Vitamin K is essential for blood clotting, bone strength, and keeping your arteries flexible. Without it, your blood literally cannot form a clot, which is why newborns receive a vitamin K injection within hours of birth. Adults need 90 to 120 micrograms daily, and most people get enough from leafy greens and other common foods.
How Vitamin K Makes Blood Clot
Vitamin K’s most critical job is activating the proteins your body needs to stop bleeding. Four clotting factors (known in medicine as II, VII, IX, and X) depend entirely on vitamin K to function. Without it, these proteins exist in your blood but sit idle, unable to do their job. Vitamin K chemically modifies them through a process that lets them bind to calcium, which is the trigger that starts the chain reaction of clot formation.
Vitamin K also activates two proteins, protein C and protein S, that work in the opposite direction: they prevent excessive clotting. So vitamin K doesn’t just help you stop bleeding. It also keeps your clotting system in balance, preventing clots from forming where they shouldn’t.
Building Stronger Bones
Your bones contain a protein called osteocalcin, produced by bone-building cells. Osteocalcin needs vitamin K to become active. Once activated, it binds calcium to the mineral structure of bone, strengthening the matrix that gives bones their density and resilience. When vitamin K levels are low, osteocalcin circulates in an inactive form and can’t do this work. In fact, the level of inactive osteocalcin in your blood is considered one of the more sensitive markers of vitamin K deficiency.
Protecting Your Arteries
Calcium belongs in your bones, not in the walls of your blood vessels. Vitamin K helps keep it that way. A protein called Matrix Gla Protein, secreted by cells in your artery walls, acts as a powerful local inhibitor of vascular calcification, the stiffening and hardening of arteries that raises cardiovascular risk. But this protein only works after vitamin K activates it. Without enough vitamin K, the protein can’t do its job, and calcium deposits can build up in artery walls over time.
K1 vs. K2: Different Jobs, Different Sources
Vitamin K comes in two main forms, and they behave differently in your body.
Vitamin K1 (phylloquinone) is found in green leafy vegetables and some plant oils. After you eat it, your liver preferentially holds onto it to keep your clotting factors active. It rarely makes it to other tissues in significant amounts.
Vitamin K2 (menaquinones) comes from animal foods like butter, egg yolks, and certain cheeses, as well as fermented foods. The long-chain forms of K2, particularly MK-7 found in fermented soy (natto), have a longer half-life in your bloodstream than K1. That means they circulate longer and reach tissues beyond the liver, including bone and blood vessel walls. This is why K2 is more closely associated with bone and cardiovascular benefits, while K1 primarily supports clotting.
Your gut bacteria also produce some K2, though very little of it gets absorbed because it stays bound to bacterial membranes in the intestine.
Best Food Sources
Vitamin K1 is abundant in dark leafy greens. A single cup of cooked spinach delivers roughly 890 micrograms, which is several times the daily adequate intake. Cooked turnip greens provide between 425 and 850 micrograms per cup depending on preparation. Other strong sources include kale, collard greens, broccoli, and Brussels sprouts.
For K2, your best options are fermented foods (natto is the richest source by far), aged cheeses, egg yolks, and butter from grass-fed animals. Because both forms of vitamin K are fat-soluble, eating them with some dietary fat improves absorption. A salad with olive oil dressing, for example, will deliver more vitamin K than the same greens eaten plain.
How Much You Need
There isn’t enough evidence to set a precise recommended dietary allowance for vitamin K, so nutrition guidelines use an “adequate intake” level instead. For adults 19 and older, that’s 120 micrograms per day for men and 90 micrograms for women, including during pregnancy and breastfeeding. Most people who eat vegetables regularly meet this threshold without trying.
What Deficiency Looks Like
Vitamin K deficiency is uncommon in healthy adults who eat a varied diet, but when it occurs, bleeding is the hallmark. You might notice unusual bruising, tiny red or purple spots on the skin (petechiae), or prolonged bleeding from minor cuts or blood draws. In clinical settings, a prolonged prothrombin time, a lab test measuring how quickly your blood clots, is the classic sign. It returns to normal rapidly after vitamin K is restored.
Deficiency is more likely in people with conditions that impair fat absorption, such as celiac disease, Crohn’s disease, or chronic liver disease, since vitamin K depends on fat for absorption.
Why Newborns Get a Vitamin K Shot
Babies are born with very little vitamin K for two reasons: it doesn’t cross the placenta efficiently during pregnancy, and a newborn’s intestines don’t yet have the bacteria needed to produce it. This leaves infants vulnerable to vitamin K deficiency bleeding (VKDB), a condition that can cause dangerous hemorrhaging in the brain, chest, or abdomen. VKDB can develop anytime in the first six months of life and is more common in exclusively breastfed infants, because breast milk contains less vitamin K than formula. A single injection at birth prevents it, which is why the shot has been standard practice in hospitals for decades.
The Warfarin Connection
If you take warfarin, a common blood-thinning medication, vitamin K intake matters more than usual. Warfarin works by blocking the enzyme that recycles vitamin K in your liver. This slows the activation of clotting factors, which is the whole point of the drug. But if you suddenly eat a large amount of vitamin K-rich food, you can partially override warfarin’s effect and increase your clotting risk. Conversely, eating much less vitamin K than usual can make the drug too potent.
The goal isn’t to avoid vitamin K. It’s to keep your intake consistent from day to day so your medication dose stays calibrated. If you eat a salad with spinach most days, keep doing that. Problems arise when intake swings dramatically. In cases of warfarin overdose, vitamin K is actually given as the antidote, because it rapidly restores the body’s ability to clot.