Understanding the Forms of Vitamin K
Vitamin K is not a single compound but a family of fat-soluble vitamins essential for many bodily functions. The two main types found in our diet are Vitamin K1 and Vitamin K2, each with distinct characteristics regarding sources, metabolism, and absorption.
Vitamin K1 (Phylloquinone)
- Sources: Primarily found in leafy green vegetables like kale, spinach, broccoli, and collard greens.
- Function: Best known for its crucial role in blood coagulation. The liver rapidly retains and utilizes K1 to activate clotting factors.
- Absorption: K1 from plants is notoriously poorly absorbed, with estimates suggesting less than 10% absorption. Its effect is mainly concentrated in the liver.
Vitamin K2 (Menaquinones)
- Sources: Produced by bacteria and found in fermented foods and animal products. It is further categorized into subtypes, known as MK-n, based on the length of their side chain.
- Subtypes: Two notable forms are MK-4 and MK-7, with different origins and physiological effects. MK-4 is found in some animal products (eggs, liver, meat), while long-chain menaquinones like MK-7 are abundant in fermented foods such as natto.
- Function: Supports extrahepatic (outside the liver) functions, particularly bone mineralization and the inhibition of arterial calcification.
Bioavailability: Why the Form Matters
The key difference between vitamin K forms lies in their bioavailability and half-life, which determine how well and for how long the body can use the vitamin.
Bioavailability refers to the proportion of a nutrient that is absorbed and utilized by the body. The half-life is the time it takes for half of the substance to be eliminated from the bloodstream. This is where MK-7 demonstrates a significant advantage.
Studies have shown that while both K1 and MK-7 are absorbed, the concentration of MK-7 in the bloodstream reaches much higher levels and lasts for days, unlike K1, which is rapidly cleared within hours. This prolonged presence allows MK-7 to be distributed more effectively to extrahepatic tissues throughout the body, including bone and blood vessel walls, where it can exert its unique health benefits.
In contrast, MK-4 is less bioavailable than MK-7 at nutritional doses and has a very short half-life, making it less effective at increasing systemic vitamin K levels.
The Role of Dietary Fat in Vitamin K Absorption
As a fat-soluble vitamin, all forms of vitamin K are absorbed more efficiently when consumed with dietary fat. The presence of bile salts and pancreatic enzymes, stimulated by fat intake, helps create micelles that transport the vitamin across the intestinal wall. This is one reason why the K2 found in fatty animal products and fermented foods is generally better absorbed than the K1 from leafy greens, which contain minimal fat. For those eating K1-rich vegetables, adding a source of healthy fat, like olive oil in a salad dressing, can significantly improve absorption.
Comparison of Key Vitamin K Forms
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (MK-4) | Vitamin K2 (MK-7) |
|---|---|---|---|
| Primary Source | Green leafy vegetables, plant oils | Animal products (eggs, liver, meat) | Fermented foods (natto, some cheeses) |
| Absorption | Poorly absorbed, especially from plants | Variable, poor absorption at nutritional doses | Excellent absorption and high bioavailability |
| Half-Life | Short (hours) | Very short | Long (days) |
| Distribution | Primarily liver | Extrahepatic tissues (via K1 conversion) | Broad extrahepatic distribution (bones, arteries) |
| Key Function | Blood clotting | Tissue-specific roles; converted from other K forms | Bone mineralization, arterial health, extrahepatic functions |
How to Ensure Optimal Vitamin K Intake
Given the differences in bioavailability and function, aiming for adequate intake of both K1 and K2 can be beneficial for comprehensive health.
Dietary Strategies
- For Vitamin K1: Regularly consume a variety of leafy greens. Always include a source of fat with these meals to maximize absorption.
- For Vitamin K2: Include fermented foods like natto or certain cheeses in your diet. Egg yolks and chicken are also good sources of the MK-4 subtype.
Supplementation
For those with dietary restrictions or specific health goals, supplementation is a viable option. MK-7 is the most common form of K2 in supplements due to its superior absorption and long half-life, which allows for consistent blood levels and effective tissue distribution. Supplementation may be particularly relevant for bone and heart health.
Conclusion: The Final Verdict
While all forms of vitamin K are important for health, the evidence is clear: vitamin K2, specifically the MK-7 subtype, is the best absorbed form of vitamin K. Its long half-life and efficient distribution throughout the body make it more effective for supporting extrahepatic health, including robust bone and cardiovascular function. For maximizing absorption, consuming vitamin K with dietary fats is essential, whether from food or supplements. For broad systemic benefits, focusing on MK-7, through either food sources or supplements, offers the most significant advantages.
This article is for informational purposes only and does not constitute medical advice. Consult a healthcare professional before starting any new supplement regimen.
Factors Influencing Vitamin K Absorption
The crucial importance of dietary fat
As a fat-soluble vitamin, vitamin K relies on dietary fat for efficient absorption in the small intestine, similar to other fat-soluble vitamins like A, D, and E. When consuming vitamin K-rich foods or supplements, including a source of healthy fats—such as olive oil, nuts, or avocados—is essential for optimizing absorption into the lymphatic system. Without sufficient fat, absorption can be severely limited. This is particularly relevant for the poorly absorbed K1 from leafy greens.
Gastrointestinal health and its impact
Efficient absorption of vitamin K depends on a healthy gastrointestinal tract. Conditions that cause malabsorption syndromes, such as Crohn's disease, celiac disease, or short bowel syndrome, can impair the body's ability to absorb vitamin K and other fat-soluble vitamins. Those with such conditions or who have undergone bariatric surgery may require closer monitoring of their vitamin K status.
Drug interactions and intestinal bacteria
Certain medications, particularly bile acid sequestrants used for cholesterol management and some antibiotics, can interfere with vitamin K absorption. Additionally, antibiotics can disturb the gut microbiome, which produces some menaquinones (vitamin K2), potentially affecting K2 status. This makes it important for individuals on long-term medication to discuss their vitamin K intake with a healthcare provider.
The difference in food matrix
The form of vitamin K and its source also affect absorption. The K1 in plant chlorophyll is tightly bound within chloroplast membranes, which may contribute to its lower bioavailability compared to K2, which is present in a different matrix in fermented foods. The higher fat content of K2-rich foods like cheese and egg yolks also naturally aids in its absorption.
