The Different Forms of Vitamin K
Vitamin K is not a single compound but a family of fat-soluble vitamins crucial for proper blood clotting and bone health. This family includes two naturally occurring forms, K1 and K2, and one synthetic form, K3.
- Vitamin K1 (Phylloquinone): This form is made by plants and found primarily in green leafy vegetables like kale, spinach, and broccoli.
- Vitamin K2 (Menaquinone): This group consists of several subtypes (known as MK-4, MK-7, etc.) and is produced by bacteria. Sources include fermented foods like natto, as well as some animal products like eggs and hard cheeses.
- Vitamin K3 (Menadione): This is a synthetic, man-made form of the vitamin that does not occur naturally in foods. Menadione is a precursor that the body can convert to other forms of vitamin K, but its use in humans is problematic.
The Danger of Menadione (Vitamin K3)
So, why won't you find a K3 supplement for humans on store shelves? The reason is simple and based on solid scientific evidence from research conducted in the 1980s and 1990s: menadione is toxic to humans. The U.S. Food and Drug Administration (FDA) has banned its sale as a human dietary supplement because of its potential to cause severe health problems.
- Liver Damage: Studies showed that vitamin K3 can damage hepatic (liver) cells.
- Hemolytic Anemia: It can lead to the destruction of oxygen-carrying red blood cells.
- Interference with Antioxidants: Menadione interferes with the function of glutathione, a crucial natural antioxidant in the body. This can increase oxidative stress and cellular damage.
- Allergic Reactions: Overdoses were also associated with allergic reactions.
Where is Vitamin K3 Used?
Despite its dangers for human consumption, menadione is still used in other applications. It is commonly added to livestock and pet feed in regulated, safe doses, as animals can safely convert it to K2. This makes it a cost-effective and stable vitamin source for animal nutrition. Some industrial applications also use it. However, this is strictly separate from the market for human dietary supplements.
Comparison of Vitamin K Forms
| Feature | Vitamin K1 (Phylloquinone) | Vitamin K2 (Menaquinone) | Vitamin K3 (Menadione) |
|---|---|---|---|
| Source | Plants, especially leafy greens | Bacteria, fermented foods, some animal products | Synthetic (man-made) |
| Human Supplements | Widely available and safe | Widely available and safe | Banned for human use due to toxicity |
| Toxicity | Very low potential for toxicity, even at high doses | Very low potential for toxicity | Harmful to humans, causing liver damage and other issues |
| Main Function | Primarily involved in blood clotting | Supports bone and cardiovascular health, in addition to clotting | Precursor for K2 in animals, but toxic to humans |
| Bioavailability | Readily absorbed from oils, less so from leafy greens | High, especially the MK-7 form, which has a longer half-life | Not applicable for human consumption |
| Metabolism | Converted to MK-4 in extrahepatic tissues | Multiple subtypes (MK-4, MK-7, etc.) with different properties | Converted by animals to K2 |
Focusing on Safe Vitamin K Supplementation
Because K3 is unsafe, if you need a vitamin K supplement, you should focus on K1 or K2. The specific form you choose may depend on your health goals:
- K1 for Coagulation: While both K1 and K2 are involved in blood clotting, K1 is the primary form that governs this process in the liver. This is why phylloquinone (K1) injections are routinely given to newborns to prevent bleeding.
- K2 for Bone and Arterial Health: K2 is particularly noted for its role in directing calcium to the bones and preventing its accumulation in soft tissues like arteries. The MK-7 form of K2 is often highlighted in supplements due to its higher bioavailability and longer half-life in the body compared to K1 and MK-4.
Who Might Need a Supplement?
While vitamin K deficiency is rare in healthy adults with a balanced diet, supplementation may be necessary for some individuals. Those with malabsorption disorders, individuals on certain medications like long-term antibiotics or blood thinners (coumarin-type anticoagulants like warfarin), and newborns are common cases where vitamin K is medically administered. For anyone on anticoagulant medication, it is crucial to consult a doctor before taking any vitamin K supplements, as it can interfere with treatment.
It is important to remember that most people can meet their vitamin K needs through diet alone by regularly consuming leafy greens and fermented foods. However, if you are considering supplementation, always choose products containing the natural and safe forms, K1 or K2.
Conclusion
In summary, the notion of a K3 supplement for human use is a misconception. The synthetic compound, menadione, was long ago proven to be toxic to humans and is no longer included in dietary supplements. Instead, safe and natural forms of vitamin K, namely K1 (phylloquinone) and K2 (menaquinone), are readily available in both food and supplement form. For those concerned about their vitamin K intake, the focus should be on dietary sources like green leafy vegetables and fermented foods, or on supplementing with K1 or K2 after consulting a healthcare professional. To learn more about vitamin K, visit the NIH Health Professional Fact Sheet.
