Vitamin K1
TL;DR
Essential vitamin for blood clotting - activates clotting factors II, VII, IX, and X
Abundant in leafy green vegetables like spinach, kale, and broccoli
Different from vitamin K2 (menaquinone) in both source and primary functions
Deficiency is rare in adults but can occur with malabsorption or antibiotic use
Critical interaction with warfarin and other blood thinners
Best for: People with malabsorption, those on long-term antibiotics, or dietary deficiency
What it is
Vitamin K1 (phylloquinone) is a fat-soluble vitamin essential for blood coagulation. It serves as a cofactor for gamma-glutamyl carboxylase, the enzyme that activates vitamin K-dependent clotting factors including prothrombin (factor II), factors VII, IX, and X. Without adequate K1, these proteins remain in their inactive forms, leading to impaired blood clotting.
Unlike vitamin K2 (menaquinone), which is primarily produced by bacteria and focuses on calcium metabolism, K1 comes mainly from plant sources and concentrates in the liver where clotting factors are produced. Dark leafy greens are the richest sources, with a single cup of cooked spinach providing over 800 mcg - well above daily requirements.
What the research says
Vitamin K1 on Amazon
*Link is just for your convenience - we do not make any fees
Safety
Vitamin K1 is extremely safe with no established upper intake limit. Unlike water-soluble vitamins, excess K1 is stored in the liver, but toxicity has never been reported from dietary sources or oral supplements. Injectable forms used medically can rarely cause allergic reactions or hemolysis in newborns with G6PD deficiency, but oral supplementation poses virtually no risk.
Interactions
• Warfarin and other coumarins - K1 directly counteracts these medications by promoting clotting factor activation. Patients on warfarin must maintain consistent K1 intake • Direct oral anticoagulants (dabigatran, rivaroxaban, apixaban) - less affected than warfarin but high K1 intake may still reduce effectiveness • Antibiotics (broad-spectrum) - prolonged use can reduce vitamin K-producing gut bacteria, potentially increasing K1 needs • Fat malabsorption drugs (orlistat, cholestyramine) - can reduce absorption of fat-soluble K1
Dosing
Adequate Intake: 90 mcg daily for women, 120 mcg daily for men
Therapeutic doses: 1-10 mg for deficiency correction, under medical supervision
Food sources preferred: One cup cooked spinach (889 mcg), kale (1062 mcg), or broccoli (220 mcg) easily meets daily needs
Supplementation: Typically unnecessary unless malabsorption present. Take with fats for optimal absorption.
Cost
Vitamin K1 supplements are very inexpensive, typically $5-10 monthly, though most people don't need them. Since K1 is abundant in common vegetables, dietary sources are the most cost-effective approach. Multivitamins often contain small amounts, and specific K1 supplements are available but rarely necessary for healthy individuals eating vegetables regularly.
The bottom line
Vitamin K1 supplementation is rarely needed for healthy adults eating any vegetables, as deficiency is uncommon outside of specific medical conditions. The main concern is not deficiency but rather the critical interaction with blood thinning medications. If you're on warfarin, work with your healthcare provider to maintain consistent K1 intake rather than avoiding it entirely. For most people, eating leafy greens regularly provides all the K1 needed for proper blood clotting.
References
- National Institutes of Health Office of Dietary Supplements. Vitamin K Fact Sheet for Health Professionals. Updated March 2021.
- Schurgers LJ, Vermeer C. Determination of phylloquinone and menaquinones in food. Effect of food matrix on circulating vitamin K concentrations. Haemostasis. 2000;30:298-307.
Sources for this page include published meta-analyses, systematic reviews, and NIH dietary supplement fact sheets. All claims reflect the evidence as of early 2026.
This is not medical advice. Consult your healthcare provider before starting any supplement, especially if you take medications.