Vitamin K is essential for activating proteins that control blood clotting—helping produce clotting factors II, VII, IX and X—and for bone health by enabling osteocalcin to bind calcium, supporting bone mineralization. It also helps prevent arterial calcification via matrix Gla protein and is obtained from green leafy vegetables (K1) and fermented foods or bacteria-produced K2.

Leafy green vegetables are richest in vitamin K1—examples include kale, spinach, collard greens, broccoli, Brussels sprouts, and parsley. Vitamin K2 is found in fermented or animal foods such as natto, certain hard cheeses, egg yolks, and liver. Because vitamin K is fat‑soluble, consuming these foods with a little dietary fat improves absorption.

Low vitamin K often causes impaired blood clotting leading to easy bruising and excessive or prolonged bleeding — from cuts, gums, nosebleeds, heavy menstrual bleeding, or blood in urine or stools. Newborns risk severe bleeding, including intracranial hemorrhage. Chronic deficiency may contribute to poorer bone health and higher fracture risk. Symptoms primarily reflect clotting problems rather than fatigue or weight change.

Vitamin K’s five major functions are: enabling blood coagulation by activating clotting factors (II, VII, IX, X); activating anticoagulant proteins C and S to regulate clotting; promoting bone health via osteocalcin carboxylation and mineralization; preventing arterial and soft‑tissue calcification through matrix Gla protein activation; and supporting calcium binding and protein carboxylation in multiple tissues.

No, vitamin K is not a blood thinner. It’s essential for producing clotting factors (II, VII, IX, X) and other proteins that promote coagulation, so it helps blood clot. Blood-thinning drugs like warfarin work by blocking vitamin K’s effects. If you’re on anticoagulants, don’t change vitamin K intake or start supplements without consulting your doctor.

Bananas contain only trace amounts of vitamin K — about 0.6–0.8 micrograms in a medium banana — so they are not a significant source. Adult daily needs are around 90–120 µg, so one banana provides well under 1% of that. For people managing vitamin K intake (e.g., on anticoagulants), bananas are unlikely to meaningfully affect levels compared with leafy greens.