Understanding the Different Forms of Vitamin K
Vitamin K is a fat-soluble nutrient that is not a single entity but a group of compounds. The two primary, naturally occurring forms are vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). A third, synthetic form known as menadione (K3) is not used in human supplements due to toxicity concerns. The functional differences between the main forms, K1 and K2, largely stem from their distinct chemical structures, sources, absorption rates, and distribution within the body.
Vitamin K1 (Phylloquinone): The Coagulation Vitamin
Vitamin K1 is the most abundant form of vitamin K in the human diet, primarily sourced from green leafy vegetables. It is well known for its critical role in regulating blood clotting, with the liver preferentially using K1. K1 from plants has relatively low absorption, which improves when consumed with fat. K1 also has a much shorter half-life in the bloodstream compared to some forms of K2.
Common K1 Food Sources:
- Kale
- Spinach
- Collard and turnip greens
- Broccoli
- Brussels sprouts
- Lettuce
- Vegetable oils
Vitamin K2 (Menaquinones): The Bone and Heart Health Vitamin
Vitamin K2 refers to a group of compounds synthesized by bacteria. K2 has a longer side chain than K1, influencing its absorption and transport. Unlike K1, which mainly functions in the liver, K2 is more widely distributed to tissues like bones and blood vessels. This makes K2 effective for activating proteins involved in bone metabolism and preventing vascular calcification.
Within the K2 family, two forms, MK-4 and MK-7, are commonly discussed.
MK-4 (Menaquinone-4)
MK-4 is a short-chain menaquinone found in animal products. It is also synthesized by the body from K1. However, supplemental MK-4 has a very short half-life and is often only effective at high, frequent doses.
MK-7 (Menaquinone-7)
MK-7 is a long-chain menaquinone primarily found in fermented foods, especially natto. Due to its longer chemical structure, MK-7 has a significantly longer half-life, circulating for several days. This extended bioavailability allows for more consistent activation of proteins for bone and cardiovascular health.
Common K2 Food Sources:
- Natto (richest source of MK-7)
- Fermented foods like sauerkraut and some cheeses
- Egg yolks
- Liver and other organ meats
- Grass-fed butter
- Salami and other cured meats
Comparison of Vitamin K Forms
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (MK-4) | Vitamin K2 (MK-7) |
|---|---|---|---|
| Primary Sources | Leafy green vegetables, vegetable oils | Animal products (meat, eggs, dairy) | Fermented foods (natto, cheese, sauerkraut) |
| Primary Function | Blood coagulation in the liver | Extrahepatic support, primarily converted from K1 | Extrahepatic support (bones, arteries) |
| Bioavailability | Low absorption from plants, improved with fat | Short half-life, requires high/frequent dosing | High absorption and longer half-life (up to 72 hours) |
| Body Distribution | Concentrated in the liver | Wider tissue distribution | Wider tissue distribution and stays in circulation longer |
| Impact on Bone Health | May support bone quality, but evidence is mixed | Requires high doses for significant effects | Demonstrated strong effects on bone mineral density and strength |
| Impact on Heart Health | Studies show mixed results | Less effective due to short half-life | Strongly associated with reduced vascular calcification |
Which Form is Best for You?
For most people, a combination of both K1 and K2 is ideal through a varied diet. However, for specific health goals, one form may be more effective.
- For Blood Clotting: Vitamin K1 is the most effective form for liver function. For those on blood thinners like warfarin, maintaining consistent K1 intake under medical supervision is crucial.
- For Bone and Heart Health: Vitamin K2, especially MK-7, shows superior benefits for these tissues. Its long half-life allows it to activate proteins for strengthening bones and preventing arterial calcification. Since many Western diets are low in K2, MK-7 supplementation is often recommended for optimal benefits.
- For Vegans: Vegans can get K1 from greens. The best vegan K2 source is natto; fermented vegetables contain some. A vegan MK-7 supplement can also be considered.
Conclusion
Both K1 and K2 are essential but have different primary roles. K1 is vital for blood clotting and found in greens. For bone and heart health, K2, particularly MK-7, is superior due to its bioavailability and longer half-life. The "best" form depends on your health needs and diet, often combining dietary K1 with supplemental K2. Consult a healthcare professional for personalized advice. Read more about vitamin K research from the National Institutes of Health.
