Vitamin K is unique among the vitamins in several respects. It is the only vitamin that can be produced within the human body, but not by the body (to be defined as a vitamin, a substance cannot be produced by human tissue).   Beneficial bacteria in the human intestine produce about 75% of the vitamin K the body absorbs each day, with the other 25% coming from dietary sources. The amount of vitamin K absorbed each day from both sources usually is equal to the minimum amount required for normal bodily function.

Vitamin K is a fat soluble vitamin, but unlike the other fat-soluble vitamins, however, vitamin K is not stored in the body. Taken together, these factors explain why the net daily balance of vitamin K is so delicate. As people live longer and vitamin K-dependent processes are discovered in more and more tissues, more scientists are suggesting that vitamin K is needed in larger quantities than what was once thought, particularly in aging adults.

 Regulating Blood Coagulation

 Vitamin K activates many of the molecules that are essential to coagulation (pro-coagulants).  When triggered by a stimulus, these proteins work together to create the dense mesh of fibrin that traps platelets and stanches the flow of blood and creates a clot.

 Effects on Bone Mineralization

 Adequate intake or supplementation with vitamin D and calcium is required to prevent osteoporosis.  Neither vitamin D nor calcium, however, can produce healthy bone mineralization without adequate supplies of vitamin K. Bone is a complex living structure comprising cells, mineral crystals, and thick matrix proteins that, like glue, hold the entire bone together. The chief bone matrix protein, osteocalcin, is a protein that is dependent on vitamin K for its production. A deficiency of vitamin K causes impaired activation of osteocalcin and reduced activity of bone-forming cells, thereby resulting in decreased new bone formation. 

 Critical Role in Vascular Health

 Scientists are continuing to learn more about the process by which atherosclerotic arteries become calcified. Calcification is now recognized not merely as an accumulation of calcium similar to build-up inside a pipe, but as an active biological process virtually identical to bone mineralization. Crucial to both processes, vitamin K produces opposite effects in bone and blood vessels: matrix proteins in bone increase mineralization when activated by vitamin K, while similar proteins in blood vessel walls decrease vascular calcification. Both actions are healthy responses that maintain strong bone and supple blood vessels.   The simultaneous loss of calcium from bone and deposition of calcium in arteries has been called the “calcification paradox.”

 Deficiency

Overt vitamin K deficiency results in impaired blood clotting, usually demonstrated by laboratory tests that measure clotting time. Symptoms include easy bruising and bleeding that may be manifested as nosebleeds, bleeding gums, blood in the urine, blood in the stool, tarry black stools, or extremely heavy menstrual bleeding. In infants, vitamin K deficiency may result in life-threatening bleeding within the skull (intracranial hemorrhage) and newborn infants are often given Vitamin K.   Vitamin K deficiency is uncommon in healthy adults for a number of reasons: 1) vitamin K is widespread in foods  2) the vitamin K cycle conserves vitamin K; and 3) bacteria that normally inhabit the large intestine synthesize menaquinones (vitamin K₂), although it is unclear whether significant amounts are absorbed and utilized.  Adults at risk of vitamin K deficiency include those taking vitamin K antagonist anticoagulant drugs such as Coumadin (Warfarin) and individuals with significant liver damage or disease.