Dietary Sources: Plant-Based Vitamin K1 (Phylloquinone)
Dietary intake is the primary way most people acquire vitamin K. This comes in two main forms: K1 and K2. Vitamin K1, or phylloquinone, is the most common form found in the diet and is naturally produced by plants. This makes green leafy vegetables and certain oils particularly excellent sources. For example, just a single serving of some leafy greens can provide your entire daily requirement.
Some of the best dietary sources of vitamin K1 include:
- Leafy Greens: This category is a powerhouse for vitamin K1. Examples include kale, spinach, turnip greens, collard greens, and Swiss chard, which offer some of the highest concentrations.
- Cruciferous Vegetables: Broccoli, Brussels sprouts, and cabbage are also substantial contributors to K1 intake.
- Vegetable Oils: Oils made from soybeans and canola contain significant amounts of phylloquinone. Salad dressings prepared with these oils can also be a good source.
- Certain Fruits: Some fruits, though generally containing lower amounts than leafy greens, still contribute to your intake. These include blueberries, figs, kiwi, and grapes.
When consuming K1-rich foods, it is important to remember that it is a fat-soluble vitamin. Pairing these foods with a source of fat, such as olive oil in a salad, can significantly improve its absorption in the small intestine. However, the bioavailability of K1 from plants can still be relatively low, as it is tightly bound within chloroplast membranes.
Dietary Sources: Animal and Fermented Vitamin K2 (Menaquinones)
Vitamin K2, or menaquinone, is less common in the Western diet but is a vital form of the vitamin. It is primarily found in animal products and fermented foods. Unlike K1, menaquinones consist of various subtypes (e.g., MK-4, MK-7) that differ in their chemical structure.
Key sources of vitamin K2 include:
- Fermented Foods: Nattō, a traditional Japanese food made from fermented soybeans, is an exceptionally rich source of MK-7, a highly bioavailable form of K2. Other fermented foods like some cheeses (hard and soft) also contain menaquinones.
- Animal Products: Small to modest amounts of K2, specifically the MK-4 subtype, can be found in meat, poultry (especially chicken), eggs, and dairy products like butter and some cheeses. Animals can convert some dietary K1 into K2 (MK-4) in their tissues.
The Role of Gut Bacteria in Vitamin K Supply
Beyond dietary sources, the human body has an endogenous source of vitamin K. Bacteria living in the large intestine synthesize menaquinones, a significant contributor to the body's vitamin K supply. Species like Escherichia coli and certain Bifidobacterium are known producers of K2. This bacterial production helps account for the rarity of clinically significant vitamin K deficiency in healthy adults. However, this endogenous production might not be sufficient on its own, and its total contribution to the body's vitamin K status is still being researched. Conditions that affect gut health, such as certain malabsorption disorders or prolonged antibiotic use, can disrupt this bacterial production and lead to lower vitamin K levels.
Supplementation and Clinical Administration
For individuals with dietary restrictions, malabsorption issues, or other medical needs, vitamin K can be supplied through supplements and clinical administration. Supplements are available over-the-counter and typically contain phylloquinone (K1), menaquinone-4 (MK-4), or menaquinone-7 (MK-7). MK-7 is often highlighted for its higher bioavailability and longer half-life compared to K1, making it a popular choice.
Clinical administration is most famously used for newborns to prevent a serious bleeding condition. Since vitamin K does not cross the placenta efficiently and breast milk contains low levels, a single intramuscular dose of vitamin K1 is routinely administered shortly after birth to protect against Vitamin K Deficiency Bleeding (VKDB). Additionally, supplementation is sometimes required for individuals with specific gastrointestinal disorders or for those on certain medications that interfere with vitamin K metabolism, such as the anticoagulant warfarin.
How Vitamin K is Absorbed and Utilized
As a fat-soluble vitamin, dietary vitamin K is absorbed along with other fats in the small intestine. This process is facilitated by bile and pancreatic juices. Once absorbed, it is transported in the bloodstream via lipoproteins and stored primarily in the liver, brain, heart, pancreas, and bones. The body requires vitamin K to activate specific proteins through a process called gamma-carboxylation. These activated proteins are crucial for various physiological processes, including:
- Blood Coagulation: Vitamin K is a co-enzyme for the production of critical blood-clotting factors, including factors II (prothrombin), VII, IX, and X.
- Bone Health: It is essential for producing proteins like osteocalcin and Matrix Gla Protein (MGP) that help bind calcium and support bone mineralization, potentially reducing osteoporosis risk.
- Cardiovascular Health: MGP, activated by vitamin K, helps inhibit arterial calcification, which is a risk factor for heart disease.
| Source | Primary Vitamin K Form | Key Food Examples | Notes on Absorption |
|---|---|---|---|
| Dietary Plants | K1 (Phylloquinone) | Kale, spinach, broccoli, vegetable oils | Absorption is lower due to binding in chloroplasts; improved with dietary fat. |
| Dietary Animal/Fermented | K2 (Menaquinones) | Nattō, cheese, eggs, meat, poultry | Often more bioavailable than K1; absorption varies by specific menaquinone subtype. |
| Gut Bacteria | K2 (Menaquinones) | (Produced endogenously) | Varies greatly by gut microbiome health; contributes to overall status but may not be sufficient alone. |
| Supplements | K1 (Phylloquinone), K2 (MK-4, MK-7) | Various multivitamin or single-nutrient products | Designed for effective absorption, with MK-7 offering superior bioavailability and a longer half-life. |
| Clinical Prophylaxis | K1 (Phylloquinone) | Intramuscular injection for newborns | Highly effective for preventing VKDB in infants; bypasses initial absorption challenges. |
Conclusion
Vitamin K is supplied to the human body through a multifaceted process involving dietary intake, endogenous production by gut bacteria, and, when necessary, supplementation. Plant-based foods are excellent sources of vitamin K1, while animal products and fermented foods offer menaquinones (K2). The body's ability to recycle and store the vitamin also contributes to stable levels, explaining why deficiency is rare in healthy adults. However, specific populations, including newborns and those with malabsorption issues, require targeted intervention to ensure an adequate supply. Maintaining a balanced diet rich in diverse sources of both K1 and K2 is the most effective strategy for ensuring adequate vitamin K levels and supporting blood clotting, bone health, and overall cardiovascular function.
