Vitamin K

Vitamin K is a group of molecules known as 2 methyl-1,4-naphthoquinones.¹ Phylloquinone is the plant version of vitamin K, and bacteria produce vitamin K known as menaquinones.¹ Menadione is a synthetic version of vitamin K.¹

Absorption and Circulation: All forms of vitamin K enter into chylomicrons within the absorptive cells of the small intestine, entering lymph and then systemic circulation.¹ Once in hepatocytes, vitamin K in chylomicron remnants is transported into VLDL once metabolized. Menaquinones are metabolized and transported out of the liver slower than phylloquinone.

Functions:  

Vitamin K is known for its function in blood clotting, a series of activating reactions that trigger the formation of fibrin to stop hemorrhaging by using clotting factors. Clotting factors IX, VII, X, and II (prothrombin used to activate fibrin) are dependent on vitamin K for both intrinsic and extrinsic pathways for clotting.¹ Proteins involved in blood clotting, known as proteins C, M, S, and Z, are also vitamin K dependent.¹ Other than its role in blood clotting, vitamin K is found in or used for:

• Bone tissue and kidney tissue contain vitamin K-dependent proteins: bone Gla protein and matrix Gla protein in bone, and kidney Gla protein in the kidneys.¹ Gla refers to glutamic acid whose carboxylation reactions use vitamin K as a coenzyme for enzymes, such as vitamin K carboxylase.¹ These proteins are found in bone, cartilage, and teeth.¹ Matrix Gla protein is associated with inhibition of vascular calcification.¹

• A vitamin D and K-dependent protein hormone, known as osteocalcin, is found within bone and dentin in teeth.¹

• In the liver, Vitamin K carboxylase is responsible for the carboxylation reactions of glutamic acid residues of many proteins.¹ This is done through the vitamin K cycle.

• The vitamin K cycle is when hydroquinone (reduced vitamin K) is oxidized into vitamin K 2,3-epoxide using vitamin K carboxylase.¹ This allows for the carboxylation of proteins with glutamic acid residues and results in the synthesis of new proteins such as prothrombin.¹ To regenerate, vitamin K 2,3 epoxide is then reduced into quinone form of vitamin K by epoxide reductase  (blocked by anticoagulant medications). The enzyme quinone reductase reduces quinone into hydroquinone to complete the cycle.¹ 

Dietary Reference Intake (DRI) - The adequate intake of vitamin K for individuals 14-18 years is 75 micrograms/day.² For women 19 years old and older, the recommended AI is 90 micrograms.² For men 19 years old and older, 120 micrograms is recommended.²

Deficiency - Vitamin K deficiency is uncommon but can occur in those with absorptive disorders or regular antibiotic use since it may reduce bacteria producing menaquinones. 

Toxicity - Vitamin K toxicity is uncommon naturally but can occur with excessive supplement intake.

Dietary Sources of Vitamin K

Vitamin K is found abundantly in fruits and vegetables but is also found in grains, meat, and dairy. Phylloquinones are found in leafy vegetables and oils while more menaquinones are found in meat, dairy, egg, and fermented foods.² Examples of vitamin K-containing foods include:

Oils/Fats/Dairy - soybean oil, canola oil, olive oil

Nuts/Seeds - pine nuts, cashews, mixed nuts 

Vegetables - collard and turnip greens, spinach, kale, broccoli 

Fruits - blueberries, grapes, figs 

Meat/animal products- chicken, ground beef, ham, milk, cheese, egg

Source(s):

1. Denise M Medeiros and Robert E.C. Wildman, Advanced Human Nutrition, 4th ed. (Burlington, MA: Jones & Bartlett, 2019).

2. https://ods.od.nih.gov/factsheets/VitaminK-HealthProfessional/