Vitamin K is the vitamin most people never think about until something quietly stops working. It powers the clotting proteins that seal a paper cut, and it activates a bone protein called osteocalcin that keeps your skeleton strong. When levels run low, the first hints usually show up on the skin, in the mouth, or in the toilet bowl.
Two forms do the work. K1 (phylloquinone) comes from leafy greens, while K2 (menaquinones) comes from fermented foods and, importantly, from bacteria in your own gut. That gut source is why long courses of antibiotics can pull adult levels down, and why the Merck Manuals list "certain medications, including antiseizure medications, and some antibiotics" as recognized causes.
Why the body needs it
Vitamin K's headline job is clotting. As Harvard's Nutrition Source explains, the vitamin "helps to make four of the 13 proteins needed for blood clotting, which stops wounds from continuously bleeding so they can heal." It also supports bone, through "the production of proteins in bone, including osteocalcin, which is needed to prevent the weakening of bones."
The recommended intake for adults is 120 micrograms a day for men and 90 micrograms for women, according to Harvard. Overt deficiency is uncommon in healthy adults, but it can develop quietly in people with the right combination of medications, gut disease, or a very low-fat diet.
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The 7 warning signs to watch for
- Unexplained easy bruising. Small clotting failures show up in the skin first. A StatPearls clinical review lists "cutaneous bleeding: ecchymoses, petechiae, purpura" as classic signs.
- Cuts that bleed longer than they should. Baylor Scott & White notes that when vitamin K is low, "bleeding from small cuts may take longer to stop."
- Frequent nosebleeds. Recurrent epistaxis is on the same clinical list of mucosal bleeding tied to low vitamin K.
- Bleeding gums. Cleveland Clinic points out that deficiencies in vitamins C and K "can also cause bruising, bleeding gums or spots on your skin called petechiae."
- Blood in the urine. Hematuria is a documented sign in adults with impaired clotting from low vitamin K, per the StatPearls review.
- Blood in the stool. Merck describes bleeding "in the stomach, or in the intestine" as a main manifestation, which can appear as dark, tarry stools or visible blood.
- Heavy menstrual periods. Both Ada and Baylor Scott & White list unusually heavy periods among the signs, since the same clotting factors are needed to control menstrual bleeding.
None of these signs is proof on its own. But two or three together, especially in someone with a known risk factor, is a reason to have a doctor check a clotting panel.
Who is most likely to run low
Three groups stand out in the clinical literature.
People on long courses of broad-spectrum antibiotics. These drugs can wipe out the gut bacteria that make K2. As StatPearls puts it, "broad-spectrum antibiotics can reduce intestinal flora responsible for synthesizing menaquinones (vitamin K2)."
People with fat-malabsorption conditions. Vitamin K needs dietary fat to be absorbed. Celiac disease, Crohn's disease, cystic fibrosis, and biliary or pancreatic disease all interfere with that step. Ada lists celiac, Crohn's, and tropical sprue among the conditions that impair absorption.
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People taking warfarin or related anticoagulants. These drugs work by blocking vitamin K's clotting action. Merck notes that warfarin "interferes with the synthesis of clotting factors." Anyone on such a medication who suddenly changes their intake of leafy greens should tell their prescriber, because it can shift how the drug performs. If you take blood thinners, a Welltica guide walks through foods that could raise your bleeding risk.
Diet is the main defense. Leafy greens are the richest sources of K1 (kale, spinach, collard greens, broccoli, Brussels sprouts). For K2, natto (fermented soybeans) is the standout single source, with smaller amounts in meat, cheese, and eggs. Because vitamin K is fat-soluble, a drizzle of olive oil on a salad is not just for flavor, it helps absorption.
Anyone noticing repeated unexplained bruising, prolonged bleeding, or unusually heavy periods, especially while taking antibiotics, blood thinners, or living with a gut condition, should raise it with their doctor rather than reach for a supplement.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.