Vitamin K is a fat-soluble vitamin your body needs to form blood clots and build strong bones. Without enough of it, even a small cut could bleed far longer than it should. Adults need between 90 and 120 micrograms per day, an amount easily reached by eating green vegetables, though many people fall short.
The Two Main Forms: K1 and K2
Vitamin K isn’t a single molecule. It comes in two naturally occurring forms that behave differently in your body.
Vitamin K1 (phylloquinone) is made by plants and is the form you eat most of. Leafy greens like spinach, kale, and broccoli are the richest sources. A 100-gram serving of cooked spinach delivers roughly 500 micrograms, well over a full day’s requirement in a single side dish.
Vitamin K2 (menaquinones) is a family of related compounds, labeled MK-4 through MK-13 based on their chemical structure. Some are produced by bacteria in your gut; others come from fermented foods and animal products. Natto, a Japanese fermented soybean dish, is the most concentrated dietary source by a wide margin, delivering about 998 micrograms of the MK-7 form per 100 grams. Cheese, butter, milk, and egg yolks provide smaller amounts of the MK-4 form. In the United States, poultry products contain MK-4 as well, because poultry feed includes a synthetic precursor that chickens convert into it.
How It Works in Blood Clotting
Vitamin K acts as a helper molecule for an enzyme that modifies certain proteins so they can bind calcium. This modification is essential for blood clotting. Four clotting factors (known in medicine as factors II, VII, IX, and X) depend on vitamin K to function. Without it, these proteins sit in your bloodstream in an inactive form and can’t participate in building a clot when you’re injured.
Your body also uses vitamin K to activate two natural anticoagulant proteins, protein C and protein S, which prevent clots from growing too large. So vitamin K doesn’t simply “thicken” blood. It keeps the entire clotting system in balance, supporting both the formation and the regulation of clots.
Bone Strength and Arterial Health
Beyond clotting, vitamin K activates proteins that direct where calcium goes in your body. This matters for two reasons.
In bones, vitamin K activates a protein called osteocalcin. Once activated, osteocalcin binds calcium and locks it into the bone matrix, contributing to mineral density and skeletal strength. When vitamin K is low, osteocalcin can’t do this job effectively, and long-term deficiency has been linked to osteoporosis and poor bone development.
In blood vessels, vitamin K activates a different protein (matrix Gla protein) that prevents calcium from depositing in arterial walls. This protein binds to calcium crystals and stops them from growing, essentially keeping arteries flexible rather than stiff and calcified. A 2024 meta-analysis of 17 clinical trials found that vitamin K supplementation, particularly the K2 form, improved insulin resistance compared to placebo. However, the same analysis found no significant effects on inflammation markers, blood sugar, cholesterol, or body weight, so the cardiovascular benefits of supplementation beyond a normal diet remain uncertain.
How Much You Need
The current adequate intake set by health authorities is 120 micrograms per day for adult men and 90 micrograms per day for adult women. These aren’t hard minimums that trigger immediate problems if you miss them. They’re estimates of the amount that keeps clotting and bone metabolism running normally in most people.
Because vitamin K1 is so concentrated in green vegetables, people who regularly eat salads, cooked greens, or cruciferous vegetables typically get plenty. People who eat very few vegetables, follow restrictive diets, or have conditions that impair fat absorption are more likely to run low. Since vitamin K is fat-soluble, your body needs dietary fat present in the gut to absorb it efficiently.
Signs of Deficiency
True vitamin K deficiency is uncommon in healthy adults, but when it occurs, the hallmark sign is abnormal bleeding. This can show up as easy bruising, bleeding gums, heavy menstrual periods, blood in urine or stool, or wounds that take unusually long to stop bleeding. In severe cases, bleeding can be spontaneous, occurring without any injury.
People at higher risk include those with liver disease, inflammatory bowel disease, celiac disease, or other malabsorption conditions that interfere with fat digestion. Certain medications, particularly blood thinners that work by blocking vitamin K’s recycling pathway, also increase risk. Over the long term, chronic low intake contributes to weaker bones and may raise cardiovascular risk through unchecked arterial calcification.
Why Newborns Get a Vitamin K Shot
Babies are born with very little vitamin K stored in their bodies, and breast milk provides only small amounts. This puts newborns at risk for a condition called vitamin K deficiency bleeding, which can cause dangerous internal hemorrhaging, including bleeding in the brain.
To prevent this, hospitals routinely give newborns a single injection of vitamin K within the first six hours after birth. The standard dose is 1.0 milligram for most infants, or 0.5 milligrams for very small babies weighing under about 3.3 pounds. For parents who decline the injection, an oral alternative exists: 2.0 milligrams given at the first feeding, repeated at 2 to 4 weeks and again at 6 to 8 weeks. The injection is preferred because it provides more reliable, longer-lasting protection with a single dose.
Vitamin K and Blood Thinners
If you take warfarin or a similar blood thinner that works by opposing vitamin K, the single most important dietary rule is consistency. You don’t need to avoid vitamin K-rich foods. You need to eat roughly the same amount of them from week to week so your medication dose stays properly calibrated.
A sudden spike in vitamin K intake, like eating a large spinach salad when you normally don’t, can reduce the drug’s effectiveness and raise clotting risk. A sudden drop can intensify the drug’s effect and increase bleeding risk. If you plan to change your diet significantly, that’s something to discuss with whoever manages your dosing. For people whose blood-thinning levels fluctuate without an obvious cause, clinicians sometimes recommend a small daily vitamin K supplement (100 to 200 micrograms) to create a steady baseline, with close monitoring and dose adjustments as needed.
Best Food Sources at a Glance
- Spinach (cooked): 480 to 544 mcg per 100 g, depending on preparation. One of the most concentrated K1 sources available.
- Kale, collard greens, Swiss chard: All deliver several hundred micrograms per cooked serving, making any dark leafy green a reliable choice.
- Broccoli and Brussels sprouts: Lower than spinach but still substantial, and easier for some people to eat in large quantities.
- Natto: The richest known source of K2 (MK-7) at roughly 998 mcg per 100 g. Its strong flavor limits appeal outside of Japanese cuisine, but even a small serving delivers a large dose.
- Cheese, butter, and egg yolks: Modest sources of K2 (MK-4). Not enough on their own to meet daily needs, but they contribute when combined with other foods.
Cooking does not significantly destroy vitamin K, so steamed or sautéed greens are just as useful as raw ones. Adding a source of fat, even a small drizzle of olive oil, helps your body absorb the vitamin more effectively.