2. Methods
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The analysis of PK and dK in U.S. food samples is an ongoing collaborative agreement of the Vitamin K Laboratory at Tufts University with the U.S. Department of Agriculture (USDA). Food samples were obtained from the USDA Nutrient Data Laboratory as part of the National Food and Nutrient Analysis Program (Haytowitz et al., 2008). Samples were collected in 12 cities in the U.S. and combined to form either brand-specific or subnational composites as appropriate to the food item. This food sampling plan provides aliquots of homogenized foods that are representative of key foods consumed in the U.S. (Pehrsson et al., 2000). Food samples were shipped to the Food Analysis Laboratory Control Center at Virginia Polytechnic Institute and State University in Blacksburg, Virginia, for the preparation of aliquots and quality-control materials. Aliquots of the foods were then shipped on dry ice to the Vitamin K Laboratory at Tufts University, Boston, Massachusetts, and stored at 80 8C until analyzed. Vitamin K is stable under these conditions for a minimum of five years (unpublished data).