Vitamin K: what it does, how much you need, and how much is too much
By Vita · fact-checked against NIH ODS
Vita is Vitaminico's AI nutrition coach. Every number here is checked against the NIH Office of Dietary Supplements; these pages have not yet been reviewed by our registered dietitians.
The vitamin that lets your blood clot, and helps keep bones and arteries healthy.
Vitamin K: key facts
- Everyday need (RDA/AI)
- Adequate Intake: 120 mcg for men, 90 mcg for women (no RDA established)
- Safe upper limit (UL)
- No established UL
- Measured in
- mcg
- Best foods
- Spinach, kale, broccoli and other green leafy vegetables, vegetable oils (soybean, canola), natto
Adult values from the NIH ODS Vitamin K fact sheet. Needs differ in pregnancy, lactation and childhood.
What does vitamin k do?
Vitamin K is a fat-soluble vitamin your body uses to switch on proteins that make blood clot, so a cut or scrape stops bleeding. It works as a helper (coenzyme) for an enzyme that activates clotting factors like prothrombin, plus bone proteins such as osteocalcin and matrix Gla protein that support bone building and help keep calcium out of blood-vessel walls. It comes in two main forms: K1 (phylloquinone) from green leafy vegetables and K2 (menaquinones) from fermented and animal foods, with some K2 also made by bacteria in your gut.
Why you might be low on vitamin k
- Malabsorption conditions that block fat absorption, such as celiac disease, cystic fibrosis, ulcerative colitis, or short bowel syndrome
- Having had bariatric (weight-loss) surgery, which can lower vitamin K status
- Long courses of antibiotics, which can kill the gut bacteria that make vitamin K (especially cephalosporins like cefoperazone)
- Certain medications that reduce fat-soluble vitamin absorption: bile acid sequestrants (cholestyramine, colestipol) and the weight-loss drug orlistat (Alli, Xenical)
- Being a newborn (vitamin K crosses the placenta poorly and breast milk is low in it) which is why babies are given vitamin K at birth
- A diet very low in green vegetables and vegetable oils, though true deficiency is rare in healthy adults
Signs of low vitamin k
These are common signals, not a diagnosis — a blood test and your clinician confirm a real gap.
- Easy bruising
- Bleeding that is slow to stop from cuts, gums, or the nose
- Blood in the urine or stool
- Heavier-than-usual bleeding
- A prolonged prothrombin time, the lab measure of how long blood takes to clot (the main clinical sign)
- In newborns, bleeding from the umbilical stump, gut, skin, or nose (vitamin K deficiency bleeding); note these signs appear only in severe deficiency
Best food sources of vitamin k
| Food | Typical amount |
|---|---|
| Natto (fermented soybeans), a K2 source | 850 mcg per 3 oz |
| Collard greens, frozen, boiled | 530 mcg per 1/2 cup |
| Turnip greens, frozen, boiled | 426 mcg per 1/2 cup |
| Spinach, raw | 145 mcg per 1 cup |
| Kale, raw | 113 mcg per 1 cup |
| Broccoli, chopped, boiled | 110 mcg per 1/2 cup |
| Soybean oil | 25 mcg per 1 tablespoon |
How much vitamin k is too much?
Vitamin K has no established upper limit. No UL because no adverse effects have been reported from food or supplements; however, vitamin K can interfere with warfarin and other blood thinners.
Vitamin K from food and supplements has very low toxicity, and experts set no upper limit because no harmful effects have been reported from consuming it. The real caution is for people on warfarin (Coumadin) or similar blood thinners: extra or wildly changing vitamin K can dangerously weaken the drug, so keep your intake steady and check with your doctor before taking supplements.
Stacking a multivitamin, a single supplement and fortified foods adds up faster than it looks — run your full dose through the free Supplement Safety Checker before adding anything new.
Vitamin K FAQ
I take warfarin (Coumadin), do I have to avoid greens and vitamin K?
No, you don't have to avoid them. What matters is consistency: eat about the same amount of vitamin K-rich foods from day to day rather than swinging between none and a huge salad, since sudden changes can throw off how well the drug works. Talk to your doctor before starting any vitamin K supplement.
What's the difference between vitamin K1 and K2?
K1 (phylloquinone) is the main form in green leafy vegetables and supplies most of the vitamin K in a typical diet. K2 (menaquinones) comes from fermented foods like natto and some animal foods, and gut bacteria also make it. Both count as vitamin K.
Do I need a vitamin K supplement?
Most healthy adults get enough from food and gut bacteria, and true deficiency is rare. Supplements mainly matter for newborns (given a dose at birth), people with fat-malabsorption conditions, or when a doctor specifically recommends one, not as a routine add-on.
Source
Every RDA/AI, upper limit and unit on this page is drawn from the NIH Office of Dietary Supplements Vitamin K fact sheet ›.
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Educational, not medical advice. The upper limits shown are Tolerable Upper Intake Levels for healthy adults from all sources combined; needs differ in pregnancy, lactation, childhood and with some conditions or medications. This page does not diagnose a deficiency or set your dose — talk to your clinician before starting any high-dose supplement.