with atrial fibrillation), apixaban compared with warfarin cut stroke or systemic embolism by 21%, major bleeding by 31%, and deaths by 11%. Several similar large trials pointed the same way, so today's atrial fibrillation guidelines put DOACs first.
So why is warfarin still around: there are situations where DOACs cannot stand in, such as a mechanical heart valve, high-risk antiphospholipid syndrome, and severely reduced kidney function. There is also a very practical reason: warfarin is much cheaper, and DOACs cost a good deal more. For these people, understanding how K1 works still matters.
Common questions:
I take aspirin; can I eat more spinach? Yes, absolutely; aspirin does not run through the vitamin K cycle.I have just started warfarin; do I have to give up greens? No. Eat about the same amount every day, tell your doctor how much you usually eat, and the dose will be set against that.I have just switched to apixaban; can I go back to the natto I gave up on warfarin? Yes; DOACs are not affected by vitamin K.An older relative is on warfarin, eats fewer vegetables in winter, and the INR is running high. What now? This is a very real situation; test INR more often, and let the doctor lower the dose if needed.Will a K2 () supplement weaken warfarin? Yes. K2 can also supply KH₂, and every form of vitamin K works against warfarin, so tell your doctor before starting K2 and adjust the dose accordingly. For vitamin K's other half of the job, in bone and blood vessels, see Vitamin K2.
K1 is the clotting key in green leaves: food supplies plenty, and healthy people do not need a dedicated supplement. The real practical question is not whether you get enough but how it fits with your medicines.
References · 6
National Institutes of Health, Office of Dietary Supplements. (2021). Vitamin K — Fact Sheet for Health Professionals. Fact sheet (updated March 29, 2021; Wayback snapshot 19 September 2026): free phylloquinone is about 80% absorbed, but absorption from food is much lower - the body absorbs only 4% to 17% as much from spinach as from a tablet; Table 2: natto, 3 ounces, 850 mcg (as MK-7); egg, hard boiled, 1 large, 4 mcg (as MK-4) (fact sheet). ods.od.nih.gov/factsheets/VitaminK-HealthProfessional
Stafford, D. W. (2005). The vitamin K cycle. Journal of Thrombosis and Haemostasis, 3(8), 1873-1878. Reviews GGCX γ-carboxylation of factors II/VII/IX/X and VKORC1-mediated recycling of vitamin K epoxide — the molecular target of warfarin. 10.1111/j.1538-7836.2005.01419.x
Shearer, M. J. (2009). Vitamin K deficiency bleeding (VKDB) in early infancy. Blood Reviews, 23(2), 49-59. 10.1016/j.blre.2008.06.001
American Academy of Pediatrics, Committee on Fetus and Newborn (Hand, I., Noble, L., & Abrams, S. A.). (2022). Vitamin K and the newborn infant. Pediatrics, 149(3), e2021056036. Recommends 0.5–1 mg IM phylloquinone within 6 h of birth to prevent VKDB. 10.1542/peds.2021-056036
Granger, C. B., Alexander, J. H., McMurray, J. J. V., Lopes, R. D., Hylek, E. M., Hanna, M., et al. (2011). Apixaban versus warfarin in patients with atrial fibrillation (ARISTOTLE). New England Journal of Medicine, 365(11), 981-992. 10.1056/NEJMoa1107039
Booth, S. L., & Centurelli, M. A. (1999). Vitamin K: A practical guide to the dietary management of patients on warfarin. Nutrition Reviews, 57(9 Pt 1), 288-296. & supporting AHA 2003 statement on warfarin + dietary vitamin K consistency. Note: the id says 2004 but this record is the 1999 paper; text that cites 'Booth 2004' means this record. Abstract: patients on warfarin are advised to keep dietary vitamin K intake constant, and a constant intake meeting the then-current recommendation of 65-80 µg/day is the most acceptable practice; a dose-response of vitamin K on warfarin anticoagulation had not been established; plant oils and foods prepared with them (baked goods, margarines, salad dressings) are also sources (abstract, PMID 10568341). 10.1111/j.1753-4887.1999.tb01815.x