Warfarin prevents blood clots by interfering with the synthesis of vitamin K-dependent coagulation factors. Vitamin-K, in turn, inhibits the effect of warfarin thus necessitating the use of higher doses of the drug in order to attain adequate anti-coagulation effect. Researchers at the University of Iowa have found that many common nutrition and sports bars contain vitamin K in amounts high enough to interfere with warfarin action. A Luna bar, for example, contains 60 micrograms of vitamin K or about half the amount found in ½ cup of broccoli. Other bars such as Viactiv soft calcium chews, Balance and Clif bars also contain significant amounts of vitamin K as does Boost and Ensure nutritional supplements. The researchers recommend that physicians inform their warfarin-using patients about these sources of vitamin K as they can significantly affect INR values.
Annals of Pharmacotherapy, Vol. 37, February 2003, p. 302