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abnormal (des-g-carboxyglutamyl) prothrombin in the plasma can now be measured directly. The ratio of the two may be a useful indicator of marginal or incipient vitamin K deficiency in the absence of an observable defect in blood clotting (Blanchard et al., 1981; Corrigan et al., 1981). The 24-hour urine excretion of y-carboxyglutamic acid along with plasma vitamin K concentration have also been used to assess vitamin K status (Sadowski et al., 1988).

In vitamin K-depleted adult subjects fed a diet containing small amounts of vitamin K (10 µg/day) and treated with neomycin for 4 weeks, daily intravenous dosages of 1.5 µg of vitamin K per kilogram of body weight restored normal plasma prothrombin levels, whereas 0.1 µg/kg daily did not (Frick et al., 1967). In a study of four adults fed 0.4 µg/kg daily and treated with antibiotics for 5 weeks, plasma concentrations of prothrombin fell but remained at 70% or more of normal values (O'Reilly, 1971).

Suttie et al. (1988) reported studies of 10 college-aged male subjects who consumed a self-selected diet that eliminated foods high in vitamin K (mainly green leafy vegetables and liver) for 21 days. Such a diet provided the subjects with an average of approximately 50 µg phylloquinone per day. Serum phylloquinone values decreased during this period, but prothrombin time remained in what was considered the normal range. By the end of the period of vitamin K restriction, however, there was a significant increase in the ratio of abnormal prothrombin to active prothrombin. Similarly, a decrease in urinary g-carboxyglutamic acid was observed during the period of reduced vitamin K consumption.

Supplementation of the subjects with 50 or 500 µg of phylloquinone per day for 12 days eliminated the abnormal ratios of active to abnormal prothrombin and restored g-carboxyglutamic acid excretion to normal values. The supplement of 50 µg of phylloquinone did not raise plasma phylloquinone levels to prerestriction levels, although the 500 µg supplement was effective in raising serum phylloquinone levels to about double normal values. This study (Suttie et al., 1988) shows that simple elimination of foods high in vitamin K from a normal diet can result in signs of vitamin K inadequacy. It also suggests that bacterial synthesis of menaquinones was not sufficient to eliminate the need for dietary vitamin K in subjects consuming approximately 50 µg of phylloquinone per day.

Given the results of Frick et al. (1967) and the more recent results of Suttie et al. (1988) discussed above, it appears that a dietary intake of about 1 µg/kg body weight per day should be sufficient to maintain normal blood clotting time in adults. Thus, the RDA for a 79-kg man is 80 µg per day, and for a 63 kg woman, it is 65 µg.