Vitamin K is essential for blood clotting, bone strength, and cardiovascular health. Without it, your body can’t produce the proteins that stop bleeding, build bone, or protect arteries from calcium buildup. Adults need between 90 and 120 micrograms daily, and most people get enough from leafy greens and other common foods.
Blood Clotting
Vitamin K’s most critical job is making your blood clot. Your liver needs it to produce four specific clotting factors. Without these proteins, even a minor cut or bruise could lead to prolonged, dangerous bleeding. This is the role vitamin K was originally discovered for (the “K” comes from the German word for coagulation), and it remains the most clinically important one.
This clotting function is also why newborns receive a vitamin K injection shortly after birth. Babies are born with very little vitamin K in their systems. It doesn’t transfer well across the placenta during pregnancy, and a newborn’s gut doesn’t yet have enough bacteria to produce it. Without the shot, some infants develop vitamin K deficiency bleeding, a condition that can cause uncontrolled bleeding in the brain and other organs, sometimes with no warning signs beforehand.
Bone Strength
Vitamin K activates a protein called osteocalcin, which is responsible for binding calcium into the bone matrix. In its inactive form, osteocalcin has limited ability to latch onto calcium. Vitamin K transforms it into its active form, allowing it to do its job. This makes vitamin K a key player alongside calcium and vitamin D in maintaining bone mineral density.
Both forms of vitamin K (more on that below) show up in significant concentrations in bone tissue, suggesting both play a role in skeletal health. While calcium and vitamin D get most of the attention in conversations about osteoporosis, vitamin K’s contribution as a cofactor for bone-building proteins is well established.
Cardiovascular Protection
Vitamin K also activates a protein in artery walls that prevents calcium from depositing where it shouldn’t. When this protein stays inactive due to low vitamin K levels, calcium can accumulate in blood vessels, stiffening them over time. This arterial calcification is a recognized risk factor for cardiovascular disease. Keeping vitamin K levels adequate helps your body run its own built-in defense against hardening of the arteries.
Brain Health
A less well-known role: vitamin K is required for producing sphingolipids, a class of fats found in high concentrations in brain cell membranes. These aren’t just structural components. They also function as signaling molecules, helping brain cells communicate. Research linking vitamin K status to cognition and behavior is still limited, but the biological pathway is clear, and certain medications that interfere with vitamin K (like the blood thinner warfarin) may affect this process.
K1 vs. K2: Two Forms, Different Jobs
Vitamin K comes in two main forms. K1 (phylloquinone) is found in green leafy vegetables and is the primary form in most people’s diets. K2 (menaquinone) comes from fermented foods and animal products, and is also produced by bacteria in your gut. Both are biologically active, but they behave differently in the body.
K1 is used up relatively quickly by the liver, turning over two to three times faster than K2. K2, particularly longer-chain versions, lingers in the body longer and reaches tissues beyond the liver more effectively. This is why K2 is often associated with the bone and cardiovascular benefits: it circulates long enough to activate proteins in those tissues. Your liver stores are predominantly made up of the longer-chain K2 forms produced by gut bacteria.
Best Food Sources
Dark leafy greens are by far the richest sources of vitamin K1. Just half a cup of boiled collard greens delivers 530 mcg, more than four times the daily recommendation for men. Here are some top sources:
- Collard greens (½ cup, boiled): 530 mcg
- Turnip greens (½ cup, boiled): 426 mcg
- Spinach (1 cup, raw): 145 mcg
- Kale (1 cup, raw): 113 mcg
- Broccoli (½ cup, boiled): 110 mcg
For K2, the standout food is natto, a Japanese fermented soybean dish, which packs 850 mcg per 3-ounce serving. Beyond natto, K2 sources are more modest: chicken breast provides about 13 mcg, ground beef about 6 mcg, cheddar cheese around 4 mcg per serving, and a hard-boiled egg about 4 mcg. If you eat leafy greens regularly, you’re likely covered on K1. Getting meaningful amounts of K2 is harder without fermented foods or supplements.
How Much You Need
The recommended adequate intake is 120 mcg per day for adult men and 90 mcg per day for adult women. These targets are easy to hit with even a modest intake of green vegetables. A single cup of raw spinach or kale gets you there. People who rarely eat vegetables or who have conditions affecting fat absorption (vitamin K is fat-soluble) are most at risk for falling short.
Vitamin K and Blood Thinners
If you take warfarin, vitamin K requires some attention. Warfarin works by blocking vitamin K’s clotting activity, so large swings in your vitamin K intake can make the medication less effective or overly potent. The key principle is consistency, not avoidance. You don’t need to cut out leafy greens. You need to eat roughly the same amount of vitamin K-rich foods from week to week so your medication dose stays calibrated. If you have a salad with spinach three times a week, keep doing that. Problems arise when intake is erratic, like avoiding greens for a week and then eating a large serving of collards.