What Is Vitamin K? Forms, Functions, and Food Sources

Vitamin K is a fat-soluble vitamin best known for its role in blood clotting, but it also helps regulate bone mineralization and protect arteries from calcium buildup. It comes in two main natural forms found in food, and the body handles each one differently. The story of vitamin K is richer than the clotting connection most people associate it with, and the way you eat it turns out to matter almost as much as how much you eat.

The Two Natural Forms

Vitamin K is not a single molecule. The name covers a family of compounds that share a common chemical backbone but differ in a crucial way. All vitamin K molecules have the same ring-shaped head, called a naphthoquinone ring. What sets them apart is the length and structure of the carbon chain attached to that ring.¹

Vitamin K1, called phylloquinone, is made by plants and is the form you get from green vegetables. It has a single double bond in its carbon tail. Vitamin K2, called menaquinone, is produced mainly by bacteria and shows up in fermented foods and some animal products. Its tail contains multiple double bonds, and it comes in several subtypes labeled by chain length: MK-4, MK-7, MK-9, and so on, where the number refers to how many repeating units make up the tail.¹ These chain-length differences are not just chemistry trivia; they change how the body absorbs, transports, and stores each form.

There is a third form, vitamin K3 (menadione), which is synthetic. It has been studied in cancer research as an experimental agent, but it is not used as a nutritional supplement in humans because of toxicity concerns at therapeutic doses.² For practical purposes, K1 and K2 are the forms that matter for your health.

What Vitamin K Actually Does Inside Your Body

Vitamin K’s job is to act as a helper molecule for a single enzyme called gamma-glutamyl carboxylase, or GGCX. This enzyme is the only one in the human body that uses vitamin K as a cofactor.³ What it does is modify certain proteins by adding a chemical group to specific spots on them. Without that modification, those proteins cannot bind calcium properly and cannot do their jobs.

The process works like a cycle. Vitamin K starts in its active form (a hydroquinone), gets used up during the modification of a protein, and is converted into an inactive form (an epoxide). Another enzyme then recycles the spent vitamin K back to its active form so it can be used again.⁴ This recycling is efficient enough that you need only tiny amounts of vitamin K each day compared to most other vitamins. It also explains why anticoagulant drugs like warfarin are so effective: they block the recycling enzyme, which grinds the whole cycle to a halt.

Blood Clotting

The most well-known vitamin K-dependent proteins are the clotting factors. When you cut yourself, a cascade of protein interactions leads to a clot that stops the bleeding. Several of the proteins in that cascade, including factors II, VII, IX, and X, need vitamin K to become functional.⁵ Without enough vitamin K, these proteins are produced in an undercarboxylated form that cannot participate in clotting properly.

But the clotting system is not just about forming clots. It also has built-in brakes. Protein C and protein S, two natural anticoagulants that prevent excessive clotting, are also vitamin K-dependent.⁵ This means vitamin K does not simply “thicken” your blood in one direction. It supports the entire balancing act between clotting and anti-clotting, which is why the consequences of deficiency can show up as unexpected bleeding rather than as a one-sided shift.

Bones and Arteries

Beyond blood clotting, vitamin K activates proteins involved in calcium regulation in two important tissues: bone and blood vessels.

In bone, the key protein is osteocalcin. Made by bone-building cells, osteocalcin needs vitamin K-dependent carboxylation before it can bind calcium and hydroxyapatite, the mineral that gives bones their hardness.⁶ When vitamin K is scarce, a larger fraction of osteocalcin circulates in its inactive, undercarboxylated form. Observational studies have linked higher levels of this undercarboxylated osteocalcin with age-related bone loss. That said, clinical trials have not consistently shown that vitamin K supplements reduce fractures in the general population, so the relationship between vitamin K intake and bone strength is not as straightforward as supplement marketing sometimes suggests.⁷

In blood vessels, the relevant protein is matrix Gla protein (MGP). MGP is widely considered a potent inhibitor of arterial calcification, the stiffening of artery walls that contributes to cardiovascular disease.⁸ Like osteocalcin in bone, MGP must be carboxylated by vitamin K to do its job. When vitamin K is insufficient, inactive forms of MGP accumulate, and elevated levels of these inactive forms have been associated with vascular calcification.⁹ A Mendelian randomization study in a Flemish population found that genetically driven increases in inactive MGP were causally related to adverse cardiovascular outcomes, strengthening the case that this is not just a coincidental association.¹⁰

The bone and vascular functions hint at something interesting: vitamin K helps direct calcium to the places it belongs (bone) and away from the places it does not (artery walls). This idea, sometimes called the “calcium paradox,” has generated a lot of research interest, though the picture is still incomplete.

Where You Get It From Food

Vitamin K1 is concentrated in dark leafy greens. Spinach, kale, collard greens, broccoli, and Brussels sprouts are among the richest sources. In studies measuring the vitamin K1 content of vegetables, levels ranged from about 35 to over 500 micrograms per 100 grams depending on the type of vegetable.¹¹ Plant oils, particularly soybean and canola oil, also contribute meaningful amounts of K1 to many diets.

Vitamin K2 sources are quite different. Fermented foods are the standout category. Natto, a Japanese dish made from fermented soybeans, contains remarkably high levels of the MK-7 form, measured at about 900 micrograms per 100 grams in one analysis.¹² Other fermented foods contribute K2 as well, but at much lower concentrations. Kimchi was found to contain about 42 micrograms per 100 grams of K1 but considerably less K2, while kefir contributed some MK-9. Fermented soybean pastes from China contained a broader mix of menaquinone subtypes, including MK-6, MK-7, and MK-8.¹² Animal products like egg yolks, liver, and certain cheeses contain MK-4 and longer-chain menaquinones, but typically in modest amounts compared to natto.

Absorption Is Not Automatic

Because vitamin K is fat-soluble, your body needs dietary fat present in the gut to absorb it efficiently. This matters more than many people realize. In a study where participants consumed the same dose of vitamin K1 from spinach with butter, spinach without butter, or a pharmaceutical preparation, the results were striking. Circulating K1 levels after eating spinach without butter were about 24 times lower than after the pharmaceutical concentrate. Adding butter improved absorption, but levels were still roughly 7.5 times lower than the concentrate.¹³ The K1 in plant cells is tightly bound inside membranes, which makes it much harder to absorb than free vitamin K.

The practical takeaway: eating your greens with some fat, whether that is olive oil on a salad or butter on steamed broccoli, can substantially improve how much vitamin K you actually absorb. A raw spinach salad with fat-free dressing may look healthy on paper, but your body extracts only a fraction of the vitamin K it contains.

The different forms of vitamin K also behave differently after they are absorbed. K1 and the short-chain form MK-4 have relatively short half-lives, remaining detectable in blood for about 8 to 24 hours. Longer-chain menaquinones like MK-7 stick around much longer, with a half-life of several days, which means they have more time to reach tissues beyond the liver.¹⁴¹⁵ K1 concentrates heavily in the liver, where it supports clotting factor production. K2 forms, particularly the longer-chain menaquinones, distribute more to tissues like bone and blood vessels.¹⁶ This tissue-distribution difference is one reason researchers have become increasingly interested in K2’s potential benefits for bone and cardiovascular health independent of K1.

Gut Bacteria and Vitamin K Production

Your intestinal bacteria produce menaquinones as a byproduct of their metabolism. For decades, this was assumed to be a significant source of vitamin K for humans, but the actual contribution remains surprisingly unclear. Much of the bacterially produced K2 sits in the colon, where fat absorption is limited, so how much of it actually gets into your bloodstream is debated.

Research using isotope-labeled vitamin K in mice has confirmed that gut bacteria do modify dietary vitamin K and produce their own menaquinone forms, including MK-4, MK-10, MK-11, and MK-12.¹⁷ This finding is notable because it shows the gut microbiome does not just passively produce vitamin K but actively transforms the vitamin K you eat. Still, the picture in humans is incomplete, and relying on gut bacteria as your primary vitamin K source would be unwise, especially given that antibiotics can disrupt this production dramatically.

Why Newborns Are at Special Risk

Vitamin K does not cross the placenta well, and breast milk contains relatively little of it. This combination leaves newborns with very low vitamin K stores, putting them at risk for a condition called vitamin K deficiency bleeding (VKDB). Without a vitamin K injection at birth, roughly one in 59 newborns develops some form of VKDB.¹⁸ The condition can range from minor bruising and bleeding from the umbilical stump (classic VKDB, appearing in the first week) to devastating intracranial hemorrhage (late VKDB, appearing between weeks two and twelve).¹⁹

A single intramuscular injection of 1 milligram of vitamin K after birth drops the risk dramatically, to about one in 100,000.¹⁸ Surveillance data from multiple countries confirm that intramuscular vitamin K prophylaxis reduces the risk of late VKDB to near zero.²⁰ This is one of the most effective single-dose preventive interventions in medicine, and it is the reason maternity wards worldwide give newborns a vitamin K shot within hours of birth. Oral vitamin K regimens exist as alternatives in some countries, but the evidence for intramuscular administration is stronger, particularly for preventing late-onset bleeding.

The Warfarin Connection

If you or someone you know takes warfarin (sold under brand names like Coumadin), vitamin K is not just a nutritional topic; it directly affects how the medication works. Warfarin blocks the enzyme that recycles vitamin K, deliberately impairing the clotting cycle to prevent dangerous blood clots in people at risk. Vitamin K intake from food and supplements is a controllable factor that influences how much warfarin someone needs. Large swings in vitamin K consumption, like suddenly eating a week’s worth of kale or abruptly cutting out all green vegetables, can push the anticoagulant effect too low or too high.²¹

The practical advice is not to avoid vitamin K-rich foods while on warfarin. It is to keep your intake consistent from week to week so your dose can be calibrated around it. People on warfarin who eat a consistently moderate amount of leafy greens tend to have more stable anticoagulation than those who swing between zero and a lot.

What Causes Deficiency in Adults

Outright vitamin K deficiency is uncommon in healthy adults who eat a varied diet, because the vitamin is widespread in foods and the body recycles it efficiently. When deficiency does occur, it usually involves one of a few recognizable situations:

Malabsorption: Conditions that impair fat absorption, such as celiac disease, Crohn’s disease, short bowel syndrome, or chronic liver disease, reduce vitamin K uptake because the vitamin depends on fat for absorption.
Prolonged antibiotic use: Extended courses of broad-spectrum antibiotics can wipe out gut bacteria that produce menaquinones, while simultaneously reducing appetite and dietary intake. Case reports have documented severe vitamin K deficiency coagulopathy triggered by antibiotic-associated disruption of the gut microbiome, particularly when combined with poor nutrition and persistent diarrhea.²²
Extremely restricted diets: People who eat almost no vegetables or fermented foods for prolonged periods can develop subclinical deficiency, especially if they also have limited fat intake.

Subclinical deficiency, where you have enough vitamin K for normal clotting but not enough for full activation of bone and vascular proteins, is thought to be more common than overt bleeding problems. This subclinical state is harder to detect and is an active area of research.

How Doctors Measure Vitamin K Status

There is no single perfect test for vitamin K status, which is one reason research in this area can be tricky. The most commonly used marker is serum phylloquinone (K1) concentration, with levels below 0.15 micrograms per liter considered deficient. The downside of this test is that it only measures K1, ignores K2 forms entirely, and fluctuates with recent meals.²³

A more functional approach measures proteins that vitamin K failed to activate. PIVKA-II (a clunky abbreviation for “protein induced by vitamin K absence”) reflects how well the liver is using vitamin K for clotting factor production. Using both serum K1 and PIVKA-II together gives a more complete picture, capturing both how much vitamin K is circulating and how well it is being used.²³ For bone-specific vitamin K status, the percentage of undercarboxylated osteocalcin can be measured as well.²⁴ These tests are not routine in standard blood panels; they are typically ordered when deficiency is clinically suspected or in research settings.

Does Cooking Destroy Vitamin K?

This is a common concern for people trying to maximize their intake from vegetables. The answer is reassuring but a little complicated: cooking generally preserves or even increases the measurable vitamin K content of most vegetables. One study found that phylloquinone levels increased in most vegetables after blanching, boiling, steaming, or grilling.¹¹ The likely explanation is that cooking breaks down plant cell walls, releasing vitamin K from the membrane structures where it is trapped and making it more accessible.

The story is different for some specific vegetable-and-method combinations. Microwave cooking caused substantial vitamin K losses in certain leafy vegetables like crown daisy and mallow, while spinach and chard retained their vitamin K well under the same method.²⁵ Boiling reduced vitamin K retention in legumes.¹¹ So the impact of cooking depends heavily on what you are cooking and how.

For cooking oils, the main threat to vitamin K is not heat but light. Vitamin K1 in oils was relatively stable during heating but was rapidly destroyed by exposure to daylight and fluorescent light. Storing oils in amber or dark glass containers protected the vitamin K from this degradation.²⁶ If you keep your olive or canola oil in a clear bottle on a sunny countertop, you may be losing some of its vitamin K content over time.

Vitamin K and the Brain

An emerging area of research connects vitamin K to neurological health. Vitamin K influences the production of certain sphingolipids, a class of fats that are abundant in brain cell membranes and play roles in cell signaling and survival. Researchers have proposed that vitamin K levels in the body are associated with changes involved in cognitive decline and dementia, partly through effects on the brain’s sphingolipid profile.²⁷ The evidence here is still early stage, mostly drawn from animal models and observational data in humans. Nobody should start taking vitamin K supplements to prevent Alzheimer’s disease based on current evidence, but the connection between vitamin K and brain lipid metabolism is a genuine area of active investigation, not fringe speculation.

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