subjects (Allison et al., 1987; Bach et al., 1996; Binkley et al., 1999; Booth et al., 1999a; Suttie et al., 1988).

On the basis of its relatively short half-life (approximately 6 hours), factor VII activity has been used to assess vitamin K status. Allison and colleagues (1987) maintained 33 healthy subjects, some given antibiotics, for 2 weeks on a low vitamin K diet (less than 5 μg/day of phylloquinone) and observed a decrease from the normal range of plasma factor VII in seven of the subjects. However, in the absence of antibiotic treatment, factor VII activity is not a sensitive indicator of vitamin K status as it does not usually respond to changes in vitamin K intake in healthy individuals (Bach et al., 1996; Ferland et al., 1993).

Both phylloquinone and the menaquinones have been used to assess status, with phylloquinone as the vitamer usually studied because it is the primary source of dietary vitamin K in western countries (Booth and Suttie, 1998). Serum or plasma phylloquinone concentration reflects recent intakes and has been shown to respond to changes in dietary intake within 24 hours (Sokoll et al., 1997). However, given the distribution of vitamin K in the food supply, a single day plasma (serum) phylloquinone concentration may not reflect normal dietary intake. Positive correlations between circulating phylloquinone concentration and dietary intake have been reported, but the strength of this association has varied according to studies, possibly due to differences in intake assessment methodology (i.e., number of diet record days) (Booth et al., 1995, 1997b). In healthy individuals, phylloquinone concentrations are higher in older subjects than in younger subjects, irrespective of dietary intake (Booth et al., 1997b; Ferland et al., 1993; Sokoll and Sadowski, 1996). Strong positive correlations between plasma (serum) phylloquinone and triglyceride concentrations have been reported (Kohlmeier et al., 1995; Sadowski et al., 1989; Saupe et al., 1993), a finding that likely explains the higher vitamin K concentrations observed in older individuals (Sadowski et al., 1989). Normal ranges for plasma phylloquinone concentration in healthy adults aged 20 to 49 years (n = 131) was 0.25 to 2.55 nmol/L; for those aged 65 to 92 years (n = 195), 0.32 to 2.67 nmol/L (Sadowski et al., 1989).