Introduction
Vitamin K has not received as much attention as most of the other vitamins. Up to now it's been thought that its chief actions are concerned with synthesizing the liver proteins that controls blood clotting, such as prothrombin. Vitamin K is often given to premature babies, who may be at risk of hemorrhages due to a deficiency of the vitamin. However this function of vitamin K is not usually of great importance in adults. Perhaps other functions are more relevant to good health? Dr Saltzmann has summarized our recent knowledge of these.

What is vitamin K?
In fact, vitamin K is really a family of chemical compounds, the most common of which is vitamin K1, found as phylloquinone in a variety of plant sources - chiefly green leafy vegetables and vegetable oils. One form, menaquinone (vitamin K2), is found in some animal foods and fermentation products (e.g. cheeses), and can be converted to vitamin K1 in body tissues.

How is it measured?
The classical way to detect vitamin K deficiency is by determining the prothrombin time (a measure of the amount of available prothrombin), but this is a little crude. It's possible to measure the serum vitamin K level directly, but this only reflects recent intake, not the general level in the body. Plasma triglycerides (part of your lipid profile) also influence the level of circulating vitamin K. To make it more complicated, different body tissues preferentially store different members of the vitamin K family; this may help explain why vitamin K has different activities in the body.

What does vitamin K do?
Apart from its function in helping the formation of important blood-coagulation factors, vitamin K is responsible for assisting in the formation of several proteins produced outside the liver, namely osteocalcin, matrix Gla protein, and protein S. As the name implies, osteocalcin is involved in the mineralization of bone, while matrix Gla is concerned with calcification in cartilage and vessel walls. The brain and spinal cord have relatively high tissue levels of vitamin K, but whether it has a role to play there is so far unknown.

Vitamin K and bone health
In recent years it's been shown that adequate vitamin K intake and vitamin K body levels are linked to bone mineral density and the risk of fracture (see first link below). However, only a few studies have been done on the potential benefits of vitamin K supplements. In one, the addition of vitamin K (1 mg daily) to a regimen of vitamin D and calcium in post-menopausal women resulted in reduced bone loss in the hip over a 3-year period. In another, a menaquinone given at a dose of 45 mg daily resulted in increased bone mineral density and reduced fracture rates over a 2-year period. Dr Saltzmann calls for further studies of vitamin K supplementation in people already consuming enough calcium and vitamin D amounts.

Vitamin K and the heart
In 2004, several studies were published showing that people with low vitamin K intake are more likely to have coronary heart disease. It's hard to determine if this is a 'cause-and-effect' phenomenon, as the cardiac benefits of eating green leafy vegetables are widely accepted, without any attention paid to their vitamin K content; maybe the association seen in recent studies merely reflects an increased intake of substances similar to, or coincidentally containing, vitamin K. Laboratory experiments indicate that blocking normal vitamin K function causes abnormal calcification in arteries. Only one human study points in this direction: low vitamin K levels in the body have been found to be linked with increased carotid artery calcium in post-menopausal women. Further clinical studies are clearly necessary.

Does vitamin E present a problem for vitamin K?
One of the side effects of vitamin E overdose is bleeding, if the subject has an already-deficient coagulation system. A similar situation can arise in warfarin-users, who are taking the anticoagulant to prevent thromboses in various medical conditions. Two studies of vitamin E supplementation (1000 units daily for 12 weeks) found significantly increased levels of proteins that are induced by vitamin K lack; however, plasma levels of vitamin K were not affected by the vitamin E supplementation. This suggests that vitamin E doesn't interfere with absorption of vitamin K, but it does seem to interfere with vitamin K-dependent proteins, such as prothrombin.

The bottom line (for now)
In addition to its beneficial effects on blood coagulation in low-prothrombin conditions, vitamin K may well have a protective role in bone and blood vessel health. One should keep an eye on the recommended adequate intake levels - 120 micrograms daily for men, 90 micrograms daily for women. These values, however, were based on the coagulation effect of the vitamin, and may not be adequate for any protective action on bone or blood vessels. If you aren't taking warfarin (Coumadin®) make sure to eat plenty of leafy vegetables; these are good for other things, too, as they are high in other naturally occurring vitamins, minerals, and antioxidants.

It doesn't appear that a high intake of vitamin K is harmful - e.g. it won't promote thrombosis. People taking warfarin, however, should try to keep their intake of vitamin K-rich foods fairly constant, to help stabilize the warfarin dosage regimen. Values for the vitamin K content of different foods can be found in the second link below.

Source
Saltzmann E. Vitamin K: The Last Frontier in Vitamins. Cyberounds/Nutrition December 9, 2004, website accessed 1/4/05 at (free registration as health professional required): http://www.cyberounds.com/conf/nutrition/2004-12-09/print.html