Understanding the Vitamin K Family
Vitamin K is a group of fat-soluble vitamins vital for several physiological processes, most notably blood coagulation and calcium metabolism. The two primary forms are phylloquinone (Vitamin K1) and menaquinones (Vitamin K2). While they share a common chemical structure—a 2-methyl-1,4-naphthoquinone ring—their side chains differ, leading to distinct functions and bioavailability within the human body.
Phylloquinone (Vitamin K1): The Plant-Derived Form
Phylloquinone is the most common form of vitamin K in the human diet, primarily from green leafy vegetables and certain plant oils. Key sources include kale, spinach, broccoli, collard greens, and soybean and canola oils. Absorption from vegetables can be improved by consuming them with fat. Once absorbed, phylloquinone is taken up by the liver to activate proteins needed for blood clotting. It is used medically to treat clotting disorders and as an antidote to certain anticoagulant drugs.
Menaquinone (Vitamin K2): The Bacterial and Animal-Based Form
Menaquinones are found in fermented and animal products and are known for functions beyond blood clotting. Menaquinones are a family of compounds (MK-n) with varying isoprenoid units in their side chain. MK-4 is found in animal products and can be made in the body from phylloquinone. Long-chain menaquinones (like MK-7, MK-8, MK-9) are produced by bacteria and found in fermented foods, especially natto. Menaquinones, particularly long-chain forms, have a longer half-life and distribute to extra-hepatic tissues like bone and arteries. They activate proteins like osteocalcin for bone mineralization and Matrix Gla Protein (MGP) to prevent arterial calcification.
The Key Differences: A Comparison Table
| Feature | Phylloquinone (Vitamin K1) | Menaquinone (Vitamin K2) |
|---|---|---|
| Primary Source | Plants (e.g., leafy greens) | Bacteria (fermented foods) and animal products |
| Chemical Structure | Contains a phytyl side chain | Contains a varying number of isoprenoid side chains (MK-n) |
| Bioavailability | Lower absorption, particularly from plants; shorter half-life | Higher absorption, especially MK-7; longer half-life (days) |
| Distribution | Primarily transported and used by the liver | Distributed to extra-hepatic tissues, such as bone and arteries |
| Main Function | Essential for activating blood-clotting proteins in the liver | Crucial for bone mineralization and inhibiting arterial calcification |
| Examples | Spinach, kale, broccoli, canola oil | Natto, hard cheeses, egg yolks, chicken |
The Conversion of Phylloquinone to Menaquinone-4
The body can convert dietary phylloquinone into menaquinone-4 (MK-4) in certain tissues. This process involves removing the phylloquinone side chain and adding a new one. This conversion helps ensure tissues that need MK-4 have it, even with low dietary intake. However, the body does not convert K1 into longer-chain menaquinones like MK-7.
Health Implications and Choosing a Supplement
The differences in K1 and K2 suggest distinct health benefits. Menaquinone, particularly MK-7, is often targeted for bone and cardiovascular health. Individuals on warfarin need consistent vitamin K intake to avoid interfering with medication. Consult a healthcare provider before taking vitamin K supplements, especially if on medication.
Conclusion
Phylloquinone (K1) and menaquinone (K2) differ in origin, structure, and metabolism, leading to distinct roles. K1, from plants, is mainly for liver blood clotting, while K2, from bacteria and animal sources, supports bone and cardiovascular health in other tissues. While K1 can convert to MK-4, dietary intake of varied sources or K2 supplementation may be needed for optimal extra-hepatic benefits. A balanced diet with both leafy greens and fermented/animal foods is recommended. For more information, visit the National Institutes of Health Office of Dietary Supplements.
