Vitamin K: Wallflower Sister of the Famous Fat Solubles
Vitamin K is the least glamorous of the fat soluble vitamins, waiting in the wings while her sisters take repeated bows on stage for their disease-fighting performances. However, the name gives some indication of its importance; "K" for the Danish word, koagulation, or for Americans, coagulation, its primary function in blood. And if you're unfortunate enough to be bleeding, this is one vitamin you don't want to be missing because it helps your blood to clot to stop the bleeding. So while there doesn't seem to be any intriguing studies linking the vitamin to disease prevention as there are for its fat soluble relatives, our need for it is critical.
Vitamin K was discovered in 1935 by the Danish scientist Dam who recognized it as a compound present in green leaves which prevented hemorrhaging in animals fed a low-fat diet. He later isolated the vitamin from alfalfa in 1939. Vitamin K occurs in nature in two groups of compounds: phylloquinone and menaquinone. A third group is related to the synthetic compound menadione, which is twice as biologically active as the two natural forms. The two natural groups are produced by many bacteria, and fortunately for us humans, the bacteria living in our intestinal tract possess this talent.
Vitamin K's blood-clotting ability is related to its role in the formation of prothrombin and five other proteins that play key roles in the complex process of coagulation. The vitamin is also needed to make proteins present in plasma, bone, and the kidney. Interestingly, when a person becomes deficient in vitamin K, other compounds fill in for these functions. It is the blood-clotting ability that suffers from a deficiency of this vitamin.
The process of blood coagulation involves over forty different substances, some of which promote clotting while others inhibit it. It's the balance of these opposing forces that determines whether blood will clot, and in the normal state, the anticoagulant compounds run the show-a good thing, too, since you'll recall from the discussion on heart attacks, clot formation in the blood is a triggering event. Clotting appears to occur in three stages, which include the formation of prothrombin activator, conversion of prothrombin into thrombin, and conversion of fibrinogen to fibrin threads. Scientists call the process a cascade, much like a large snowball rolling down a hill picking up speed and more snow as it goes along unchecked.
The first step starts with some kind of injury, like the blowout of a blood vessel, which triggers the formation of prothrombin activator. Prothrombin is a protein made by the liver and is normally present in blood. The prothrombin activator, in the presence of calcium, converts prothrombin into thrombin, which acts as an enzyme. The next step involves thrombin acting on another blood protein made by the liver, fibrinogen, to convert it into fibrin. The fibrin compounds form long fibrin threads that form the framework of the clot, like a spider's web, which continues to develop by trapping platelets, blood cells, and plasma.
The threads also stick to the damaged area and the resulting clot prevents blood from flowing past it. Clot formation, or thrombosis, is critical in the case of injury to prevent blood loss. However, this protective mechanism is also responsible for heart attack and stroke when the clot formed after injury to a blood vessel prevents blood from flowing to the heart or brain.
Several compounds, called antagonists, are known to interfere with vitamin K activity because their structure is similar to that of the vitamin. The antagonists compete directly with vitamin K at the site where the vitamin exerts its important effects. Because they don't have the same biological activity, these compounds prevent coagulation, and doctors use several of these for this very purpose, as effective anticoagulants. The first antagonist researchers identified was a substance found in spoiled sweet clover, dicumaral, which is one of many compounds that work against vitamin K. Relatives of dicumaral include warfarin and tromexan, which doctors use to treat blood clots or to thin the blood of people who are at risk for blood clots.
Our Need for Vitamin K
The RDA for vitamin K is 80 and 65 micrograms for adult males and females, respectively. The levels were established based on studies that assessed the amount required to maintain plasma concentrations of prothrombin in the normal range. Food composition tables don't include vitamin K content because scientists haven't yet come up with precise techniques of analyzing foods for the vitamin. The best dietary sources are leafy greens, providing anywhere from 50 to 800 micrograms per 100 grams. Many other foods contain the vitamin, such as dairy products, meats, grains, fruits, and vegetables, but the amount is small at 1 to 50 micrograms for every 100 grams of food.
One of the most important sources of vitamin K comes from our intestinal bacteria, those friendly little organisms that live in the intestine. By itself, this source can't meet the body's total need for the vitamin; experts aren't sure how much is contributed, but some estimate that up to 30 percent of our vitamin K is from this source. Studies which have restricted dietary vitamin K produced alterations in clotting factors. Likewise, people who are on antibiotic therapy for extended periods may become deficient, because in addition to destroying disease-causing bacteria, the drugs also kill the "good" bacteria which produce vitamin K. Studies have shown that even a regimen as brief as five weeks resulted in a significant drop in plasma prothrombin levels, to about 70 percent of normal.
Elderly people in the hospital may be at risk for vitamin K deficiency. Studies have shown that in one such group, 75 percent of patients had low plasma prothrombin levels, which improved after vitamin K treatment. Scientists think that chronic diseases, drug therapy, and poor diet may make the problem worse in people who are ill and weak. In contrast to other vitamins, a recent study proves that the problem isn't related to the effects of aging on vitamin status. In trying to find the best biochemical marker for evaluating vitamin K status, researchers reported that aging didn't much affect it.
Other high-risk groups include people who have suffered trauma, kidney disease, any disease that causes fat to not be absorbed, and those who are physically debilitated. Other compounds besides antibiotics can interfere with vitamin K. One class of drugs which destroys vitamin K are the sulfas, and chronic use of these drugs is common in people with ulcerative colitis. Even other vitamins can pose problems, with studies dating back to the 1940s showing that megadoses of vitamins A and E could antagonize vitamin K.
Newborn infants get an injection of vitamin K, because they're at risk for vitamin K deficiency for a few interesting reasons. Babies are born with sterile intestinal tracts, free of vitamin K-producing bacteria. It would take up to several weeks for the bacteria to start eking out a living in the intestine. In addition, newborns have low prothrombin levels, which protect them against fatal blood clotting during the stress of birth. But these two facts could conspire against the little tykes in the event of an injury, if they didn't receive that gut-priming dose of the vitamin. There has been some concern that the vitamin K injection newborns receive might be linked to the later devel!Jpment of leukemia. A recent study evaluated hospital records and was unable to clear the vitamin or implicate it, so the jury is still out.
Vitamin K is a fat soluble vitamin, which means that excesses are not excreted too easily, but even large doses given over a long period of time aren't toxic for most people. Two exceptions are infants and pregnant women, who appear to be more sensitive. High doses of the vitamin can cause problems for people who take anticoagulant drugs to prevent clots. In that situation, vitamin K antagonizes the drug and makes it less effective at clot prevention. If you take anticoagulants, you should avoid eating too much of the foods that are high in the vitamin, most notably the nutrient-packed green leafies.
The symptoms of vitamin K overdose include jaundice, or yellowing of the skin and whites of the eyes, destruction of red blood cells, and even brain damage. Some studies have shown that high amounts of the synthetic form, menadione, can destroy red blood cells, causing a disease called hemolytic anemia. However, the form which occurs naturally in foods, phylloquinone, doesn't appear to be toxic.
Compared to what they've done for the reputations of other fat soluble vitamins, researchers haven't uncovered any exciting connections between this humble vitamin and the chronic diseases which plague mankind. But if you "spring a leak." vitamin K will be the most important thing to you aside from a good doctor. As with many of the other vitamins, scientists have yet to figure out every aspect of vitamin K's role in the body, so anything is possible.