Vitamin K

Medically reviewed by Prof. Giorgi Pkhakadze, MD, MPH, PhD — GMJ Editorial Board

Scientific name: Phylloquinone (K1), Menaquinones (K2)
Category: Vitamin (fat-soluble)
RDA: 120 mcg/day (men), 90 mcg/day (women)
UL: No established upper limit

What is Vitamin K?

Vitamin K is a fat-soluble vitamin essential for blood clotting and bone health. It exists in two main forms: vitamin K1 (phylloquinone), found primarily in leafy green vegetables, and vitamin K2 (menaquinones), produced by bacteria and found in fermented foods and animal products. The body can also synthesize small amounts of vitamin K2 through intestinal bacteria, though this may not meet all physiological needs.

Unlike other fat-soluble vitamins, vitamin K is rapidly metabolized and excreted, requiring regular dietary intake. The vitamin was first discovered in the 1920s and named “K” for “koagulation” due to its critical role in blood clotting.

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Health benefits & uses

Blood clotting (Strong evidence): Vitamin K is absolutely essential for the synthesis of several clotting factors, including prothrombin. Deficiency can lead to serious bleeding disorders, making adequate intake crucial for normal hemostasis.

Bone health (Moderate evidence): Vitamin K activates osteocalcin, a protein that helps bind calcium in bones. Studies suggest adequate vitamin K intake may reduce fracture risk and support bone mineral density, particularly in postmenopausal women.

Cardiovascular health (Limited evidence): Some research indicates vitamin K2 may help prevent arterial calcification by activating matrix Gla protein, which inhibits calcium deposits in blood vessels. However, more clinical trials are needed to confirm cardiovascular benefits.

How it works

Vitamin K functions as a cofactor for the enzyme gamma-glutamyl carboxylase, which activates vitamin K-dependent proteins through a process called carboxylation. This process is essential for:

• Coagulation: Activating clotting factors II, VII, IX, and X, plus proteins C and S
• Bone metabolism: Activating osteocalcin, which regulates calcium binding in bone matrix
• Vascular health: Activating matrix Gla protein, which prevents soft tissue calcification

The vitamin undergoes a recycling process in the liver, though some is lost with each cycle, necessitating continuous dietary replenishment.

Dosage

The Recommended Dietary Allowance (RDA) is 120 mcg daily for adult men and 90 mcg for adult women. No tolerable upper intake level has been established due to the lack of reported toxicity from food sources.

Therapeutic doses may range from 100-500 mcg daily, though supplementation should be medically supervised, especially for individuals on anticoagulant medications. Higher doses (1-15 mg) are sometimes used clinically to reverse warfarin effects or treat deficiency, but only under medical supervision.

Consult a healthcare provider before starting vitamin K supplements, particularly if you have bleeding disorders or take blood-thinning medications.

Food sources

Vitamin K1 is abundant in:

• Leafy greens: kale, spinach, collard greens (500-1000 mcg per cup)
• Cruciferous vegetables: broccoli, Brussels sprouts (150-300 mcg per cup)
• Herbs: parsley, cilantro, basil

Vitamin K2 sources include:

• Fermented foods: natto (very high), sauerkraut, kimchi
• Animal products: liver, egg yolks, cheese
• Poultry and meat (smaller amounts)

Cooking and processing generally preserve vitamin K content, and dietary fats enhance absorption.

Safety & side effects

Vitamin K from food sources is generally very safe with no known toxicity. Natural vitamin K supplements are also well-tolerated by most people. Rare side effects may include:

• Allergic reactions (extremely rare)
• Gastrointestinal upset with very high doses
• Interference with anticoagulant medications

Synthetic vitamin K3 (menadione) can be toxic and is not used in supplements for human consumption.

Interactions

Drug interactions:

• Warfarin and other coumarins: Vitamin K directly antagonizes these anticoagulants, potentially reducing their effectiveness
• Antibiotics: Broad-spectrum antibiotics may reduce vitamin K production by intestinal bacteria
• Bile acid sequestrants: May impair vitamin K absorption

Supplement interactions:

• Vitamin E: Very high doses may interfere with vitamin K function
• Vitamin A: Excessive amounts may reduce vitamin K effectiveness

Who should be cautious

Certain groups should exercise caution with vitamin K supplementation:

• Anticoagulant users: Those taking warfarin or similar medications need consistent vitamin K intake and medical monitoring
• Liver disease patients: Impaired vitamin K metabolism may occur
• Malabsorption disorders: Conditions affecting fat absorption may require monitoring
• Newborns: Have low vitamin K stores and typically receive vitamin K injections at birth

Frequently asked questions

Can I take vitamin K if I’m on blood thinners?

Only with medical supervision. Vitamin K can interfere with anticoagulant medications like warfarin. Your doctor may recommend maintaining consistent vitamin K intake rather than avoiding it entirely, with regular monitoring of blood clotting times.

What’s the difference between K1 and K2?

Vitamin K1 (phylloquinone) is primarily involved in blood clotting and is found in green vegetables. Vitamin K2 (menaquinones) may have additional benefits for bone and cardiovascular health and is found in fermented foods and animal products.

Do I need to supplement if I eat plenty of leafy greens?

Most people who regularly consume leafy greens get adequate vitamin K1. However, vitamin K2 intake may be lower in typical Western diets, and some individuals may benefit from K2 supplementation, though more research is needed.

Can vitamin K deficiency be serious?

Yes, severe deficiency can cause dangerous bleeding disorders. However, deficiency is rare in healthy adults with normal diets. Newborns, people with malabsorption disorders, and those on certain medications are at higher risk.

Should I take vitamin K with other vitamins?

Vitamin K works synergistically with vitamins D and calcium for bone health. Take with fats for better absorption, but avoid very high doses of vitamins A and E, which may interfere with vitamin K function.