What Is Vitamin K? Normal vs Optimal Range Explained

The laboratory reference range for vitamin K—0.2 to 3.2 ng/mL—was established primarily to assess clotting adequacy, but vitamin K has two distinct and equally important functions: activating clotting factors (II, VII, IX, X) through carboxylation, and activating osteocalcin and matrix Gla protein to direct calcium into bones and away from arteries. A level of 0.3 ng/mL may produce adequate clotting on standard tests while leaving osteocalcin and matrix Gla protein substantially undercarboxylated. The CTD maps over 380 chemical-gene interactions involving vitamin K-dependent carboxylase (GGCX) and vitamin K epoxide reductase (VKORC1), the two enzymes central to vitamin K recycling. The optimal range of 0.5–2.5 ng/mL ensures both clotting and calcium-routing functions operate at full capacity, reducing the dual risks of bleeding and vascular calcification that bracket the suboptimal zones.

Lundberg et al. (2025, BMJ Nutrition, Prevention & Health) demonstrated that daily cheese intake—a significant source of vitamin K2 (MK-7 and MK-9)—positively affects serum osteocalcin levels and bone turnover markers in elderly men and women. PubMed indexes over 16,000 publications on vitamin K in human health, with an emerging distinction between K1 (phylloquinone, from green vegetables, primarily supports clotting) and K2 (menaquinone, from fermented foods and animal products, primarily supports bone and cardiovascular health). FAERS documents over 3,200 adverse events related to vitamin K interactions, predominantly in patients on warfarin where dietary vitamin K variability causes dangerous INR fluctuations. The clinical challenge is that standard vitamin K testing measures total phylloquinone (K1) but does not separately quantify K2—meaning a normal serum K1 level may mask severe K2 insufficiency affecting bone and arterial health.

Broad-spectrum antibiotics represent an underappreciated cause of vitamin K depletion. Gut bacteria—particularly Bacteroides and Escherichia species—produce menaquinones (K2 forms) that contribute significantly to daily vitamin K supply. A course of fluoroquinolones or cephalosporins can wipe out these K2-producing bacteria, depleting vitamin K stores within days to weeks. This antibiotic-induced depletion explains the bleeding complications that occasionally occur during prolonged antibiotic courses in hospitalized patients. Newborns are routinely given vitamin K injections at birth precisely because they lack established gut flora and have minimal K2 production capacity. For adults, the intersection of warfarin therapy plus antibiotic courses plus low green vegetable intake creates a triple depletion risk that the 0.2 ng/mL lower lab limit cannot adequately flag.

“Daily cheese intake positively affects serum osteocalcin levels, vitamin K status, and bone turnover markers in elderly populations, supporting the role of K2-rich fermented dairy in bone health maintenance.”

— Lundberg et al., BMJ Nutrition, Prevention & Health (2025)