Why are patients on Warfarin (blood thinner) asked not to take green leafy vegetables?
Leafy vegetables are rich in vitamin K. Warfarin group of blood thinners prevent the clotting of blood by antagonizing the effect of vitamin K in the synthesis of blood clotting factors by the liver. Hence if green leafy vegetables are taken by a person taking Warfarin, its effectiveness will be less. Moreover the dose of Warfarin is adjusted using a test known as prothrombin time with international normalized ratio (PT-INR). When leafy vegetables are being taken along with Warfarin, INR levels will be low and the physician if unaware of the intake of green leafy vegetable, up titrates the dose of Warfarin for better therapeutic effect. After that, if the individual stops taking green leafy vegetables, the INR will increase and cause bleeding.
Note on Vitamin K
The form of Vitamin K present in green leafy vegetables is Vitamin K1 or Phylloquinone (synonym: Phytomenadione). Vitamin K produced by gut bacteria and that seen in fermented dairy products is Vitamin K2 or Menaquinone. Gut bacteria can also convert Vitamin K1 to K2. Vitamin K is present in large quantities in green leafy vegetables because of its integral role in photosynthesis. Synthetic form (Vitamin K3 or Menadiaone) is not preferred in view of adverse effects.