Vitamin K
Characteristics: "Vitamin K" is a general name for a group of fat-soluble compounds with a common chemical structure. These compounds include phylloquinone (vitamin K1) and a variety of menaquinones (vitamin K2). Menaquinones have unsaturated isoprenyl side chains and are designated as MK-4 to MK-13 based on their length. The best researched menaquinones are MK-4, MK-7 and MK-9. Phylloquinone is present primarily in green leafy vegetables and is the main dietary form of vitamin K. Menaquinones are present in small amounts in a variety of animal and fermented foods. But almost all menaquinones, especially long-chain menaquinones, are produced by bacteria in the human gut. MK-4 is unique in that it is produced by the body from phylloquinone through a conversion process that does not involve bacterial action. Vitamin K plays its irreplaceable role in blood clotting, bone mineralization and vascular protein metabolism.

Absorption: Like dietary lipids and other fat-soluble vitamins, ingested vitamin K is incorporated into micelles by the action of bile and pancreatic enzymes, absorbed by the enterocytes of the small intestine and further transported throughout the body. It is present in the liver and other body tissues, including the brain, heart, pancreas, and bones. In the circulation, vitamin K is carried mainly in lipoproteins. Compared to other fat-soluble vitamins, a very small amount of vitamin K circulates in the blood (30‒40% of the oral dose), because it is metabolized and excreted very quickly.

Dietary supplements: Several forms of vitamin K are used in dietary supplements, including vitamin K1 phylloquinone or phytonadione (a synthetic form of vitamin K1) and vitamin K2 as MK-4 or MK-7. Very little data is available on their relative bioavailability. However, one study found that both phytonadione and MK-7 are well absorbed, but MK-7 has a longer elimination half-life (remains in the body longer). Menadione, sometimes referred to as vitamin K3, is another synthetic form of vitamin K. In laboratory studies conducted in the 1980s and 1990s, it was shown to damage liver cells, so it is no longer used in dietary supplements or fortified foods.

Natural sources: Food sources of phylloquinone (K1) include vegetables, especially green leafy vegetables, vegetable oils and some fruits. Meat, dairy products, and eggs contain low levels of phylloquinone, but at least small amounts of menaquinones (K2). Menaquinones are also found in fermented foods such as cheeses or soybeans. The forms and amounts of vitamin K in these foods vary depending on the bacterial strains used to produce the foods and the conditions of their fermentation. Animals synthesize MK-4 from menadione (a synthetic form of vitamin K that can be used in poultry and pig feed). Thus, poultry and pork products contain MK-4 if menadione is added to animal feed. In general, the most common sources of vitamin K are spinach, broccoli, iceberg lettuce, fats and oils. Data on the bioavailability of the various forms of vitamin K from food are very limited. The rate of absorption of phylloquinone in free form is approximately 80%, but the rate of absorption from food is significantly lower. Phylloquinone in plant foods is tightly bound to chloroplasts, making it less bioavailable than that from oils or supplements. For example, the body absorbs only 4‒17% of phylloquinone from spinach compared to tablets. Eating vegetables with some fat improves the absorption of phylloquinone from vegetables, but the amount absorbed is still lower than from oils. Almost all menaquinones, especially long-chain menaquinones, are produced by bacteria in the human gut.
Effect: Vitamin K acts as a coenzyme for vitamin K-dependent carboxylase, an enzyme necessary for the synthesis of proteins involved in hemostasis (blood clotting) and bone metabolism. The protein prothrombin (clotting factor II), which is directly involved in blood clotting, is also dependent on vitamin K. The vitamin K-dependent matrix Gla-protein, present in vascular smooth muscle, bone, and cartilage, can help reduce abnormal calcification (deposition calcium). Osteocalcin is another vitamin K-dependent protein that is present in bone and may be involved in bone mineralization or turnover.

Deficiency: Achieving a low vitamin K intake is nearly impossible in healthy people who eat a varied diet. Bleeding is a classic symptom of vitamin K deficiency. Vitamin K deficiency reduces bone mineralization and thus contributes to osteoporosis. It may occur during the first few weeks of infancy due to low placental transfer of phylloquinone, low levels of clotting factors, and low vitamin K content in breast milk.

Recommended daily dose: adult: 120 µg for men, 90 µg for women (90 µg during pregnancy, 90 µg during breastfeeding).

Adverse Effects: There are no reported adverse effects associated with the consumption of vitamin K from food or dietary supplements in humans or animals.

Interactions: Vitamin K can have a serious and potentially dangerous interaction with anticoagulants (blood-clotting drugs) such as warfarin. These drugs antagonize (oppose) the activity of vitamin K, leading to a deficiency of vitamin K-dependent clotting factors. People taking warfarin and similar anticoagulants must maintain a consistent intake of vitamin K from food and supplements because sudden changes in vitamin K intake can increase or reduce the anticoagulant effect (bleeding or, conversely, the formation of a blood clot may occur). Antibiotics can kill vitamin K-producing bacteria in the gut, potentially reducing the amount of vitamin K. This effect may be more pronounced with cephalosporin antibiotics because these antibiotics can also inhibit the action of vitamin K in the body. Vitamin K supplements are usually not needed unless antibiotic use is long-term (more than a few weeks) and accompanied by poor vitamin K intake. Diet drugs, whether over the counter or prescription, can reduce the absorption of dietary fat and thus can also reduce the absorption of vitamins fat-soluble, such as vitamin K.

Pregnancy: Safe in usual doses.

Breastfeeding: Safe in usual doses.

Toxicity: Vitamin K toxicity is extremely rare. The only reported toxicity comes from menadione, which is not used in humans. If toxicity occurs, symptoms of jaundice, hyperbilirubinemia, and hemolytic anemia occur.