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being conducted in North America and in Europe will help elucidate this question within the next few years.

Vitamin K and Atherosclerosis

A role for vitamin K in atherosclerosis was hypothesized when proteins containing Gla residues were isolated from hardened atherosclerotic plaque (Gijsbers et al., 1990; Levy et al., 1979). These were later identified as osteocalcin and matrix Gla proteins (Ferland, 1998). In a more recent study involving 113 postmenopausal women, lower vitamin K intakes and higher ucOC levels were associated with the presence of atherosclerotic calcification in the abdominal aorta (Jie et al., 1995). Although these results are interesting, they should be considered with caution as the assessment of vitamin K status was performed 5 years after the diagnosis of atherosclerosis was made. To what extent this time lag affected the findings is unknown. Furthermore, the vitamin K intake reported for this population is quite high, in fact much higher than what is usually reported for subjects of similar age (Booth and Suttie, 1998).

A role of vitamin K in vascular health is supported by the finding of extensive arterial calcification in the matrix Gla protein knock-out mouse (Luo et al., 1997). Whether vitamin K status within the range of normal intake plays a significant role in the development of atherosclerosis requires further investigation and should be verified in studies using rigorous experimental designs.

FACTORS AFFECTING THE VITAMIN K REQUIREMENT

Bioavailability

The predominant form of vitamin K in the North American diet is phylloquinone from green leafy vegetables, and the available data on the vitamin K content of foods have been reviewed (Booth and Suttie, 1998). These data are comprehensive, but little information on the relative bioavailability of phylloquinone from various foods in human subjects is available. Gijsbers and colleagues (1996) have compared the relative bioavailability, measured as area under an absorption curve, of 1,000 μg of phylloquinone from a synthetic preparation and from a food matrix. Phylloquinone in the form of cooked spinach was reported to be 4 percent as bioavailable as that from a phylloquinone supplement. Three times as much phylloquinone was absorbed when butter was consumed with the spinach. Garber and coworkers (1999) observed that when 500 μg of phyllo-



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