Understanding the Forms of Vitamin K
Vitamin K is a fat-soluble nutrient that is essential for blood clotting and plays a vital role in bone and cardiovascular health. It exists in two primary forms: vitamin K1 (phylloquinone) and vitamin K2 (menaquinones). However, these forms differ significantly in their bioavailability, or the degree to which they are absorbed and utilized by the body.
Vitamin K1 (Phylloquinone)
Found predominantly in leafy green vegetables like spinach, kale, and broccoli, vitamin K1 is the most common form of vitamin K in the Western diet. Despite its prevalence, dietary vitamin K1 is notoriously poorly absorbed. Studies have estimated that the body absorbs less than 10% of the K1 present in plants. It also has a very short half-life, lasting only a few hours in the bloodstream, and is primarily directed to the liver to aid in blood coagulation.
Vitamin K2 (Menaquinones)
Vitamin K2 is a family of compounds known as menaquinones, synthesized by bacteria. There are several subtypes, denoted as MK-n based on their side-chain length. The most common and studied are MK-4 and MK-7.
- MK-4: Found in animal products like eggs, meat, and liver, this form is also synthesized in animal tissues from dietary K1. At nutritional doses, MK-4 has a very short half-life and does not accumulate in the blood, making it poorly bioavailable via supplementation.
- MK-7: Sourced from fermented foods, most notably the Japanese dish natto, MK-7 is known for its exceptional bioavailability. It boasts a long half-life of around 72 hours, allowing it to circulate in the bloodstream longer and distribute effectively to extra-hepatic tissues, such as bones and blood vessels.
Why MK-7 Offers Superior Bioavailability
Research comparing MK-7 to other forms of vitamin K consistently shows its superiority in absorption and systemic distribution. A key study administered nutritional doses of MK-4 and MK-7 to healthy women and found that MK-7 significantly increased serum levels for up to 48 hours, while MK-4 was undetectable. This confirms MK-7's effectiveness at nutritional doses and highlights MK-4's poor performance as a supplement.
The secret to MK-7's success lies in its long side chain and how it's transported in the body. Unlike K1, which is rapidly cleared by the liver, MK-7 is efficiently redistributed by the liver into LDL-C lipoproteins. Since LDL-C has a long half-life in the circulation, MK-7 can remain in the bloodstream for days, providing consistent and reliable access to tissues beyond the liver. This is crucial for activating extra-hepatic proteins like osteocalcin for bone health and matrix Gla protein for preventing arterial calcification.
Factors Influencing Vitamin K Absorption
Several physiological and dietary factors can impact how well you absorb vitamin K, regardless of its form:
- Presence of Dietary Fat: As a fat-soluble vitamin, vitamin K requires dietary fat for optimal absorption. Ingesting vitamin K with a meal containing some fat, such as adding olive oil to a salad or eating eggs, significantly improves absorption.
- Bile Salts: The process of absorption in the small intestine relies on the action of bile and pancreatic enzymes to form mixed micelles that carry the vitamin. Liver or pancreatic diseases that impair bile flow can lead to malabsorption.
- Intestinal Health: Conditions like celiac disease or inflammatory bowel disease that damage the small intestinal lining can impair vitamin K absorption.
- Antibiotic Use: Long-term use of broad-spectrum antibiotics can alter the gut microbiome, which is responsible for producing some menaquinones.
Vitamin K Absorption and Bioavailability at a Glance
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (MK-4) | Vitamin K2 (MK-7) |
|---|---|---|---|
| Primary Source | Leafy green vegetables, vegetable oils | Animal products, tissue conversion | Fermented foods (natto), supplements |
| Absorption Efficiency | Low (less than 10% from plants) | Poor (undetectable in serum at nutritional doses) | High (well-absorbed) |
| Blood Half-Life | Short (1-2 hours) | Short (hours, <24) | Very Long (approx. 72 hours) |
| Primary Action Site | Liver (blood clotting factors) | Local tissues (converted from K1/MK-7) | Liver, bones, arteries, and other extra-hepatic tissues |
Making the Best Choice
For individuals seeking to maximize the benefits of vitamin K for bone and cardiovascular health, the choice is clear. While a balanced diet containing both leafy greens and fermented foods is ideal, supplementation is often the most reliable way to achieve optimal vitamin K status, particularly for extra-hepatic tissues. Because of its superior bioavailability, longer half-life, and better systemic distribution, vitamin K2 as MK-7 is the optimal choice for supplementation.
Conclusion
While all forms of vitamin K are important, their bioavailability and distribution throughout the body vary significantly. MK-7 stands out as the best-absorbed form, offering a long half-life that allows it to effectively reach and benefit tissues beyond the liver, playing a critical role in supporting bone density and inhibiting arterial calcification. For those focused on systemic health benefits, particularly for bone and heart health, prioritizing MK-7 either through natto consumption or supplementation is the most effective strategy. Research continues to highlight the distinct roles and benefits of the different vitamin K forms.
