Understanding Vitamin K: K1 vs. K2
Vitamin K is a fat-soluble vitamin essential for several critical bodily functions, including blood clotting and bone metabolism. However, its role is more nuanced than a single nutrient, as it exists in two primary forms: phylloquinone (K1) and menaquinones (K2). A comprehensive strategy for acquiring sufficient vitamin K must consider both forms.
Vitamin K1 (Phylloquinone)
Vitamin K1 is the most common form in the human diet, making up 75–90% of all dietary vitamin K intake. Its primary function is in the liver, where it activates proteins responsible for blood coagulation. Plant-based foods are the main source, and its absorption is relatively poor compared to K2. To improve absorption, it is important to consume K1-rich foods alongside a dietary fat.
Top Sources of Vitamin K1
- Dark Leafy Greens: Kale, spinach, collard greens, and turnip greens are exceptionally high in K1.
- Cruciferous Vegetables: Broccoli, Brussels sprouts, and cabbage also provide significant amounts.
- Vegetable Oils: Oils like soybean and canola contain K1.
Vitamin K2 (Menaquinones)
Vitamin K2, or menaquinones, is primarily found in fermented foods and animal products. It is more effectively absorbed and circulates in the blood longer than K1, allowing it to reach and benefit extra-hepatic tissues like bones and blood vessels. While K1 is the clotting vitamin, K2 is key for bone and cardiovascular health.
Top Sources of Vitamin K2
- Natto: This traditional Japanese fermented soybean dish is the single richest source of K2 (specifically MK-7).
- Certain Cheeses: Hard cheeses like Gouda and soft cheeses like Brie contain K2 variants (MK-8 and MK-9) produced by bacteria during fermentation.
- Organ Meats: Liver (particularly beef liver) is a good source of MK-4.
- Egg Yolks: Contain modest amounts of MK-4.
- Chicken and Pork: Dark meat chicken and certain pork products contain MK-4.
Combining Diet and Supplements for the Best Approach
For most healthy adults, a balanced and varied diet is the best way to get all the necessary vitamin K. Combining K1-rich leafy greens with K2-containing fermented and animal products offers the most comprehensive approach. A salad with spinach and a cheese topping, or cooked kale with a side of chicken, are excellent examples of pairing K1 and K2 with a source of fat to enhance absorption.
Supplements can play a role, especially for those with malabsorption issues due to conditions like cystic fibrosis or ulcerative colitis, or for people on long-term antibiotics. K2 (specifically MK-7) supplements are often promoted for their long half-life and extra-hepatic benefits, but should only be considered after a doctor's consultation, particularly for those on blood-thinning medication.
Comparison of Vitamin K Forms
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinone) |
|---|---|---|
| Primary Source | Plant foods, especially dark leafy greens, vegetable oils | Fermented foods (natto, cheese), animal products (egg yolks, liver) |
| Absorption | Poorly absorbed, better with fat | Better absorbed, especially longer-chain forms like MK-7 |
| Half-Life | Short, cleared by the liver within hours | Longer, can remain in circulation for days (e.g., MK-7) |
| Primary Function | Activating clotting factors in the liver | Promoting bone mineralization and inhibiting arterial calcification |
| Distribution | Concentrated in the liver | Redistributed to extra-hepatic tissues (bone, blood vessels) |
Maximizing Vitamin K Intake and Absorption
To ensure you are getting the full benefits of vitamin K, follow these simple guidelines:
- Pair with Fat: Always consume vitamin K-rich foods with a source of healthy fat, such as olive oil in a salad dressing, avocado, or nuts.
- Vary Your Diet: Include a range of K1 sources like kale and broccoli, and K2 sources like aged cheese and pastured eggs, throughout the week.
- Explore Fermented Foods: If you can tolerate it, natto is a powerhouse of K2. Other fermented foods like sauerkraut also contain some K2.
- Consider Cooking Methods: Cooking leafy greens can concentrate the vitamin K content, though raw versions are still excellent sources.
- Consult Your Physician: Before considering supplements, especially if on blood thinners like warfarin, always speak with a doctor to maintain consistent vitamin K intake and avoid dangerous interactions.
Conclusion: A Balanced Dietary Approach is Best
The best way to get vitamin K is not through a single food or supplement, but through a varied dietary approach that incorporates both K1 and K2. By focusing on a combination of leafy green vegetables, healthy oils, and select fermented or animal products, most individuals can easily meet their needs for both forms. While deficiencies are rare in healthy adults, certain medical conditions or medications may warrant supplementation under medical supervision. Prioritizing food-based sources, and ensuring fat is present for optimal absorption, provides the most effective and natural path to ensuring you have all the vitamin K you need for healthy blood, bones, and heart.
For more information on dietary sources and daily recommendations, consult authoritative health resources such as the NIH Office of Dietary Supplements.
