Nutrition Diet: Is K1 or K2 More Bioavailable? A Look at Vitamin K Absorption

October 25, 2025 •

2 min read

Studies show that certain forms of Vitamin K2 can maintain significantly higher serum concentrations over a longer period compared to Vitamin K1. So, when it comes to the question, 'Is K1 or K2 more bioavailable?', the answer is crucial for understanding how to optimize your dietary nutrition.

Quick Summary

Vitamin K2, particularly the MK-7 form, is more bioavailable than K1 due to its superior absorption and significantly longer half-life, allowing for better distribution to extra-hepatic tissues like bone and arteries.

Key Points

K2 is more bioavailable than K1: Long-chain forms of vitamin K2, like MK-7, are absorbed more efficiently and have a longer half-life than vitamin K1.
K1 primarily serves the liver: After consumption, vitamin K1 is rapidly cleared and used by the liver for blood coagulation.
K2 targets extra-hepatic tissues: Thanks to its longer circulation time, K2 is better distributed to areas outside the liver, supporting bone and cardiovascular health.
Dietary K1 is poorly absorbed: K1 in leafy greens is tightly bound in chloroplasts, resulting in low absorption rates, though consuming it with fat can help.
Dietary sources differ greatly: K1 is abundant in green vegetables, while K2 is found in fermented foods, cheese, and animal products.
Supplementation benefits: Supplementing with MK-7, a highly bioavailable form of K2, can be an effective way to ensure optimal levels, especially for extra-hepatic functions.

Understanding Bioavailability in Vitamins

Bioavailability is the amount of a nutrient that is absorbed and used by the body after eating. This is affected by factors like the nutrient's structure, source, and other foods consumed with it. For vitamin K, the bioavailability differences between K1 and K2 are important for health.

The Fundamental Difference: K1 vs. K2

Both K1 (phylloquinone) and K2 (menaquinones) are vitamin K forms, but they have different structures and come from different sources, leading to different actions in the body.

Vitamin K1: The Plant-Based Form

Found in green leafy vegetables, K1 is the main form in Western diets. K1 is poorly absorbed unless fat is present and is primarily directed to the liver for blood clotting.

Vitamin K2: The Bacterially-Derived Form

K2 is a group of menaquinones made by bacteria and found in animal products and fermented foods. Forms like MK-7 are more bioavailable and remain in the blood longer than K1, helping them reach tissues like bones and blood vessels.

Comparative Bioavailability: Why K2 Wins

K2, particularly MK-7, is more bioavailable than K1. K1 from plants is poorly absorbed, with absorption from spinach being only 4-17%, and is quickly cleared for liver use. K2 is better absorbed with fat and has a longer half-life, allowing it to reach and benefit tissues beyond the liver.

A Comparative Look at Bioavailability and Function


Feature	Vitamin K1 (Phylloquinone)	Vitamin K2 (Menaquinones)
Primary Source	Green leafy vegetables, plant oils	Fermented foods (natto, cheese), animal products
Absorption Rate	Low (4-17% from plants), improved with fat	High, especially MK-7 form
Half-Life	Short (hours)	Long (days for MK-7)
Tissue Distribution	Primarily to the liver	Distributed widely to extra-hepatic tissues (bone, arteries)
Primary Function	Blood clotting (hepatic)	Bone health, cardiovascular health (extra-hepatic)

Dietary Considerations for Optimal Vitamin K Intake

Since K1 and K2 work differently, it's best to get both from your diet. Pairing leafy greens with fat can enhance K1 absorption. While K1 intake is often sufficient, K2 intake can be low, making it important to include K2-rich foods like aged cheeses, egg yolks, and natto. MK-7 supplements are also an option for better bioavailability.

Conclusion: How to Maximize Your Vitamin K Nutrition

To answer "Is K1 or K2 more bioavailable?", K2, particularly long-chain forms, is more bioavailable due to better absorption and longer circulation. K1 is mainly for blood clotting in the liver and is poorly absorbed from plants. For best nutrition, eat a variety of leafy greens and K2 sources. MK-7 supplements can help ensure enough K2 for overall health. For more information, consult resources like the {Link: NIH Office of Dietary Supplements https://ods.od.nih.gov/factsheets/VitaminK-HealthProfessional/}.

Frequently Asked Questions

The primary difference lies in their tissue distribution. Vitamin K1 is primarily used by the liver to activate clotting factors. Vitamin K2, due to its longer presence in the blood, is distributed more effectively to extra-hepatic (non-liver) tissues, where it plays a key role in bone and cardiovascular health.

Vitamin K1 is naturally bound tightly to chloroplasts within plant cells. This structure makes it difficult for the body's digestive system to release and absorb it effectively. Absorption is improved by consuming fat with the vegetables, but it remains relatively inefficient.

A longer half-life means that vitamin K2 stays in the bloodstream for a longer period (days, in the case of MK-7), whereas K1 is cleared in just hours. This extended circulation time allows K2 to reach and activate proteins in tissues throughout the body, such as bones and blood vessels.

For most people following a Western diet, vitamin K1 intake is generally adequate for blood clotting functions due to high consumption of leafy greens. However, dietary intake of K2 is often much lower and may be insufficient for optimal bone and cardiovascular health. A balanced diet including both K1-rich vegetables and K2-rich fermented foods or animal products is ideal.

Yes, the body can convert vitamin K1 into menaquinone-4 (MK-4), one of the forms of K2, in some tissues like the pancreas and brain. However, this conversion process does not fully replace the need for dietary intake of other forms of K2 like MK-7, which has different functions and pharmacokinetics.

The most potent food source of K2 (specifically MK-7) is natto, a traditional Japanese fermented soybean dish. Other excellent sources include fermented foods like certain cheeses and sauerkraut, as well as some animal products like egg yolks and meat.

For most healthy individuals, a balanced diet is sufficient. However, for those with certain health conditions or inadequate intake of K2, supplementation may be considered. It is crucial to consult with a healthcare provider before starting any supplement, especially if you are on blood-thinning medication like warfarin, as vitamin K can affect its function.