As part of a metabolic study aimed at inducing a subclinical vitamin K deficiency in healthy adults, we designed a diet containing less than 10 micrograms/day of vitamin K-1 (phylloquinone). Two-day menus were constructed from a vitamin K-1 food database developed in our laboratory. This diet offers palatability and provides nutrient adequacy; that is, it meets the Recommended Dietary Allowances for protein and all other vitamins and minerals. An interesting feature of the diet is that the vitamin K-1 level can be kept very low while still fulfilling a large range of energy needs (1,800 to 3,400 kcal/day). The diet proved useful in altering apparent nutritional status for vitamin K-1; after ingestion of the diet for 6 days, plasma concentrations of vitamin K-1 decreased by 80% and fell below the established normal range. Given the well documented drug-nutrient interaction between vitamin K and warfarin--a vitamin K antagonist routinely used in the management of thromboembolic diseases--the dietary information presented in this report may provide useful information for patients undergoing warfarin therapy.