The Two Natural Forms of Vitamin K
Vitamin K isn't a single compound but a family of related fat-soluble compounds. In nature, it primarily exists in two forms, which are found in different types of food. Understanding this distinction is key to building a varied diet that ensures adequate intake of both essential types.
Vitamin K1 (Phylloquinone)
This is the most common form of vitamin K in the human diet, accounting for approximately 75–90% of our intake. K1 is synthesized by plants and is therefore abundant in green plant tissues. It is primarily transported to the liver, where it plays a critical role in producing proteins necessary for blood coagulation.
Vitamin K2 (Menaquinones)
This form is found in smaller quantities in our diet, but it is highly bioavailable and has a longer half-life in the bloodstream than K1. K2 is produced by bacteria and is found in fermented foods and some animal products. Different subtypes of K2, known as menaquinones (MKs), exist depending on the length of their side chain. The most well-researched subtypes are MK-4 and MK-7.
The Richest Natural Sources of Vitamin K1 (Phylloquinone)
The highest concentrations of vitamin K1 are consistently found in green leafy and cruciferous vegetables. To maximize absorption of this fat-soluble vitamin, these foods should be consumed with a source of dietary fat.
- Dark, Leafy Greens: Kale, spinach, collard greens, mustard greens, Swiss chard, and turnip greens are all exceptionally rich sources.
- Cruciferous Vegetables: Broccoli, Brussels sprouts, and cabbage contain significant amounts of vitamin K1.
- Vegetable Oils: Soybean and canola oils are also good sources of K1, and incorporating them into cooking helps improve overall absorption.
- Other Vegetables and Fruits: Smaller amounts can be found in parsley, asparagus, green beans, kiwi fruit, blueberries, and dried prunes.
Exploring the Dietary Sources of Vitamin K2 (Menaquinones)
Unlike K1, the best sources of K2 are primarily from animal and fermented foods, where bacteria have been involved in the production process. The concentration can vary significantly depending on the specific food.
- Natto: A traditional Japanese dish made from fermented soybeans, natto is the richest food source of K2 (specifically MK-7).
- Cheese: Many cheeses, especially hard varieties like Gouda and soft cheeses like Brie, contain modest amounts of K2 (MK-8 and MK-9).
- Organ Meats: Beef liver and other organ meats are a good source of K2 (MK-4).
- Eggs: The yolk of eggs, particularly from pasture-raised chickens, contains notable amounts of K2 (MK-4).
- Fermented Products: In addition to natto, other fermented foods like sauerkraut and kefir contain lower levels of K2.
Vitamin K1 vs. Vitamin K2 Food Sources: A Comparison
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinones) |
|---|---|---|
| Primary Source | Green plants and plant oils | Animal products and fermented foods |
| Examples | Kale, spinach, broccoli, soybean oil | Natto, cheese, egg yolks, liver |
| Concentration | Generally high in leafy greens | Varies greatly; highest in natto |
| Absorption | Poor absorption, improved with dietary fat | More readily absorbed, especially with fat |
| Role | Mostly liver function (blood clotting) | Extra-hepatic tissues (bone, cardiovascular) |
| Half-Life | Short half-life, cleared from blood quickly | Long half-life, remains in circulation longer |
Understanding Absorption and Bioavailability
The way your body processes and utilizes vitamin K varies between its two main forms. The absorption rate for K1 from plant sources is relatively low, typically less than 20%. This is because it is tightly bound within the plant's chloroplasts. This is why consuming leafy greens with some fat is so important for maximizing absorption. The K2 found in animal and fermented products is generally better absorbed, as it is not bound in the same way. The intestinal bacteria also synthesize some K2, though this is not sufficient to meet all of the body's needs. The long-chain forms of K2 (MK-7, for example) also circulate longer in the blood, allowing them to benefit tissues beyond the liver, such as bones and arteries.
Conclusion: The Importance of a Varied Diet
In conclusion, what is vitamin K naturally found in is a question with a nuanced answer that points to a wide range of foods. To ensure a comprehensive intake of both vitamin K1 and K2, incorporating a variety of nutrient-rich sources into your diet is the most effective strategy. This means not only filling your plate with dark, leafy greens but also including fermented foods, dairy, and animal products where appropriate. A balanced approach ensures your body receives both K1 for blood clotting and K2 for optimal bone and cardiovascular health, contributing to overall well-being. For more detailed information, consult the National Institutes of Health Office of Dietary Supplements: Vitamin K Fact Sheet for Health Professionals.
