Vitamin K

What it does

Vitamin K helps the body use calcium in the right places. It supports proteins that bind calcium into bone and helps regulate calcium handling in blood vessels and other tissues. Vitamin D helps bring calcium in, magnesium helps keep calcium signaling balanced, and vitamin K helps make sure calcium is put to work where it belongs.

Vitamin K works across several systems, but its roles in calcium handling and blood clotting are particularly important.

Blood clotting. Vitamin K is needed to activate proteins involved in blood coagulation. Without enough vitamin K, blood clotting becomes less efficient, which can lead to excessive bleeding.

Bone health. Vitamin K helps activate osteocalcin, a protein that binds calcium into bone. This is one reason vitamin K matters for bone health, not just blood clotting.

Calcium handling in soft tissues. Vitamin K helps activate matrix Gla protein, which is involved in regulating calcium in blood vessels and other tissues. This role depends on steady vitamin K availability over time.

Why vitamin K can be inconsistent

Vitamin K shortfalls are less common than vitamin D shortfalls, but intake can still vary depending on food patterns.

K1 comes mostly from leafy greens. Spinach, kale, collard greens, and broccoli are the main sources of vitamin K1 (phylloquinone). A single serving of cooked greens can provide several times the daily recommendation. People who eat these foods regularly tend to have high K1 intake.

K2 comes from fermented foods and some animal foods. Natto (fermented soybeans) is the richest source of K2 (menaquinone). Cheese, egg yolks, and meat provide smaller amounts. K2 intake tends to be lower and more variable than K1 intake in typical Western diets.

K2 forms differ in how long they last in the body. MK-4 is a shorter-lasting form of K2 found in animal foods. MK-7 is a longer-lasting form found in fermented foods like natto. MK-7 stays in the bloodstream longer, which may support more steady vitamin K activity over time.

Fat absorption affects vitamin K uptake. Vitamin K is fat-soluble, so it absorbs better when eaten with dietary fat. Very low-fat diets or digestive conditions that affect fat absorption can reduce how much vitamin K the body uses.

Some medications interfere with vitamin K. Warfarin and other blood thinners work by blocking vitamin K activity. People taking these medications need to maintain consistent vitamin K intake and should talk to their clinician before making significant changes to their diet or supplements.

Who may need to pay closer attention

Some people are more likely to have inconsistent vitamin K intake than others:

• people who eat few leafy greens or fermented foods
• people with digestive conditions that affect fat absorption
• people eating very low-fat diets
• people taking warfarin or other blood thinners
• people taking antibiotics long-term, which can affect gut bacteria that produce small amounts of K2
• newborns, who are typically given vitamin K at birth to prevent bleeding disorders

None of these factors guarantees deficiency. They are simply reasons to pay closer attention.

Best food sources

Vitamin K appears in two main forms: K1 (phylloquinone) in plant foods and K2 (menaquinones) in fermented and animal foods.

Food	Vitamin K per serving	Form
Natto (fermented soybeans, 3 oz)	~850 mcg	K2 (MK-7)
Collard greens, cooked (1 cup)	~530 mcg	K1
Spinach, cooked (1 cup)	~890 mcg	K1
Kale, cooked (1 cup)	~540 mcg	K1
Broccoli, cooked (1 cup)	~220 mcg	K1
Brussels sprouts, cooked (1 cup)	~220 mcg	K1
Hard cheese (1 oz)	~10 mcg	K2 (MK-4)
Egg yolk (1 large)	~5 mcg	K2 (MK-4)

Natto is an extreme outlier. A single serving of natto can provide roughly a week’s worth of vitamin K. It is the richest source of MK-7, but it is not a food most people eat regularly outside of certain cultural contexts.

Leafy greens dominate K1 intake. A single serving of cooked spinach or kale provides several times the daily recommendation. People who eat greens regularly tend to have very high vitamin K intake, though most of it is K1 rather than K2.

How much do you need?

Standard AI (Adequate Intake)

90 mcg per day for adult women and 120 mcg per day for adult men. These are Adequate Intake levels rather than RDAs because the data on vitamin K requirements is less precise than for some other nutrients.

Individual context matters

People who eat leafy greens regularly tend to exceed vitamin K recommendations easily. Those who avoid greens or fermented foods may have lower intake, though deficiency severe enough to cause bleeding problems is uncommon in healthy adults.

No established upper limit

No tolerable upper limit has been set for vitamin K from food or supplements. High intakes from food have not been shown to cause adverse effects in healthy people. However, people taking blood thinners should maintain consistent vitamin K intake and consult their clinician before making significant changes.

Forms and absorption

When vitamin K appears in supplements, the form matters.

Vitamin K1 (phylloquinone)

The form found in leafy greens. Well absorbed and effective for supporting blood clotting. Most multivitamins use K1.

Vitamin K2 as MK-4

A shorter-lasting form of K2 found in animal foods and some supplements. Cleared from the body relatively quickly.

Vitamin K2 as MK-7

A longer-lasting form of K2 found in fermented foods. Stays in the bloodstream longer than K1 or MK-4, which may support more steady vitamin K activity over time. This is the form often used in bone health supplements.

Take it with fat

Vitamin K is fat-soluble, so it absorbs better when taken with a meal that contains dietary fat. Taking it on an empty stomach may reduce how much your body uses.

Medication interactions

Vitamin K can interact with certain medications, particularly blood thinners.

Warfarin and other vitamin K antagonists

Warfarin works by blocking vitamin K activity to reduce blood clotting. People taking warfarin need to maintain consistent vitamin K intake because sudden increases or decreases can affect how well the medication works. This does not mean avoiding vitamin K entirely, but it does mean keeping intake stable and talking to a clinician before making significant dietary changes or starting vitamin K supplements.

Antibiotics

Long-term antibiotic use can reduce gut bacteria that produce small amounts of K2. This is rarely a problem for healthy adults eating a normal diet, but it may matter for people with already low vitamin K intake.

Nutrient context

Calcium, vitamin D, and magnesium

Vitamin K helps activate proteins involved in calcium handling, including osteocalcin in bone and matrix Gla protein in blood vessels and other tissues. This role depends more on steady vitamin K availability than on taking vitamin K at the exact same time as calcium, especially with longer-lasting forms such as vitamin K2 as MK-7.

Closing the gap

Vitamin K is widely available in food, and severe deficiency is uncommon in healthy adults. Leafy greens provide large amounts of K1, while fermented foods and some animal foods provide K2.

The goal is not to chase high doses or treat vitamin K like more is better. It is to understand whether vitamin K is showing up consistently enough in your routine and whether the forms you are eating suit your needs.

Those adjustments might include working in leafy greens, fermented foods like natto or cheese, or K2-containing animal foods more regularly. When food is not enough, a modest vitamin K supplement can help, but most people do not need to chase high vitamin K doses. People taking blood thinners should talk to their clinician before changing vitamin K intake significantly.

See how vitamin K shows up in your usual diet →

Related nutrients: Calcium · Vitamin D · Magnesium

The information on this page is educational and does not constitute medical advice. Talk to a qualified healthcare provider before making changes to your supplement regimen or interpreting lab results.