Understanding the Two Types of Vitamin K
To determine what is the best source of vitamin K, it is important to first understand that there are two main types: vitamin K1 (phylloquinone) and vitamin K2 (menaquinone). While both are crucial for human health, they have different dietary origins and varying absorption rates and biological functions.
Vitamin K1 is the most common form in the average diet and is found predominantly in plant foods. Its primary role is in supporting healthy blood coagulation.
Vitamin K2 is less common in the Western diet and is primarily found in animal products and fermented foods. K2 is especially important for bone and cardiovascular health, as it helps regulate calcium and prevents its buildup in arteries. Some of the vitamin K1 we consume can also be converted into K2 by bacteria in our gut.
Top Sources of Vitamin K1
Green leafy vegetables are the most potent sources of vitamin K1. Because this is a fat-soluble vitamin, consuming these vegetables with a source of fat, like olive oil, can significantly enhance absorption.
Here are some of the richest sources of vitamin K1:
- Kale: A powerhouse of nutrition, with a single cup of cooked kale providing several times the Daily Value (DV) for vitamin K.
- Spinach: Whether raw or cooked, spinach is a reliable and abundant source of K1.
- Collard Greens: These leafy greens offer a very high concentration of vitamin K1, especially when cooked.
- Broccoli and Brussels Sprouts: These cruciferous vegetables are excellent sources of vitamin K1 and other nutrients.
- Lettuce: Green leaf varieties contain significant amounts of vitamin K1, though less than denser greens like kale.
Top Sources of Vitamin K2
Vitamin K2 is a family of compounds (menaquinones) with different subtypes, such as MK-4 and MK-7. The most significant dietary sources come from fermented and animal-based products. Natto is, by far, the richest source of K2.
Some of the best sources of vitamin K2 include:
- Natto: This Japanese fermented soybean dish is the richest source of K2, particularly MK-7. A single serving can provide over 100% of the DV.
- Hard and Soft Cheeses: Certain cheeses contain menaquinones, with the content varying based on the fermentation process and ripening time. Aged Gouda and Munster are noted sources.
- Goose Liver Pâté: Goose liver is one of the highest animal-based sources of K2 (MK-4).
- Egg Yolks: Eggs, especially those from pasture-raised chickens, can be a moderate source of vitamin K2.
- Chicken and Beef Liver: Organ meats like liver can provide small to moderate amounts of vitamin K2.
Maximizing Vitamin K Absorption
Since vitamin K is fat-soluble, its absorption is enhanced when consumed with a meal containing dietary fat. This is particularly important for vitamin K1 from plant sources, where absorption can be low unless fat is present. For example, adding olive oil to a kale salad or cooking greens with a bit of butter can make a difference. The bioavailability of K2, particularly the MK-7 form found in natto, is higher and has a longer half-life in the body, which contributes to its effectiveness.
Comparison Table: K1 vs. K2
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinone) |
|---|---|---|
| Primary Sources | Green leafy vegetables, some oils | Fermented foods, animal products, gut bacteria synthesis |
| Main Role | Blood clotting (hepatic) | Bone and cardiovascular health (extra-hepatic) |
| Best Source | Kale and spinach | Natto (fermented soybeans) |
| Bioavailability | Lower absorption from plant matrix, higher with dietary fat | Higher bioavailability, especially MK-7 |
| Body Storage | Concentrates mainly in the liver | Found in higher concentrations in the brain and kidneys |
| Half-Life | Shorter half-life | Longer half-life, more persistent in the body |
Potential Health Benefits Beyond Clotting
Beyond its well-established role in blood coagulation, adequate vitamin K intake is linked to several other health benefits. Research has shown that vitamin K can contribute to stronger bones by activating osteocalcin, a protein that binds calcium to the bone matrix. A study published in the American Journal of Clinical Nutrition found an association between higher vitamin K2 intake and a reduced risk of coronary heart disease. By helping to prevent calcium buildup in arteries, vitamin K—particularly K2—supports cardiovascular health.
Conclusion: The Best Source of Vitamin K
Ultimately, there is no single "best" source of vitamin K, as an optimal intake involves a balance of both vitamin K1 and K2. For a high concentration of readily available K1, dark leafy greens like kale and spinach are excellent choices. To access potent K2, fermented foods like natto are unparalleled. Since K2 has higher bioavailability and specific benefits for bone and heart health, including foods like natto, certain cheeses, and eggs is also beneficial. The best strategy is to consume a variety of vitamin K-rich foods from both plant and animal sources, ensuring they are eaten alongside a small amount of dietary fat to maximize absorption. Combining K1-rich leafy greens with K2-rich foods provides a comprehensive approach to meeting your body's vitamin K needs.
For more detailed information on vitamin K's functions and dietary requirements, consult the NIH Office of Dietary Supplements' fact sheet.
