Vitamin K, a fat-soluble nutrient, plays a critical role in blood clotting, bone metabolism, and heart health. While Vitamin K1 is primarily involved in coagulation, Vitamin K2 (menaquinones) is particularly important for directing calcium to the bones and preventing its accumulation in soft tissues like arteries. Within the K2 family, two forms, MK-4 and MK-7, are the most widely discussed, yet they differ significantly in their bioavailability and how the body utilizes them. For those seeking optimal benefits, identifying what is the most bioavailable form of K2 is essential.
The Difference Between MK-4 and MK-7
Vitamin K2 exists as several subtypes, called menaquinones, which are designated by a number (n) indicating the length of the side chain. MK-4 and MK-7 are the most common forms found in supplements, but their structure and origin vary significantly.
- MK-4 (Menaquinone-4): This is a shorter-chain menaquinone found in animal products like meat, eggs, and dairy. It has a very short half-life, meaning it is cleared from the bloodstream within hours after consumption. Because of this, it is not effective at nutritional doses and requires much higher intake to be beneficial. While some MK-4 is produced in the body from K1, dietary MK-4 has poor bioavailability at normal nutritional levels.
- MK-7 (Menaquinone-7): A longer-chain menaquinone, MK-7 is produced by bacteria and is found in fermented foods, most notably the Japanese dish natto. MK-7 has a much longer half-life, allowing it to circulate in the blood for days. This longer circulation time enables the body to achieve stable blood levels, providing consistent support for bone and cardiovascular health.
Why Bioavailability is Critical
High bioavailability ensures a nutrient is effectively absorbed and utilized by the body, and for Vitamin K2, this is key to activating vital proteins. The primary reason MK-7 is the more bioavailable form is its longer half-life, which leads to superior health outcomes.
- Consistent Activation: The key function of Vitamin K2 is to activate specific proteins, such as osteocalcin (which binds calcium in bones) and Matrix Gla Protein (MGP) (which prevents arterial calcification). MK-7's long half-life means these proteins receive a steady supply of activation, a process that is much less effective with the short-lived MK-4.
- Accumulation: Unlike MK-4, MK-7 can accumulate in the body's tissues, creating a reserve that provides continuous support. This sustained presence allows for greater and more stable activation of K2-dependent proteins throughout the body.
MK-4 vs. MK-7: A Comparative Table
| Feature | Vitamin K2 (MK-4) | Vitamin K2 (MK-7) |
|---|---|---|
| Source | Animal products (meat, dairy, eggs) | Bacterial fermentation (natto, cheese) |
| Bioavailability | Poor at nutritional doses | High; absorbed efficiently |
| Half-Life | Short (cleared in hours) | Long (circulates for days) |
| Effective Dose | High, requires frequent intake | Lower, provides sustained benefits |
| Main Role | Supports various tissue functions | Optimal for systemic health, especially bones and arteries |
| Serum Levels | Does not significantly increase serum levels at nutritional doses | Significantly increases and maintains serum levels |
The All-Trans Isomer Advantage
For maximum bioavailability, the form of MK-7 is also important. Naturally occurring MK-7 is found in the "all-trans" isomer form, which is the biologically active configuration. Inferior or unstable production methods can result in a mix of trans and cis isomers, with the cis form being less biologically effective. High-quality supplements, often featuring patented MK-7 like MenaQ7® or K2VITAL®, ensure a high percentage of the active all-trans isomer.
Practical Implications for Supplementation
When choosing a vitamin K2 supplement, the science points to MK-7 as the superior choice due to its higher bioavailability and longer-lasting effects.
- Look for MK-7: Prioritize supplements that explicitly state they contain menaquinone-7.
- Check for the all-trans isomer: Reputable brands will often specify that their MK-7 is in the highly active all-trans form.
- Combine with a fat source: Since Vitamin K2 is fat-soluble, taking your supplement with a meal containing fat (such as avocado or olive oil) will further enhance its absorption.
- Pair with Vitamin D3: Vitamin K2 and Vitamin D3 work synergistically. Vitamin D promotes calcium absorption, while K2 helps direct it to the bones, away from the arteries.
A Note on Dietary Sources
While supplementation is often the most direct way to ensure adequate intake of highly bioavailable K2, certain foods are excellent sources.
Foods Rich in Vitamin K2 (MK-7):
- Natto (fermented soybeans) is the richest dietary source.
- Sauerkraut
- Fermented dairy, like certain cheeses (Gouda, Edam) and kefir
- Goose liver
Incorporating these foods into your diet can provide a natural source of MK-7, complementing any supplementation regimen. For those who do not eat fermented foods regularly, particularly natto, supplementation offers a reliable alternative for achieving optimal vitamin K2 status.
Conclusion
Based on extensive research and its physiological behavior in the human body, menaquinone-7 (MK-7) is the most bioavailable form of K2. Its longer half-life and ability to accumulate in tissues ensure a sustained supply that effectively activates vitamin K-dependent proteins critical for bone and cardiovascular health. While MK-4 has a role, its poor bioavailability at nutritional doses makes it less effective for supplementation. Consumers should seek high-quality MK-7 supplements that specify the all-trans isomer to ensure they are getting the most effective form of this vital nutrient.
Understanding Bioavailability
Bioavailability is a pharmacological term that describes the proportion of a drug or other substance which enters the circulation when introduced into the body and so is able to have an active effect. For nutrients like Vitamin K2, a high bioavailability means the body can efficiently absorb and use the nutrient. The superior bioavailability of MK-7 over MK-4 is a key differentiator in their effectiveness at nutritional doses. MK-7's fat-soluble nature and longer residence time in the bloodstream allow for broader distribution to extra-hepatic tissues like bones and arteries, where it exerts its most significant health benefits. In contrast, the shorter half-life of MK-4 limits its systemic reach and impact at typical supplement levels. This scientific understanding is why health professionals often recommend MK-7 for targeted support of bone and cardiovascular health.
