Short answer: Vitamin K is a family of fat-soluble compounds with three principal classes. K1 (phylloquinone) is the plant-source form found in green leafy vegetables. K2 (menaquinones) is a group of compounds with side-chain lengths designated MK-4 through MK-13 — MK-4 from animal-source ingredients, MK-7 to MK-13 from microbial fermentation per Shearer 2008 review. K3 (menadione) is the synthetic form used as the standard pet-food supplement, typically delivered as menadione sodium bisulfite complex (MSBC) or menadione nicotinamide bisulfite. Per AAFCO 2024 expert panel 2021 menadione safety review, K3 is the AAFCO-affirmed standard supplement form. Per Booth 2012 (Nutr Rev), MK-7 has longer plasma half-life than MK-4 (approximately 3 days vs 1–2 hours) with implications for bone-matrix Gla-protein research. The KibbleIQ rubric recognizes K1, K2 menaquinones, and K3 menadione as AAFCO-compliant vitamin K sources.

The 3-class family — K1, K2, K3

Per Shearer 2008 (J Lipid Res) vitamin K review and the AAFCO 2024 ingredient definitions, the vitamin K family shares a 2-methyl-1,4-naphthoquinone head group with side-chain variants. K1 (phylloquinone): single isoprenoid side chain similar to alpha-tocopherol; the dominant form in plant-source dietary ingredients (alfalfa, spinach, kale, broccoli). K2 (menaquinones): multi-isoprenoid side chain of variable length, designated MK-n where n is the number of isoprene units. MK-4 is the dominant K2 form in animal-source ingredients (liver, egg yolk, fermented dairy) and is the form generated by tissue conversion from K1 and K3 per Okano 2008 (J Biol Chem). MK-7 through MK-13 are produced by microbial fermentation in the colon and in certain fermented foods (natto produces MK-7; some cheeses produce MK-8 / MK-9). K3 (menadione): synthetic 2-methyl-1,4-naphthoquinone without side chain, the precursor that tissues convert to MK-4 in vivo.

Per NRC 2006 Nutrient Requirements of Dogs and Cats, the canine vitamin K requirement is approximately 1 mg/kg DM, typically met by a combination of background gut-microbial menaquinone synthesis, dietary K1 from plant-source ingredients, and supplemental K3 in commercial formulations. AAFCO 2024 does not list a separate vitamin K minimum because the multi-source supply is generally adequate; however, AAFCO does recognize menadione as a permitted supplement and defines its safe use under the 2021 expert panel review.

Gamma-carboxylation — the vitamin K mechanism

Per Suttie 1985 (Annu Rev Biochem) vitamin K biochemistry review and standard biochemistry references, vitamin K functions as a cofactor for the gamma-glutamyl carboxylase enzyme, which catalyzes the post-translational gamma-carboxylation of specific glutamate residues in vitamin K-dependent proteins. The gamma-carboxylation reaction adds a carboxyl group to the glutamate gamma-carbon, generating gamma-carboxyglutamate (Gla) residues that confer calcium-binding capacity. The reaction couples vitamin K hydroquinone oxidation to vitamin K 2,3-epoxide; vitamin K epoxide reductase (VKOR) then regenerates the reduced cofactor, completing the vitamin K cycle.

The vitamin K cycle is the basis for the anticoagulant mechanism of warfarin and the warfarin-class rodenticides (brodifacoum, bromadiolone): these compounds inhibit VKOR, depleting reduced vitamin K and impairing coagulation factor gamma-carboxylation. Per AAHA 2022 toxicology references, rodenticide ingestion is a common canine emergency; treatment is vitamin K1 (phylloquinone) at 2.5–5 mg/kg/day orally for 4–6 weeks depending on the rodenticide pharmacokinetics. The clinical relevance: vitamin K supplementation is therapeutically meaningful in rodenticide toxicity, not commonly nutritionally relevant in healthy dogs on AAFCO-compliant diet.

The menadione (K3) safety controversy — resolved by AAFCO 2024

Per AAFCO 2024 expert panel 2021 menadione safety review and FDA 21 CFR 573.620 affirmation, menadione sodium bisulfite complex (MSBC) and menadione nicotinamide bisulfite (MNB) are AAFCO-affirmed safe for use in dog food at typical formulation concentrations. The expert panel formally addressed the historical safety controversy that had been driving consumer concern. The historical concern: 1960s–1980s reports of hemolysis and kernicterus in human neonates given supraphysiological injectable menadione doses raised toxicity questions; the case-series data does not generalize to oral menadione at AAFCO-typical concentrations in adult dogs.

Per the AAFCO 2024 expert review, oral menadione is rapidly converted to MK-4 menaquinone in the intestinal mucosa and liver, bypassing the toxicity mechanism associated with parenteral high-dose injection. AAFCO-typical inclusion is well below toxic thresholds, and decades of commercial use in dog food have not produced clinical adverse-effect signals. The marketing controversy persists in consumer-facing discussion but is not supported by the regulatory or veterinary nutrition consensus. See our menadione explainer for additional detail on the K3 supplement form.

MK-4 vs MK-7 — pharmacokinetics and bone-matrix research

Per Booth 2012 (Nutr Rev) menaquinone review and Schurgers 2007 (Blood) comparative bioavailability data, MK-4 and MK-7 differ substantially in pharmacokinetics. MK-4 has a plasma half-life of approximately 1–2 hours; MK-7 has a plasma half-life of approximately 3 days. The longer half-life of MK-7 produces more sustained plasma concentrations and more efficient gamma-carboxylation of slow-turnover proteins such as osteocalcin (bone matrix) and matrix Gla protein (vascular calcification regulation).

The bone-matrix research significance: per Booth 2012 and the Inoue 2014 (Asia Pac J Clin Nutr) human osteoporosis review, MK-7 supplementation has demonstrated effects on osteocalcin gamma-carboxylation status that K1 supplementation does not consistently replicate. The clinical relevance in dogs is less established; AAFCO 2024 does not differentiate vitamin K source for bone-health claims, and commercial dog food typically delivers K3-derived MK-4 rather than direct MK-7. Bone-health-positioned formulations may include K1 or K2 alongside calcium, vitamin D, and skeletal-mineral support; the K2 contribution is structural in the formulation narrative rather than dose-dependent at typical commercial levels.

How KibbleIQ scores vitamin K

The KibbleIQ Dry Kibble Rubric recognizes vitamin K1 (phylloquinone), K2 (menaquinones MK-4 / MK-7 etc.), and K3 (menadione sodium bisulfite complex / menadione nicotinamide bisulfite) as AAFCO-compliant vitamin K sources. The rubric does not penalize menadione (K3) presence because AAFCO 2024 expert panel 2021 affirmed K3 safety at typical formulation concentrations; the historical marketing controversy is not aligned with the regulatory and veterinary nutrition consensus.

The rubric does not currently differentiate K2 menaquinone source for bone-health credit because the canine bone-health evidence for MK-7-vs-MK-4 differentiation is less established than the human-medicine evidence base per Booth 2012. Skeletal-support and senior formulations carrying vitamin K alongside calcium, vitamin D, glucosamine + chondroitin (joint support), and omega-3 EPA + DHA earn the broader joint-and-bone-support tier credit. See our menadione explainer (the K3 sister page) and mixed tocopherols explainer (the vitamin E sister page). To check your dog’s food, paste the ingredient list into the KibbleIQ analyzer.