Dietary Menaquinone-9 Supplementation Does Not Influence Bone Tissue Quality or Bone Mineral Density in Mice

Vitamin K has been implicated in skeletal health because vitamin K-dependent proteins are present in bone. While there are multiple forms of vitamin K, most research has focused on phylloquinone, which is found mainly in plant-based foods, and its metabolite menaquinone-4 (MK4). However, there are additional forms of vitamin K that are bacterially produced that appear to influence bone health but have not yet been studied extensively. Herein, we evaluated the effects of menaquinone-9 (MK9), a bacterially produced form of vitamin K on bone tissue quality and density in young mice. Four-week-old male (n=32) and female (n=32) C57BL/6 mice were supplemented with 0.06 mg/kg diet or 2.1 mg/kg diet of MK9 for 12 weeks. During week 11, a sub-group of mice (n=7/sex/group) received daily deuterium-labeled MK9 to trace its metabolic fate in bone. Liver MK4 and MK9 were significantly higher in mice fed 2.1 mg MK9/kg compared to those receiving 0.06 mg MK9/kg, regardless of sex (all p [≤] 0.017). MK4 was the only vitamin K form detected in bone, with 63-67% of skeletal MK4 in mice fed 2.1 mg MK9/kg derived from deuterium-labeled MK9. Femoral tissue strength, maximum bending moment, section modulus, and bone mineral density did not differ significantly across diet groups in either sex (all p[≥]0.083). Cross-sectional area (p=0.003) and moment of inertia (p=0.001) were lower in female mice receiving 2.1 mg MK9/kg compared to those receiving 0.06 mg MK9/kg, but no differences were found in male mice. Higher bone MK4 concentrations did not correlate with higher bone tissue quality or density. Despite dietary MK9 being a dietary precursor to MK4 in bone, dietary MK9 supplementation did not affect bone tissue quality or bone mineral density. Lay summaryMost research about vitamin K and bone health has focused on phylloquinone, the plant-based vitamin K form, and its metabolite menaquinone-4. Because interest in bacterially produced forms of vitamin K, which are abundant in the intestinal microbiome, is growing, we evaluated the effect of menaquinone-9 (a bacterially-produced form of vitamin K) on skeletal health. We supplemented mice with low and high doses of menaquinone-9 and also used stable-isotope labeled menaquinone-9 to trace its conversion to menaquinone-4 in bone. We found menaquinone-9 served as a precursor to menaquinone-4 in bone, but menaquinone-9 supplementation did not improve bone health.