What is Vitamin K?
Vitamin K is a fat-soluble vitamin essential for several critical bodily functions, most famously its role in blood coagulation. The "K" in its name comes from the German word Koagulation. It is a group of compounds, with two forms being most significant for human nutrition: phylloquinone (vitamin K1) and menaquinone (vitamin K2). Vitamin K1 is primarily found in plant foods, while K2 is found in some animal products and fermented foods and is also produced by bacteria in the human gut. The body stores vitamin K in the liver and other tissues, meaning it is not needed in the diet every single day, but a consistent intake is necessary to maintain adequate levels.
Adequate Intake Recommendations for Vitamin K
Because there is insufficient evidence to establish a Recommended Dietary Allowance (RDA), the Food and Nutrition Board of the National Academies of Sciences, Engineering, and Medicine has set an Adequate Intake (AI) for vitamin K based on typical healthy dietary consumption.
- Adult Men (19+ years): 120 mcg daily
- Adult Women (19+ years): 90 mcg daily
- Pregnant or Lactating Women: 90 mcg daily
- Adolescents (14-18 years): 75 mcg daily
- Children (9-13 years): 60 mcg daily
- Children (4-8 years): 55 mcg daily
- Children (1-3 years): 30 mcg daily
These recommendations are generally met by eating a varied and healthy diet, as clinically significant vitamin K deficiency is rare in the general adult population.
Vital Functions of Vitamin K in the Body
Vitamin K's roles extend beyond its well-known clotting function. It serves as a cofactor for enzymes that activate several key proteins in the body.
Blood Coagulation
Vitamin K is essential for the liver to produce certain proteins known as clotting factors, including prothrombin (factor II), and factors VII, IX, and X. Without sufficient vitamin K, the blood's ability to clot is impaired, leading to excessive bleeding and bruising. This is particularly critical in newborns, who are born with low vitamin K levels and are at risk for Vitamin K Deficiency Bleeding (VKDB) if they do not receive a prophylactic vitamin K injection at birth.
Bone Health
Vitamin K plays a crucial part in bone metabolism and mineralization by activating a protein called osteocalcin. Osteocalcin helps bind calcium to the bone matrix, which in turn supports bone density and reduces the risk of fractures, especially in older adults. Research suggests that vitamin K2 may be particularly beneficial for bone health.
Cardiovascular Health
Another vitamin K-dependent protein, matrix Gla-protein (MGP), helps prevent soft tissue calcification, including in the arteries. Inadequate vitamin K status can lead to undercarboxylated MGP, which some studies link to an increased risk of arterial calcification, a major contributor to heart disease.
Key Dietary Sources of Vitamin K
Both forms of vitamin K are found in different food sources, and a varied diet is the best way to ensure adequate intake. It is also important to note that because vitamin K is a fat-soluble vitamin, consuming it with some dietary fat can improve absorption.
K1 (Phylloquinone) Sources
- Green leafy vegetables: Kale, spinach, collard greens, turnip greens, Swiss chard
- Cruciferous vegetables: Broccoli, Brussels sprouts, cabbage
- Vegetable oils: Soybean and canola oil
- Certain fruits: Blueberries and figs
K2 (Menaquinone) Sources
- Fermented foods: Natto (fermented soybeans) is an exceptionally rich source.
- Animal products: Egg yolks, cheese (especially soft cheese and blue cheese), and fatty meats.
Vitamin K1 vs. Vitamin K2
While both forms contribute to overall vitamin K status, they have distinct metabolic properties that influence their physiological effects.
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinone) |
|---|---|---|
| Primary Source | Plant-based foods (leafy greens, vegetable oils) | Animal products (egg yolks, dairy) and fermented foods (natto) |
| Synthesis | Synthesized by plants | Synthesized by bacteria in the gut; also available in some foods |
| Absorption Rate | Low, as little as 10%, often consumed without fat | Higher, as it is often consumed with fat |
| Half-Life | Short; cleared from the blood within hours | Long; can circulate in the blood for several days |
| Role in the Body | Primarily used by the liver for blood clotting | More active outside the liver, contributing significantly to bone and heart health |
Risks of Deficiency and Toxicity
For healthy individuals, vitamin K deficiency is rare and unlikely to occur from diet alone because it is widespread in foods and produced by gut bacteria. However, certain conditions and medications can increase risk. Symptoms of a significant deficiency include easy bruising, excessive bleeding from wounds or gums, and blood in the urine or stool. In infants, it can lead to life-threatening bleeding.
High intakes of natural vitamin K1 and K2 from food or supplements have not been shown to be toxic, and no tolerable upper intake level has been established for them. However, the synthetic form, menadione (vitamin K3), is toxic and has been banned for sale as a human dietary supplement in the US. It is also critical for anyone taking anticoagulant medications like warfarin to maintain a consistent daily intake of vitamin K, as sudden, large changes can interfere with the medication's effectiveness. Always consult with a healthcare provider for personalized guidance.
Conclusion
Understanding how much vitamin K is needed everyday is key to a healthy diet, with specific Adequate Intake levels set for different age groups and genders. Primarily sourced from green leafy vegetables (K1) and fermented foods or animal products (K2), this essential nutrient is vital for blood clotting, bone mineralization, and cardiovascular health. While deficiency is uncommon in healthy adults, special attention is needed for infants and individuals on anticoagulant medications. A balanced diet rich in varied whole foods ensures sufficient intake, and high-dose supplementation is generally unnecessary and should be discussed with a doctor, especially for those on specific medications.
Visit the NIH Office of Dietary Supplements for more information
