This 2019 review focused on one Vitamin K-deficiency biomarker. All parts I’ve quoted are outside the liver, so Vitamin K deficiency ≈ Vitamin K₂ deficiency.

This is a hard read with many technical details, but sometimes that’s how researchers do it:

“Active MGP (matrix Gla protein), once released into extracellular space, acts as a local inhibitor of calcification. Widespread expression of MGP points to a role of MGP that by far exceeds its well-known function as local inhibitor of calcification.

Recent research confirmed this concept, usually by measuring plasma dp-ucMGP (desphospho-uncarboxylated MGP), a biomarker reflecting poor vitamin K status:

Vitamin K plays a pivotal role in maintaining bone health. Increasing evidence also implicates MGP in maintaining bone health.

In the Health, Aging and Body Composition study, 791 older community-dwelling adults underwent magnetic resonance imaging to measure bilateral knee structural features. The highest [25%] compared with the lowest fourth of the dp-ucMGP distribution had higher odds of having:

• Meniscus damage;
• Osteophytes;
• Bone marrow lesions; and
• Subarticular cysts.

Regarding Vitamin K supplementation:

• Studies showed a dose-dependent decrease in circulating dp-ucMGP with an 86% decrease already observed after 4 weeks of substitution by 360 μg menaquinone-7 [in 50 hemodialysis patients];
• In a randomized double-blind trial of 244 postmenopausal women followed up for 3 years, arterial stiffness as captured by aortic pulse wave velocity or stiffness index β, decreased in intervention compared with control group.

These results should be considered as hypothesis-generating in view of small sample size, and because there were no between-group differences in vitamin K–induced changes in elastic properties of the carotid artery.

Plasma dp-ucMGP levels ranging from 1.4 to 4.6 μg/L were optimal in terms of risk of mortality and macrovascular cardiovascular illness (4.6 μg/L threshold corresponding to the 65th percentile of dp-ucMGP distribution).

Vitamin K supplementation before irreversible organ damage sets in might find its application in prevention of a wide range of disabling diseases. Circulating dp-ucMGP levels might be measured over time to track risk of vascular complications.”

https://www.ahajournals.org/doi/10.1161/HYPERTENSIONAHA.119.12412 “Vitamin K–Dependent Matrix Gla Protein as Multifaceted Protector of Vascular and Tissue Integrity”

I usually don’t give 5+ stars to reviews. This one was different.

Yes, there could be factors other than this one Vitamin K deficiency biomarker involved in study findings. Sure, these coauthors cited their own studies. Its overall purpose, though, was to inform readers.

I’ll summarize this paper as providing evidence for a biomarker of Vitamin K₂ deficiency being implicated in the development and progression of many diseases.