Vitamin K2 (Menaquinone) Encyclopedia — Scientific Background

What Is Vitamin K2?

Vitamin K2 (CAS 11032-49-8), also known as menaquinone, is a series of prenylated naphthoquinones sharing a common 2-methyl-1,4-naphthoquinone (menadione) structure, characterized by a variable-length isoprenoid side chain at the 3-position designated by the number of isoprenoid units (MK-n). The compound is chemically characterized by the general formula C₁₆H₁₆O₂·(C₅H₈)ₙ. The most common commercially available form, menaquinone-4 (MK-4), has the molecular formula C₃₁H₄₀O₂ and a molecular weight of 444.65 g/mol.

The Vitamin K2 Subtypes

Vitamin K2 exists in multiple subtypes, with menaquinone-4 (MK-4) and menaquinone-7 (MK-7) being the most widely used commercially:

MK-4 is a short-chain menaquinone that is rapidly absorbed but has a short half-life, requiring higher doses (15–45 mg/day) and more frequent administration. MK-7 is a long-chain menaquinone primarily produced through fermentation by Bacillus subtilis natto and has a significantly longer half-life in circulation, with superior extrahepatic tissue distribution. A comparative study in rats demonstrated that vitamin K2 levels were much higher in the blood and liver of MK-7-fed rats than in MK-4-fed rats, indicating that MK-7 is superior to MK-4 as a nutritional supplement. MK-7 has a 1.5-fold better absorption and remains at high levels for a significantly longer period of time.

Physiological Functions of Vitamin K2

Vitamin K2 is critical for several physiological functions:

• Bone Health — Vitamin K2 activates osteocalcin, a vitamin K-dependent protein that binds calcium to the bone matrix. Carboxylated osteocalcin is essential for proper bone mineralization and bone mineral density. Vitamin K2 can promote bone formation and inhibit bone resorption, participating in the maintenance of bone health and demonstrating potential clinical application value in the prevention and treatment of osteoporosis.

• Cardiovascular Health — Vitamin K2 activates matrix Gla protein (MGP), a potent inhibitor of vascular calcification. Activated MGP prevents calcium deposition in arterial walls, supporting arterial elasticity and cardiovascular health. Vitamin K2 also protects mitochondrial function, improves insulin resistance, and promotes cellular autophagy.

• Muscle Function — Vitamin K-dependent proteins are involved in muscle function and Energy Metabolism. Clinical trials have evaluated vitamin K2 for muscle recovery after exercise and for managing nocturnal leg cramps.

• Calcium Metabolism Regulation — Vitamin K2 directs calcium to bone tissue and away from soft tissues, helping maintain healthy calcium homeostasis.

• Blood Coagulation — Like all vitamin K forms, vitamin K2 supports the gamma-carboxylation of coagulation factors II (prothrombin), VII, IX, and X in the liver.

Mechanisms of Action

Vitamin K Cycle and Gamma-Carboxylation

Vitamin K functions as a cofactor for the enzyme γ-glutamyl carboxylase (GGCX), which catalyzes the post-translational carboxylation of specific glutamate residues in vitamin K-dependent proteins (VKDPs) to γ-carboxyglutamate (Gla) residues. This carboxylation reaction enables these proteins to bind calcium ions and become biologically active. The vitamin K cycle regenerates the reduced form of vitamin K from the epoxide form through the action of vitamin K epoxide reductase (VKORC1), allowing continuous recycling of the vitamin.

Osteocalcin Activation and Bone Mineralization

Osteocalcin is the most abundant non-collagenous protein in bone and is synthesized by osteoblasts. In its carboxylated form (Gla-osteocalcin), it binds calcium and hydroxyapatite crystals, facilitating bone matrix mineralization. Undercarboxylated osteocalcin (ucOC) is a sensitive marker of vitamin K status. Vitamin K2 supplementation has been shown to increase the carboxylation of circulating osteocalcin, supporting bone health.

Matrix Gla Protein Activation and Vascular Protection

Matrix Gla protein (MGP) is a vitamin K-dependent protein synthesized by vascular smooth muscle cells that functions as a potent inhibitor of vascular calcification. Activated MGP (Gla-MGP) binds calcium and prevents its deposition in arterial walls. Vitamin K2 contributes to cardiovascular health by preventing arterial calcification through its role in activating Gla-proteins.

The Bioavailability Challenge

Despite its essential physiological functions, conventional oral vitamin K2 presents significant bioavailability challenges. As a highly lipophilic molecule, vitamin K2 has poor water solubility, is sensitive to light, heat, and oxygen, and is prone to degradation. Its absorption is dependent on bile salt-mediated micelle formation and is influenced by dietary fat intake and individual gastrointestinal conditions. Moreover, directly ingested vitamin K2 has low bioavailability, which severely limits its application in the food and nutraceutical industry.

These limitations have driven the development of advanced delivery systems, including liposomal encapsulation. A Liposomal vitamin K2 delivery system optimized at a phospholipid-to-vitamin K2 mass ratio of 1:4 achieved an encapsulation efficiency of 98.13% ± 1.01% with remarkable environmental stability. In vitro digestion studies demonstrated that free fatty acid release rate and bioaccessibility were approximately threefold and twofold higher, respectively, than unencapsulated vitamin K2 oil. A human clinical study reported that liquid liposomal vitamin K2 had an oral bioavailability 37 times greater than a conventional tablet formulation.

Liposomal Vitamin K2

Liposomal encapsulation of vitamin K2 provides a fundamentally different delivery mechanism from conventional forms. The phospholipid bilayer surrounds the vitamin K2 molecule, forming a protective shell that enhances stability, bioavailability, and cellular uptake. This delivery system:

• Achieves 98.13% ± 1.01% encapsulation efficiency with 127.67 ± 3.65 nm particle size

• Demonstrates remarkable stability under extreme conditions including pH 1.5–11, 500 mmol/L NaCl, and 95°C thermal treatment

• Increases free fatty acid release rate and bioaccessibility approximately threefold and twofold, respectively, compared to unencapsulated vitamin K2 oil

• Achieves an oral bioavailability 37 times greater than conventional tablet formulations in human studies

• Boosts intestinal absorption 5–10 times over free forms through endocytosis and membrane fusion

Clinical Evidence

Nocturnal Leg Cramps — Randomized Controlled Trial

A randomized clinical trial in China evaluated the effectiveness of vitamin K2 (menaquinone) for managing idiopathic nocturnal leg cramps in 200 people aged 65 or older. Participants were randomized to receive 180 μg of vitamin K2 or placebo daily for 8 weeks. The study demonstrated significant reductions in cramp frequency and severity in the vitamin K2 group.

Muscle Recovery — TAKEOVER Randomized Controlled Trial

The TAKEOVER randomized controlled trial evaluated the effects of vitamin K2 (MK-7, 240 μg/day) on recovery from muscle-damaging resistance exercise in healthy young (18–40 years) and older (65+ years) adults. Participants were randomly assigned to either vitamin K2 (240 μg/day) or placebo for 12 weeks in this double-blind randomized controlled trial. The study assessed markers of muscle damage and recovery.

Knee Osteoarthritis — Ongoing Clinical Trial

The Role of Vitamin K on Knee Osteoarthritis Outcomes (KONAKION) study is a pilot randomized clinical trial in older adults with knee OA. Participants are randomized to vitamin K1 500 μg, vitamin K1 1,000 μg, vitamin K2 (MK-7) 300 μg, or placebo. The most commonly used forms and doses in intervention studies include menaquinone-7 (MK-7) at 300 μg daily.

Cystic Fibrosis — Randomized Multi-Center Trial

A randomized multi-center trial evaluated fat-soluble vitamin supplementation using a liposomal formulation containing vitamin K2 (MK-7, 400 μg) compared with standard treatment in pancreatic-insufficient cystic fibrosis patients. The liposomal fat-soluble vitamin supplement containing vitamin K2 was superior to the standard form in delivering vitamin D3 and E in pancreatic-insufficient patients with CF.

Safety and Regulatory

Vitamin K2 has a well-established safety profile. No tolerable upper intake level (UL) has been established for vitamin K due to its low toxicity. The European Food Safety Authority concluded that vitamin K2 (menaquinone) poses no safety concern when used in foods for the general population (including food supplements), with no adverse effects observed at doses of 900 μg/day or 2,000 μg three times per week.

Regulatory Status:

• United States — Vitamin K2 (MK-7) is Generally Recognized as Safe (GRAS) for use in nutritional beverages, with multiple GRAS notifications receiving FDA "No Objection Letter" responses. MK-7 is regulated as a dietary supplement ingredient.

• European Union — Vitamin K2 (menaquinone series) is authorized as a novel food ingredient in the European Union under Commission Implementing Regulation (EU) 2017/2470, with MK-7 being a permitted source of vitamin K for nutritional purposes. The safety of Vitamin K2 (MK-7) has been assessed positively by EFSA.

• Global Standards — Vitamin K2 (MK-7) is available with Halal, Kosher, Vegan, ISO 9001, ISO 22000, and USP certifications. It is produced industrially through Bacillus subtilis fermentation, the most employed microorganism for dietary MK-7 supplement production.

Precautions: Vitamin K2 is generally well tolerated at recommended doses. However, individuals taking anticoagulant medications (e.g., warfarin, dabigatran, rivaroxaban, apixaban) should consult a healthcare professional before use, as vitamin K can antagonize the effects of these medications. Pregnant or lactating women and children should consult a healthcare professional before use. Vitamin K2 should be discontinued at least 2 weeks before surgery.

Testing protocols must adhere to international standards including USP, European Pharmacopoeia, and FAO/WHO Codex Alimentarius. Formulators should verify local regulatory requirements prior to market entry.

This product is intended for research, nutraceutical, and functional food applications. Not intended to diagnose, treat, cure, or prevent any disease.

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