Overview
The vitamin your blood needs in order to clot, and your bones need in order to bind calcium into their structure. K1 comes from plants; K2 comes from fermented foods and gut bacteria.
Why your body needs it
Required to make several clotting factors — the K is for Koagulation. Also activates proteins that bind calcium into bone.
Food sources
- Kale, spinach and other dark greens
- Broccoli and Brussels sprouts
- Vegetable oils
- Natto and fermented foods (K2)
- Hard cheeses (K2)
Fat-soluble. Gut bacteria make some K2.
Recommended intake
120 µg (men) · 90 µg (women) for most adults. The upper limit, past which harm becomes likely, is None established.
Reference intakes are population figures. Pregnancy, illness, age and medication all move an individual’s needs, which is one of several reasons this page is information rather than advice.
Low vitamin k / deficiency
Most at risk: Newborns, people with fat malabsorption, and those on long courses of antibiotics.
Possible causes
Uncommon in adults. Fat malabsorption, prolonged broad-spectrum antibiotics, and liver disease. Newborns are born with very little, which is why a vitamin K injection at birth is routine.
Signs & symptoms
Easy bruising and prolonged bleeding. Rare in adults; newborns are given vitamin K at birth because they are born with very little.
Every sign here has other, often likelier, explanations. A list of symptoms is a reason to look into something, never a diagnosis.
Testing / diagnosis
Rarely measured directly. Prothrombin time or INR reflects the functional consequence and is the usual practical measure.
Food strategies
Kale, spinach, broccoli, Brussels sprouts and other green leaves for K1; natto, hard cheeses and fermented foods for K2. Fat alongside improves absorption.
Supplements
Seldom needed from a supplement in adults with ordinary digestion. The important point is consistency rather than quantity for anyone on warfarin.
Safety
No known toxicity from food or supplements.
Interactions: Directly opposes warfarin. Anyone on warfarin should keep their intake steady rather than avoiding greens — sudden changes in either direction destabilise INR.
When to seek professional advice
Speak to your doctor before changing your intake of green vegetables or taking vitamin K if you take warfarin — it directly opposes the medication, and swinging intake destabilises control. All newborns should receive vitamin K at birth.
This page is general reference information, not medical or dietetic advice, and it cannot account for your health history or medications. Decisions about testing, supplements or significant dietary change belong in a conversation with your doctor, pharmacist or a registered dietitian.