Vitamin K1, also called phylloquinone, is a fat-soluble vitamin found primarily in green plants. Its main job is helping your blood clot properly, but it also plays a role in bone metabolism. Adults need between 90 and 120 micrograms per day, an amount easily reached through a diet that includes leafy greens or vegetable oils.
What Vitamin K1 Does in Your Body
Vitamin K1’s most critical function is activating the proteins your body needs to form blood clots. Without it, four specific clotting factors (known as factors II, VII, IX, and X) can’t do their job. When you cut yourself, these proteins are what stop the bleeding. Vitamin K1 acts as a helper molecule in a recycling process: it gets converted into an active form, donates its chemical energy to activate clotting proteins, then gets recycled back to its original form to be used again. This loop is called the vitamin K cycle.
Beyond clotting, vitamin K1 helps activate a bone protein called osteocalcin. This protein binds calcium into bone tissue. A recent meta-analysis of clinical trials found that vitamin K supplementation significantly improved the activation of osteocalcin, converting more of it into its functional form. However, the effect on actual bone mineral density was less convincing. Supplementation showed a possible benefit at the lumbar spine, but no significant improvements at the hip, femoral neck, or wrist. In short, vitamin K1 keeps your bone-building machinery working, though taking extra beyond what you need may not dramatically strengthen bones on its own.
How It Differs From Vitamin K2
Vitamin K comes in two main forms: K1 (phylloquinone), which comes from plants, and K2 (menaquinone), which comes from fermented foods and animal products. Both support blood clotting, but they behave differently once absorbed.
Vitamin K1 is carried through the bloodstream primarily by fat-rich lipoproteins and clears from the body relatively quickly, remaining detectable in plasma for only about 8 to 24 hours after you eat it. Long-chain forms of K2, such as those found in fermented soy (natto), stick around much longer, staying in the blood for up to 96 hours. This longer presence allows K2 to reach tissues beyond the liver more effectively. K1 is primarily taken up by the liver, where it fuels clotting factor production, while certain K2 forms are redistributed from the liver into other tissues like bone and blood vessels. Both forms matter, but they have somewhat different roles based on where they end up in the body.
Best Food Sources
Green leafy vegetables are by far the richest sources of vitamin K1. A single cup of raw kale, spinach, or collard greens can deliver several hundred micrograms, far exceeding the daily recommendation in one serving. Broccoli, Brussels sprouts, and green leaf lettuce are also strong sources. Vegetable oils, particularly soybean and canola oil, contribute meaningful amounts as well, and these are common enough in cooking that many people get vitamin K1 without thinking about it.
One important detail: vitamin K1 from leafy greens is less bioavailable than vitamin K1 from oils or supplements. Your intestines absorb it more efficiently when fat is present in the same meal. Adding olive oil to a salad or cooking greens in butter isn’t just tastier; it genuinely helps you absorb more of the vitamin. This is true of all fat-soluble vitamins, but it’s especially relevant for K1 because its richest sources (raw greens) are naturally very low in fat.
How Much You Need
The recommended adequate intake for vitamin K varies by age and sex. For adult men 19 and older, the target is 120 mcg per day. For adult women in the same age range, including those who are pregnant or breastfeeding, it’s 90 mcg per day. Children need less: 30 mcg for ages 1 to 3, 55 mcg for ages 4 to 8, 60 mcg for ages 9 to 13, and 75 mcg for teenagers.
These values are set as “adequate intakes” rather than strict recommended dietary allowances because researchers didn’t have enough data to establish a precise average requirement. In practice, most people who eat vegetables regularly meet the target without supplements. Newborns are the notable exception, which is why vitamin K supplementation at birth is standard practice (more on that below).
Signs of Deficiency
Vitamin K1 deficiency is uncommon in healthy adults, but when it occurs, the primary symptom is abnormal bleeding. This can show up as easy bruising, bleeding gums, blood in urine or stool, or wounds that take unusually long to stop bleeding. In more severe cases, internal bleeding becomes a risk.
Deficiency is most likely in people with conditions that impair fat absorption, such as celiac disease, Crohn’s disease, or chronic liver disease. Long-term antibiotic use can also contribute, since some gut bacteria produce small amounts of vitamin K. Doctors diagnose deficiency by checking how long your blood takes to clot. A significantly prolonged clotting time, combined with normal levels of other blood components like platelets and fibrinogen, strongly suggests vitamin K deficiency. A blood test can also measure phylloquinone directly; levels below 0.15 mcg/L indicate low body stores.
Vitamin K1 and Newborns
Babies are born with very low vitamin K stores because the vitamin doesn’t cross the placenta efficiently, and newborns haven’t yet developed the gut bacteria that contribute to vitamin K production. This puts them at risk for a condition called vitamin K deficiency bleeding, which can cause dangerous, sometimes life-threatening hemorrhage in the first weeks or months of life.
To prevent this, medical guidelines recommend a single intramuscular injection of vitamin K1 within the first six hours after birth. The standard dose is 1.0 mg for infants weighing more than 1,500 grams and 0.5 mg for smaller newborns. For parents who decline the injection, an oral alternative exists: 2.0 mg given at birth, then 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.
Interaction With Blood Thinners
If you take warfarin or a similar blood-thinning medication, vitamin K1 intake matters more than usual. Warfarin works by blocking the vitamin K cycle, which slows clot formation. Eating a large amount of vitamin K1 in one sitting can counteract the medication, while eating very little can make it too potent. The result in either case is an unstable response to your medication.
The key recommendation is consistency, not avoidance. Older guidelines that told patients to eat a low-vitamin-K diet are now considered outdated. Current advice is to maintain your usual eating pattern and notify your healthcare team about any planned dietary changes. For people whose blood-thinning response fluctuates unpredictably, some clinicians suggest a small daily vitamin K supplement (100 to 200 mcg) to create a steady baseline, with close monitoring and dose adjustments to the medication as needed.