Vitamin K and bone health

Townsend Letter for Doctors and Patients,  April, 2005  by Gina L. Nick

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For many years after its discovery in 1935, vitamin K was recognized only as a necessary cofactor in the production of prothrombin, a precursor to thrombin, which is a necessary component of the coagulation cascade. A synthetic form of vitamin K was subsequently created by the pharmaceutical industry and earned FDA approval for treatment of coagulation disorders due to vitamin K deficiency or interference with vitamin K activity, as occurs in certain bowel disorders. This preparation has caused severe and sometimes fatal anaphylactoid reactions when administered intravenously and is not indicated for preventive use.

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Recent research has identified another and equally important role for vitamin K, which is to contribute substantially to the integrity of bones. Knowledge of this function is not sufficiently advanced to recommend pharmaceutical treatment of bone disorders such as osteoporosis with vitamin K, but there is no doubt as to its significance. However, prevention with natural forms of vitamin K will contribute to bone health and forestall the development of this type of bone disease. Natural forms of the vitamin are plentiful in common foods, are completely safe, and are therefore available as preventives, so common sense would dictate the wider use of these natural preparations to decrease the incidence of deficiency disorders and diminish the need for treatment with pharmaceutical agents with deleterious side effects.

Vitamin K

A Danish scientist working with hemorrhagic disease of newly hatched chickens first discovered Vitamin K in 1935. The "K" comes from the Danish spelling of "koagulation." (1) There are three forms of vitamin K, about which there appears to be some confusion in the literature. Names such as phylloquinone, phytonadione, menaquinone, menatetrenone, menaphthone and menadione are more confusing than helpful. So, for present purposes they will be identified as:

* Natural vitamin K derived from plants

* Natural vitamin K synthesized in the bowel by bacteria

* Synthetic (pharmaceutical) vitamin K

One of the most abundant sources of plant-derived vitamin K is green vegetables, since chlorophyll is high in vitamin K. Thus, the greener the vegetable, the more chlorophyll it contains. Plant-derived vitamin K is fat soluble, therefore its absorption in the intestine is facilitated by the presence of bile and pancreatic juices. Natural chlorophyll is also fat-soluble, but preparations of it available as supplements are water-soluble, making it more rapidly digestable. Tomatoes, meat, egg yolks, milk, and vegetable oils are also good dietary sources of vitamin K, but hydrogenation decreases the content of vitamin K in vegetable oils and is best avoided for this reason and because hydrogenation increases the ratio of trans-fatty acids, known to promote atherosclerosis among other pathologic conditions. Although vegetable oils contain vitamin K, an excess of fat in the bowel will bind vitamin K, decreasing its absorption.

Commensal bowel bacteria synthesize another form of vitamin K. Therefore, healthy intestines are necessary to insure an adequate supply of this form of vitamin K. Broad-spectrum antibiotics can decrease vitamin K levels, by killing off these beneficial bacteria and diminishing vitamin K production in the bowel.

All forms of vitamin K require processing by the liver before they are effective. Hence, compromised liver function can also cause a deficiency of vitamin K in the body.

Drugs that can impair blood coagulation through the vitamin K pathway are aspirin, some antibiotics, bile acid sequestrants, laxatives, orlistat, phenytoin, and, of course, warfarin. Aspirin, antibiotics, bile acid sequestrants, laxatives, and orlistat all interfere with intestinal absorption of vitamin K. Aspirin also inhibits another component of blood coagulation, platelet adhesion, thereby increasing the risk of bleeding when prothrombin levels are low. Other drugs such as phenytoin interferes with the body's ability to use vitamin K at another stage in its metabolism. Warfarin (Coumadin[R]) reduces the body's ability to synthesize prothrombin, directly antagonizing the effect of vitamin K.

Mechanisms of action in bone

Vitamin K functions as a cofactor for the enzyme that catalyzes the carboxylation of osteocalcin. (2) Osteocalcin or BGP (bone gla protein) is found exclusively in bone tissue, accounting for 10-20% of the noncollagenous protein in bone. It is a protein produced by osteoblasts and thought to be involved in calcium ion and hydroxyapatite binding. (3) Although the exact function of osteocalcin is not yet understood, it, and therefore, vitamin K, is clearly necessary for normal bone metabolism. As with most metabolic processes, a number of distinct elements are involved, so that calcium, trace elements and a variety of vitamin and endocrine agents all are necessary for normal function. This is the generally accepted explanation for the fact that nutritional elements are best acquired through natural food sources rather than as purified and isolated entities. The matrix from which vitamin K (or any other nutrient) is derived--that is, its complex found in food--is likely to be more effective than the isolated molecule.