K in some diets. The bioavailability and the relative biological activity of dihydrophylloquinone have not been determined with any certainty. The long-chain menaquinones, which are produced in substantial amounts by intestinal microorganisms, can also serve as active forms of vitamin K, but they are not widely distributed in commonly consumed foods. Green vegetables and plant oils, the major dietary sources of vitamin K, do not contain menaquinones, and only small amounts are found in animal products. Relatively large amounts (40–80 μg/100 g) can, however, be obtained from some cheeses (Schurgers et al., 1999).
The availability of reliable data on the vitamin K content of foods has now made it possible to obtain reasonable estimates of the dietary phylloquinone intake of the North American population. The results of a number of studies on phylloquinone intake that used dietary records, with or without recall or a food frequency questionnaire, have been summarized by Booth and Suttie (1998) and are presented in Table 5-2. These data are somewhat variable but indicate a mean phylloquinone intake of about 150 μg/day for older (above 55 years) and 80 μg/day for younger men and women. Gender differences were not apparent in these studies.
Data from nationally representative U.S. surveys are available to estimate vitamin K intakes (Appendix Tables C-10, C-11, E-1). Data from the Third National Health and Nutrition Examination Survey (NHANES III) shows that median intakes of dietary vitamin K ranged from 79 to 88 μg/day for women and 89 to 117 μg/day for men (Appendix Table C-10). Because of the relatively small number of foods that provide significant amounts of phylloquinone in the diet, the daily variation in intake is high, and Booth and coworkers (1995) have estimated that a 5-day record of intake is needed to get a true measure of dietary intake. Data on phylloquinone intake have recently been calculated (Booth et al., 1999c) from 14-day food diaries collected by the Market Research Corporation of America. These data reflect the intake of nearly 4,000 men and women aged 13 years or older with a demographic profile similar to that of the U.S. census. These data clearly demonstrate the large daily variation in phylloquinone intake and indicate an average intake of 70 to 80 μg/day for the U.S. adult population. The same data provide an estimate of the dihydrophylloquinone intake of this population (19 μg/day for men and 15 μg/day for women) that is about 20 to 25 percent as much as the intake of phylloquinone.