Vitamin K: What It Does, Where to Get It, and Who Should Pay Attention
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Key takeaways
- Vitamin K helps activate proteins involved in blood clotting and bone metabolism
- It comes in two main forms, K1 from leafy greens and K2 from fermented foods and animal products
- Deficiency is rare in healthy adults but a real concern for newborns and people on certain medications
- People taking blood thinners like warfarin need to keep their vitamin K intake consistent, not necessarily low
What is vitamin K?
Vitamin K is a fat soluble vitamin best known for its role in blood clotting.
The 'K' comes from the German word 'Koagulation,' a nod to its discovery through studies on bleeding.
There are two main forms.
- Vitamin K1 (phylloquinone), found mostly in leafy green vegetables
- Vitamin K2 (menaquinones), found in fermented foods and animal products, and also made in small amounts by gut bacteria
K1 makes up the bulk of dietary vitamin K in most Western diets. K2 gets more attention in research on bone and cardiovascular health, though the evidence there is still developing.
How does vitamin K work in the body?
Vitamin K acts as a cofactor for an enzyme that activates several proteins through a process called carboxylation.
The most well established of these are clotting factors made in the liver. Without enough active vitamin K, blood cannot clot normally, which is why severe deficiency shows up as excessive bleeding or bruising.
Vitamin K also activates osteocalcin, a protein involved in bone mineralization, and matrix Gla protein, which helps keep calcium out of blood vessel walls. This is the basis for ongoing research into vitamin K's role in bone density and arterial calcification.
What does the evidence show?
The clotting role of vitamin K is textbook physiology, confirmed by decades of clinical observation and the well documented effects of vitamin K antagonist drugs like warfarin.
The bone and cardiovascular angles are less settled. Some observational studies link higher vitamin K2 intake with better bone mineral density and lower risk of arterial calcification, but randomized trial evidence is mixed and not large enough to draw firm conclusions.
Vitamin K is also standard practice for newborns, who are born with low vitamin K stores and immature gut bacteria, putting them at risk of a rare but serious bleeding disorder.
How much vitamin K do you need?
There is no single universally adopted daily target, but general guidance for adults falls in the range of about 90 to 120 micrograms per day, with women typically cited at the lower end and men at the higher end.
Because vitamin K1 is so concentrated in leafy greens, many people meet this easily with a regular serving of vegetables like kale, spinach, or broccoli.
There is no established upper limit for vitamin K from food, since toxicity from dietary sources has not been documented.
Food sources of vitamin K
Vitamin K1 sources are dominated by dark leafy greens.
- Kale
- Spinach
- Collard greens
- Swiss chard
- Broccoli
- Brussels sprouts
Vitamin K2 shows up in different foods entirely.
- Natto (fermented soybeans, an especially concentrated source)
- Hard and soft cheeses
- Egg yolks
- Chicken and other meats
- Butter and full fat dairy
Cooking with a bit of fat, like olive oil, can help with absorption since vitamin K is fat soluble.
Is vitamin K safe? Who needs to be careful?
For most healthy people, vitamin K from food or a standard supplement is considered very safe, with no established toxic dose.
The major exception is people taking warfarin or other vitamin K antagonist blood thinners.
- Vitamin K directly counteracts how these medications work
- Sudden large increases or decreases in vitamin K intake can throw off blood clotting test results (INR)
- The goal for people on these medications is usually consistency in vitamin K intake, not avoidance
Anyone on blood thinners should talk with their prescribing doctor before making major dietary changes involving leafy greens or vitamin K supplements.
People with fat malabsorption conditions, such as celiac disease, cystic fibrosis, or inflammatory bowel disease, may be at higher risk of deficiency and worth discussing with a doctor.
Who might consider paying attention to vitamin K?
- Newborns, who routinely receive a vitamin K shot at birth as standard care
- People on long term antibiotics, which can disrupt the gut bacteria that produce some vitamin K2
- People with fat malabsorption disorders
- People interested in bone health research, given vitamin K's role in osteocalcin activation
- Anyone on blood thinners, for the opposite reason: keeping intake steady rather than increasing it
Bottom line
Vitamin K's role in blood clotting is well established and essential. Its potential benefits for bone and heart health are promising areas of research but not yet proven enough to build supplement habits around.
For most people, a diet with regular leafy greens and some fermented or animal foods covers the basics without needing a supplement.
Frequently asked questions
What foods are highest in vitamin K?
Dark leafy greens like kale and spinach are the richest sources of vitamin K1, while natto and some cheeses are notably high in vitamin K2.
Can too much vitamin K be harmful?
No upper limit or toxicity has been established for vitamin K from food, though people on blood thinning medication need to keep intake consistent rather than variable.
Does vitamin K interact with medications?
Yes, vitamin K can reduce the effectiveness of warfarin and similar blood thinners, so people on these medications should keep their intake steady and consistent.
Is vitamin K2 better than vitamin K1?
They serve overlapping but somewhat different roles. K1 is the dominant dietary form tied to clotting, while K2 is more studied for bone and vascular health, though evidence there is still developing.
Why do newborns get a vitamin K shot?
Babies are born with low vitamin K stores and undeveloped gut bacteria, putting them at risk for a rare bleeding disorder, so a vitamin K injection is standard preventive care at birth.
Top Food Sources
Animal and fermented foods supply K2 (menaquinones), which stay in circulation longer than K1 and are the form associated with calcium handling in arteries and bone.
Leafy greens supply K1 (phylloquinone), which the liver uses efficiently for clotting. K1 absorbs better with fat in the meal, and the body converts only a limited amount of K1 to K2.