The Two Main Types of Vitamin K: K1 and K2
To understand the origin of vitamin K, one must first differentiate between its two primary forms: vitamin K1 (phylloquinone) and vitamin K2 (menaquinone). While they share a similar core chemical structure, the distinction in their side chains dictates their source, absorption, half-life, and function within the body.
- Vitamin K1 (Phylloquinone): This is the most common form of vitamin K found in the human diet, making up 75–90% of total intake. It is synthesized exclusively by plants and algae.
- Vitamin K2 (Menaquinone): This form comprises several subtypes, known as MK-4 through MK-13, with varying side-chain lengths. It is primarily produced by bacteria and is found in animal products and fermented foods.
Where is Vitamin K1 (Phylloquinone) Produced?
Plants are the sole producers of vitamin K1, where it plays a vital role in the photosynthesis process. Consequently, the highest concentrations of this nutrient are found in the photosynthetic tissues of plants, namely the leaves. To obtain K1, humans and animals must consume plant-based foods. Some of the most potent plant-based sources include:
- Leafy Greens: Spinach, kale, collard and turnip greens, Swiss chard.
- Cruciferous Vegetables: Broccoli, Brussels sprouts, cabbage.
- Plant Oils: Soybean and canola oil.
- Fruits and other vegetables: Kiwi, blueberries, avocado, green beans, and asparagus.
Since vitamin K is fat-soluble, its absorption from plant sources is often enhanced when consumed with dietary fat, such as with olive oil in a salad.
How is Vitamin K2 (Menaquinone) Produced?
Unlike K1, vitamin K2 has multiple origins. The primary producers are bacteria, and this bacterial synthesis occurs in several contexts.
Gut Microbiota Production
Beneficial bacteria residing in the large intestine of humans, such as Escherichia coli, Lactobacillus, and Bifidobacterium, are capable of synthesizing menaquinones. The gut's contribution to a person's total vitamin K status is important, though it may not be enough to meet the entire daily requirement. The specific types and amounts of K2 produced can depend on the strains of bacteria present. Factors like antibiotic use, certain gastrointestinal diseases, and overall gut health can disrupt this bacterial production.
Fermented Foods Production
Vitamin K2 is also produced during the fermentation of certain foods by bacteria. This process explains why fermented products are such rich sources of K2. For example, Bacillus subtilis is used to ferment soybeans to make natto, a traditional Japanese dish with exceptionally high levels of K2 (specifically MK-7). Other fermented products, like certain cheeses and sauerkraut, also contain menaquinones.
Animal Products
Animals consume K1 from plants, which they can then convert into MK-4, a subtype of vitamin K2. This MK-4 is then stored in animal tissues, explaining why it is found in animal-based foods. As a result, animal products are another key source of dietary vitamin K2.
Comparison Table: Vitamin K1 vs. Vitamin K2
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinone) |
|---|---|---|
| Source | Produced by plants during photosynthesis. | Produced by bacteria (including gut microbiota and fermentation) and found in animal products. |
| Dietary Sources | Green leafy vegetables (kale, spinach), broccoli, vegetable oils. | Fermented foods (natto, cheese), egg yolks, liver, and meat. |
| Bioavailability | Poorly absorbed from plant sources unless consumed with fat. | Generally better absorbed, especially from fatty foods. |
| Distribution | Primarily retained and used by the liver for blood clotting. | Circulates longer in the blood and is available for extrahepatic tissues like bones and arteries. |
| Primary Function | Cofactor for clotting factors in the liver to promote hemostasis. | Supports bone mineralization and inhibits arterial calcification. |
| Half-Life | Shorter half-life in the bloodstream. | Longer half-life, especially long-chain menaquinones like MK-7. |
The Essential Role of a Varied Diet
Despite the body's ability to produce some vitamin K2 via gut flora, relying solely on this internal production is not recommended. Evidence suggests that a varied and balanced diet is the most effective way to ensure adequate intake of both vitamin K1 and K2. While vitamin K deficiencies are rare in healthy adults, certain conditions can affect absorption and metabolism. For instance, individuals with malabsorption disorders, liver disease, or those taking specific medications (like warfarin) may require careful monitoring and supplementation.
To optimize your vitamin K intake, incorporate a mix of leafy greens for K1 and fermented or animal-based foods for K2. This approach ensures your body receives both forms for their distinct yet equally important roles in maintaining overall health, from blood clotting to bone and heart health. For further information on recommended intake levels, you can consult resources like the National Institutes of Health.(https://ods.od.nih.gov/factsheets/VitaminK-Consumer/)
Conclusion
In summary, vitamin K is produced from both external and internal sources, each contributing different forms of the nutrient. Plants produce vitamin K1, which is abundant in leafy green vegetables. Bacteria, both in the human gut and during food fermentation, produce vitamin K2, found in fermented foods and animal products. While gut bacteria provide a valuable source of K2, dietary intake remains crucial for ensuring sufficient vitamin K for vital bodily functions like blood clotting and bone health. A balanced diet rich in both plant-based K1 and fermented/animal-based K2 is the best strategy for maintaining optimal levels of this essential fat-soluble vitamin.
