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.40,41 Crucial to both processes, vitamin K produces opposite effects in bone and blood vessels: matrix Gla-proteins in bone increase mineralization when activated by vitamin K, while similar proteins in blood vessel walls decrease vascular calcification.42 Both actions are healthy responses that maintain strong bone and supple blood vessels.
Inflammatory change in the blood vessel wall, and an increase in the size and number of smooth muscle cells, play a major role in atherosclerosis.43,44 Vitamin K-dependent matrix Gla-proteins holding vascular cells and other components together have proven to be vital in preventing calcification following an initial inflammatory injury.45
The simultaneous loss of calcium from bone and deposition of calcium in arteries has been called the “calcification paradox.”37 This double-jeopardy situation occurs frequently in postmenopausal women, apparently in part because of incomplete formation of the crucial Gla-proteins that normally both increase bone calcification and prevent arterial wall calcification.
Scientists are greatly interested in vitamin K’s role as a possible anti-atherosclerosis or anti-calcification agent.30 In an observational study of dietary vitamin K in coronary heart disease, researchers in the Rotterdam Study found that subjects with above-average intake of vitamin K2 had reduced mortality from coronary heart disease, as well as lower all-cause mortality.46 In addition, the likelihood of having severe aortic calcification was lower in subjects with higher K2 intake. The authors suggest that an adequate intake of vitamin K2 could be important in preventing coronary heart disease. Vitamin K intake earlier in life does not seem to be associated with premature coronary calcification, suggesting that vitamin K’s effects on blood vessels become more important with advancing age.47
Deficiencies in Intake and Production
Unlike the other fat-soluble vitamins A, D, and E, which can be stored in body fat, vitamin K has no significant storage pool in the body. Humans can therefore develop a vitamin K deficiency in as few as 7-10 days of a vitamin K-deficient diet, even with functioning intestinal bacteria.9
Despite the fact that older adults typically consume more dietary vitamin K than younger adults,21 elderly people may suffer from low levels of vitamin K. In one study, 12% of patients at an older adult outpatient clinic had very low plasma levels of vitamin K.48
Newborn infants represent the largest population that regularly suffers from bleeding due to overt vitamin K deficiency. Because newborns do not have any bacteria in their intestines at birth, for several days they are vulnerable to bleeding until they develop a normal intestinal population. For these reasons, the American Academy of Pediatrics recommends that all newborns receive an intramuscular dose of vitamin K at birth.49 Babies who are exclusively breast fed may require additional doses of vitamin K to support them until they develop healthy intestinal flora.50
A report of two cases of post-operative bleeding in non-infants (a 73-year-old and a 6-year-old) highlights the complex balance of vitamin K supply and demand.51 While normal gut bacteria almost always ensure ample vitamin K to prevent clotting problems, these two patients had undergone long courses of antibiotics that may have destroyed those beneficial bacteria. This internal deficiency state, added to high utilization of vitamin K-dependent pro-coagulant proteins in the post-operative period, was thought to account for the bleeding. When the patients were given vitamin K, coagulation immediately returned to normal. Such cases are mercifully rare, but they serve as additional evidence of the narrow window between vitamin K sufficiency and deficiency states.
The use of broad-spectrum anti-biotics to treat serious infections is known to produce vitamin K deficiency that can be severe enough to cause bleeding.52 Patients who undergo bone marrow transplants are often dependent on vitamin K supplementation, as their intestinal bacteria are intentionally obliterated in preparation for the destruction of their own bone marrow germ-fighting cells.53
Deficiencies in Absorption
Poor absorption of fats is generally a hallmark of liver and gall bladder diseases. Bile acids, produced in the liver and excreted by the gall bladder, function as “detergents” in the small intestine, aiding in the absorption of fats and the fat-soluble vitamins. For these reasons, people with obstructed bile ducts and decreased small intestinal bile acid concentrations almost always develop deficiencies of the fat-soluble vitamins A, D, E, and K.10,12 Fortunately, proper management of diet and vitamin supplementation usually prevents significant vitamin deficiencies in patients with chronic obstructive biliary diseases.13
Interior view of the liver, gallbladder, and associated vessels
Consumption of hydrogenated and saturated fats has well-known adverse effects on blood lipid profiles. The hydrogenation of fats produces a hydrogenated form of vitamin K called dihydrophylloquinone, which is not absorbed as well as unsaturated (normal) vitamin K and also appears to be ineffective in promoting bone production.54 These data further support avoiding foods that contain saturated fat and supplementing with vitamin K when a diet high in saturated fat cannot be avoided.
Vitamin K Antagonist Drugs
The most important form of vitamin K “deficiency” in adults is not related to intake or absorption, but rather to the use of vitamin K antagonists such as warfarin (Coumadin®).55,56 Vitamin K antagonists prevent vitamin K from activating Gla-proteins in the coagulation cascade. Warfarin is a commonly used drug among older patients, and has been approved for many conditions requiring short- and long-term anticoagulation.57 Long-term anticoagulation, however, carries certain risks, such as bleeding complications. The likelihood of these complications is greater in patients with preexisting vitamin K deficiency.
It takes several days after beginning warfarin therapy to achieve the desired antithrombotic effect.58 Almost all patients subsequently experience periods of fluctuation in their international normalized ratio (INR), a measure of the blood’s ability to clot properly. This sometimes leads to bleeding or clotting complications. Older patients generally tend to be more sensitive to warfarin’s effects, both near the beginning of treatment when coagulation status may fluctuate widely, and later during maintenance treatment.58 Genetic differences in sensitivity to warfarin account for some of the fluctuations as well. For example, Asian-Americans tend to require much lower doses of warfarin to achieve anticoagulation than do African-Americans.59,60
For people using warfarin, maintaining normal INR values is usually adequate to prevent dangerous clots without posing the risk of serious bleeding. The risk of clinically significant bleeding rises with elevation of INR to levels above the normal range.61
When warfarin causes excessive INR elevation but no active bleeding is evident, doctors may sometimes recommend skipping one or more doses of warfarin, which will slowly restore the INR level back to normal. More rapid, safe reversal can be accomplished with small oral doses of vitamin K1.57 Oral vitamin K at a dose of 1 mg (1000 mcg) daily is effective in treating patients whose INR has become too high, even up to levels that are three times higher than the upper limit of the normal range.62 In a placebo-controlled trial among patients with elevated INR but no symptomatic bleeding, 1 mg of oral vitamin K restored INR to safe therapeutic levels faster than placebo (all patients had their warfarin doses stopped). Patients treated with vitamin K also had fewer episodes of bleeding than control patients.63 This study is one of several that demonstrate the safety and effectiveness of using low-dose vitamin K in patients still on warfarin.64-66 A recent systematic review of the literature concluded that low-dose oral vitamin K is the preferred strategy for rapidly restoring INR levels in patients who were not actively bleeding.61
Fluctuating INRs can also go below the therapeutic range, putting patients at risk for clotting complications, especially when the warfarin dose is held constant.67 In 2005, University of Texas researchers studied eight patients on warfarin treatment whose INRs had been fluctuating.68 All patients were started on daily supplementation with oral vitamin K (100 mcg/day) while their INR responses were monitored. Nearly all patients experienced diminished fluctuations while taking the supplements, with more INRs in the therapeutic range than before treatment. The patients’ total amount of time with INRs within the target range also increased. These results suggest that low-dose vitamin K may help to maintain a small reserve capacity of active coagulation factors that can smooth out the variability in INR caused by warfarin, without creating excessive coagulation.