Vitamin K1, also called phylloquinone, is a fat-soluble vitamin made exclusively by plants, green algae, and certain cyanobacteria, and it serves as the primary dietary form of vitamin K for humans. Its best-known job is keeping your blood clotting system running properly, but it also plays roles in bone and vascular health that researchers have been teasing apart over the past few decades. The vitamin is concentrated in green leafy vegetables and some vegetable oils, and most adults need only a modest daily amount to stay in good shape.
What Vitamin K1 Actually Does in Your Body
Your body uses vitamin K1 as a helper molecule for a single but critical chemical step: it activates certain proteins by modifying them after they have already been built. Specifically, an enzyme called gamma-glutamyl carboxylase uses vitamin K1 to attach a carbon-dioxide-derived group onto specific proteins, switching them from inactive to active forms.1PubMed Central. Vitamin K – sources, physiological role, kinetics, deficiency, detection, therapeutic use, and toxicity Without this activation step, those proteins sit around unable to do their jobs.
The most familiar of these proteins are clotting factors II, VII, IX, and X, which form the cascade that stops you from bleeding when you cut yourself. But the same activation process also applies to proteins outside the clotting system, including osteocalcin in bone and matrix Gla protein (MGP) in blood-vessel walls. MGP is one of the body’s main defenses against calcium deposits building up inside arteries, and it only works after vitamin K helps activate it.2PubMed Central. The Inhibitory Roles of Vitamin K in Progression of Vascular Calcification This is why vitamin K’s reach extends well beyond blood clotting.
How Your Body Recycles Its Supply
Here is an elegant bit of biology: every time vitamin K1 activates a protein, it gets chemically spent and converted into a form called vitamin K epoxide. If the story ended there, you would need enormous amounts of the vitamin to keep up. Instead, an enzyme called vitamin K epoxide reductase (VKOR) converts the spent form back into the active one, ready to be used again.3PubMed. The vitamin K cycle This recycling loop is known as the vitamin K cycle, and it is so efficient that only tiny amounts of the vitamin are needed each day to keep everything working.4PubMed. VKORC1 and the vitamin K cycle
Research has shown that VKOR can handle both steps of the recycling process on its own: converting vitamin K epoxide back to vitamin K, and then reducing it further to the fully active form.5PubMed Central. Purified vitamin K epoxide reductase alone is sufficient for conversion of vitamin K epoxide to vitamin K and vitamin K to vitamin KH2 This matters medically because the anticoagulant drug warfarin works by blocking this exact enzyme, which is why warfarin and vitamin K are so closely intertwined.
Where You Get It From Food
Phylloquinone is the major dietary form of vitamin K and is concentrated in leafy green plants.6PubMed Central. Vitamin K: food composition and dietary intakes The richest sources include kale, spinach, collard greens, turnip greens, and Swiss chard. Broccoli, broccoli raab, and certain lettuces also contain more than 100 micrograms per 100-gram serving.7Journal of Food Composition and Analysis. Phylloquinone (vitamin K1) content of vegetables Beyond leafy greens, some vegetable oils, particularly soybean and canola oil, contribute meaningfully to intake because they are used so widely in cooking and processed foods.
A rough guide to where vitamin K1 shows up in the diet:
- Dark leafy greens: Kale, spinach, collards, and Swiss chard are the heaviest hitters, often providing several hundred micrograms per cooked cup.
- Cruciferous vegetables: Broccoli, Brussels sprouts, and cabbage deliver moderate to high amounts.
- Vegetable oils: Soybean oil and canola oil contain phylloquinone, and because these oils are used in many packaged foods, they contribute more to the average person’s intake than you might expect.
- Herbs: Fresh parsley, basil, and cilantro pack a lot per gram, though you typically eat them in small quantities.
- Legumes and other vegetables: Most carry lower amounts, in the range of roughly 30 to 90 micrograms per 100 grams for legumes.
Getting the Vitamin Into Your Bloodstream
Vitamin K1 is fat-soluble, so it travels the same route as dietary fat. It gets absorbed in the small intestine, packaged into fat-carrying particles called chylomicrons, and shipped through the bloodstream.8PubMed. Transport of vitamin K to bone in humans In fasting blood samples, more than half of circulating vitamin K1 is found riding on chylomicrons and their remnants rather than on the more familiar LDL particles. Blood levels of the vitamin also track closely with triglyceride levels, which makes sense given the shared transport system.
There is a catch, though. When vitamin K1 is locked inside the cell membranes of leafy greens, your body has a much harder time extracting it compared with a supplement or an oil-based preparation. One study found that circulating vitamin K1 levels after eating spinach were dramatically lower than after taking a pharmaceutical preparation of the same vitamin, and adding butter to the spinach improved absorption but still did not come close to the supplement.9PubMed. Effect of food composition on vitamin K absorption in human volunteers The practical takeaway: eating your greens with some fat, whether from olive oil, nuts, or salad dressing, helps your body pull out more vitamin K1, but the bioavailability from whole vegetables will always trail behind a concentrated source.
An interesting genetic wrinkle affects how quickly your body clears vitamin K1 from circulation. A protein called apolipoprotein E (apoE) helps ferry the chylomicron remnants carrying the vitamin to your liver. People carry different variants of apoE, and the variant that clears these remnants most efficiently is associated with lower circulating vitamin K1 and a stronger response to anticoagulant drugs.8PubMed. Transport of vitamin K to bone in humans This is one reason why people on the same diet can have meaningfully different vitamin K levels in their blood.
How Much You Need
Recommended intakes for vitamin K1 are set as “adequate intakes” rather than strict recommended dietary allowances, because the evidence base is not precise enough for the harder target. In the United States, the adequate intake is 120 micrograms per day for adult men and 90 micrograms per day for adult women. The European Food Safety Authority takes a slightly different approach, setting an adequate intake of about 1 microgram per kilogram of body weight per day, which works out to roughly 70 micrograms per day for adults, including pregnant and lactating women.10PubMed Central. Dietary reference values for vitamin K For children, the European figures range from 12 micrograms per day for toddlers up to 65 micrograms per day for teenagers.
These numbers are not hard to reach if you eat vegetables regularly. A single serving of cooked kale or spinach can easily exceed a full day’s requirement. But surveys consistently find that many people fall short, especially those who eat few vegetables or follow heavily processed diets. Unlike some fat-soluble vitamins, vitamin K1 does not accumulate in large stores in your body. The liver holds a reserve, but it turns over relatively quickly, so you depend on a fairly steady dietary supply.
There is no established upper limit for vitamin K1 from food or supplements, because toxicity from the natural form has not been documented at any tested dose. The vitamin is sometimes given in large intravenous doses (5 to 10 milligrams) to reverse the effects of warfarin, and even those clinical doses are well tolerated, though the delivery vehicle used in injectable formulations can occasionally trigger allergic-type reactions.11PubMed Central. Characterizing the Severe Reactions of Parenteral Vitamin K1
Warfarin and the Diet Question
If you or someone you know takes warfarin, you have almost certainly heard the warning: avoid leafy greens, or at least keep your intake perfectly consistent. The logic sounds straightforward: warfarin works by blocking the vitamin K recycling enzyme, so eating more vitamin K should fight against the drug. And in principle, big swings in vitamin K intake can shift how strongly the blood thins.12PubMed Central. Warfarin and vitamin K intake in the era of pharmacogenetics13PubMed. Warfarin and its interactions with foods, herbs and other dietary supplements
The reality, though, is more nuanced than the standard dietary caution suggests. A systematic review of the available studies found conflicting evidence on whether normal dietary vitamin K intake actually destabilizes anticoagulation control. The studies that did show an interaction found it kicked in mainly at intakes above roughly 150 micrograms per day, which corresponds to roughly a cup of cooked broccoli or a third of a cup of cooked spinach. The review concluded that the evidence does not support telling patients to restrict vitamin K-rich foods when starting warfarin; instead, the more sensible advice is to keep your eating habits reasonably stable so you are not introducing sudden large changes in either direction.14PubMed Central. Interaction Between Dietary Vitamin K Intake and Anticoagulation by Vitamin K Antagonists: Is It Really True? A Systematic Review Some researchers have even argued that maintaining a minimum daily level of vitamin K intake could help stabilize, rather than disrupt, anticoagulation control.
Vitamin K1 Versus Vitamin K2
Vitamin K comes in two main natural forms: K1 (phylloquinone) from plants and K2 (menaquinones) from bacteria and certain animal-derived or fermented foods. Both support the same basic protein-activation process, but they differ in their side-chain structure, which translates into differences in how the body absorbs, distributes, and retains them.15PubMed Central. Vitamin K: Double Bonds beyond Coagulation Insights into Differences between Vitamin K1 and K2 in Health and Disease
Vitamin K1 is taken up primarily by the liver, which makes sense because the liver is where most clotting factors are made. Vitamin K2 subtypes, particularly the long-chain form MK-7, tend to circulate longer in the blood and reach tissues beyond the liver, including bone and arterial walls, more efficiently. This has led to a popular claim that K2 is “better” for bone and cardiovascular health. The evidence is still being sorted out. What is well-established is that K1 is the dominant dietary form for most people, it clearly supports clotting, and in populations that eat plenty of green vegetables, K1 intake is usually sufficient to keep both clotting and non-clotting vitamin K-dependent proteins reasonably well supplied. Your body can also convert a portion of dietary K1 into the K2 subtype MK-4 in certain tissues, adding another layer of flexibility.
Why Newborns Get a Vitamin K Shot
Babies are born with very low stores of vitamin K because the vitamin does not cross the placenta efficiently, and breast milk provides only small amounts. Without supplementation, the risk of a dangerous bleeding condition called vitamin K deficiency bleeding (VKDB) is about 1 in 60.16PubMed. Prevention of Vitamin K Deficiency Bleeding VKDB can cause bleeding in the brain, gut, or skin, and the late-onset form, which appears weeks after birth, can be life-threatening.
A single intramuscular injection of 1 milligram of vitamin K1 given shortly after birth drops the risk to about 1 in 100,000.16PubMed. Prevention of Vitamin K Deficiency Bleeding A Cochrane review confirmed the effectiveness of this single-dose injection for preventing the classic early form of the bleeding disorder.17Cochrane Database of Systematic Reviews. Prophylactic vitamin K for vitamin K deficiency bleeding in neonates This is one of the clearest public health wins in neonatal medicine, and the practice has been standard in most countries for decades. Oral vitamin K regimens exist as alternatives but require multiple doses over several weeks and have a slightly less reliable track record for preventing late-onset VKDB.
Does Cooking Destroy the Vitamin?
A reasonable worry with any nutrient is whether cooking wipes it out. For vitamin K1, the answer depends on both the vegetable and how you cook it. One study comparing different cooking methods found that microwaving caused the greatest losses in some vegetables (like crown daisy and mallow) but the least loss in spinach and chard.18PubMed Central. Effect of different cooking methods on the content of vitamins and true retention in selected vegetables There is no single rule that applies across the board.
Interestingly, some cooking methods actually increase the measurable vitamin K1 content in vegetables. Blanching, boiling, steaming, and grilling all raised phylloquinone levels in many of the vegetables tested in a study of 21 vegetable types.19PubMed Central. Effect of different cooking method on vitamin E and K content and true retention of legumes and vegetables commonly consumed in Korea This likely happens because cooking breaks down cell walls and drives off water, concentrating the vitamin per gram of food. For legumes, boiling tended to reduce vitamin K1 retention, so the effect really does go both ways depending on the food.
The practical implication: you do not need to eat your kale raw to get its vitamin K1. Lightly cooking greens with a bit of fat may actually give you a double advantage, making the vitamin both more concentrated and more absorbable.
What Vitamin K1 Does for Plants
Before vitamin K1 was ever a concern for human nutrition, it was a molecule that plants evolved to use in photosynthesis. In plant cells, phylloquinone sits inside the photosystem I complex, a molecular machine that captures light energy, and acts as an electron carrier, passing electrons along the chain that ultimately converts light into chemical energy.20PubMed. Biosynthesis of vitamin K1 (phylloquinone) by plant peroxisomes and its integration into signaling molecule synthesis pathways This is why the vitamin is most concentrated in the greenest parts of a plant: those are the tissues doing the most photosynthesis. Pale inner lettuce leaves contain far less than dark outer leaves, and fruits and roots are generally poor sources.
The fact that plants make phylloquinone and animals do not is a tidy example of nutritional co-dependence between kingdoms. Plants synthesize the molecule for their own energy needs, and animals repurposed it for an entirely different function: protein activation in blood clotting and tissue maintenance.21Bentham Science / Mini-Reviews in Medicinal Chemistry. Phylloquinone (Vitamin K1): Occurrence, Biosynthesis and Functions It also helps explain a quirk of the food supply: the greener and leafier a food is, the more vitamin K1 it contains, because the vitamin is literally embedded in the photosynthetic equipment inside every chloroplast.
Signs of Deficiency in Adults
Outright vitamin K1 deficiency is uncommon in healthy adults, partly because the recycling system is so efficient and partly because the vitamin is widespread in the food supply, including in cooking oils. When deficiency does develop, the hallmark is impaired blood clotting. You might bruise more easily, notice bleeding gums, or find that small cuts take unusually long to stop bleeding. In more severe cases, internal bleeding becomes a risk.
The people most vulnerable to deficiency include those with conditions that impair fat absorption, such as celiac disease, cystic fibrosis, or short bowel syndrome. Long-term antibiotic use can also play a role, since gut bacteria produce some vitamin K2 (though not K1), and wiping them out removes that supplementary source. Certain cholesterol-lowering drugs and weight-loss medications that block fat absorption can reduce vitamin K1 uptake as well. For these groups, monitoring and supplementation may be warranted beyond what a normal diet provides.
Measuring Vitamin K1 Status
Assessing whether someone has enough vitamin K1 is less straightforward than for many other vitamins. Blood levels of phylloquinone reflect recent dietary intake more than long-term stores, so a single blood draw can be misleading. The most common functional test is the prothrombin time (PT), which measures how quickly your blood clots. But PT only becomes abnormal when vitamin K status is severely depleted, because the recycling system can compensate for a long time before clotting slows down.
A more sensitive marker is the level of undercarboxylated (inactive) forms of vitamin K-dependent proteins. For example, measuring undercarboxylated osteocalcin in blood gives a window into whether vitamin K supply is adequate to fully activate proteins outside the clotting cascade. Researchers use these markers in studies, but they are not part of routine clinical testing for most patients. Measuring phylloquinone in foods themselves requires sophisticated laboratory techniques, typically involving mass spectrometry with labeled internal standards, which helps explain why food composition data for vitamin K1 has lagged behind data for other nutrients.22PubMed. Analysis of vitamin K1 in fruits and vegetables using accelerated solvent extraction and liquid chromatography tandem mass spectrometry with atmospheric pressure chemical ionization
Supplements and Who Might Consider Them
Vitamin K1 supplements are widely available, typically in doses ranging from 100 to 1,000 micrograms. For most people who eat a varied diet that includes green vegetables, supplements are unnecessary. The groups who stand to benefit most are those with documented fat-malabsorption conditions, people on medications that interfere with vitamin K absorption, and in some cases older adults whose diets are nutritionally limited.
If you take warfarin or another vitamin K antagonist, starting a vitamin K supplement without telling your prescriber is a bad idea, not because vitamin K itself is dangerous, but because it will alter the drug’s effect and throw off your dosing. That said, the evidence reviewed earlier suggests that consistent moderate vitamin K intake is more important for anticoagulation stability than strict avoidance. Some anticoagulation clinics now recommend small daily vitamin K supplements for patients with erratic diets, precisely to smooth out the day-to-day variability that causes INR swings.
Vitamin K1 supplements absorb better when taken with a fat-containing meal, which mirrors the general absorption principle for the whole vitamin K family. Gel-cap formulations that include an oil base tend to have better bioavailability than dry tablets, for the same reason that spinach eaten with butter outperforms spinach eaten plain.