Vitamin K Requirements and Reference Intakes

Most reference intrikes are based on 0.5 to 1 /j,g phyUoquinone per kg of body weight (see Table 5.1). However, the U.S./Canadian Adequate Intake (Institute of Medicine, 2001) is 120 f g for men and 90 fMg for women, based on observed intrikes. There is some evidence that average intakes may, in fact, be inadequate to permit full carboxylation of osteocalcin Binkley and coworkers (2000) showed that giving supplements to normal healthy subjects reduced the circulating concentration of undercarboxylated osteocalcin. 

The determination of vitamin K requirements is complicated by the intestinal bacterial synthesis of menaquinones and the extent to which these are absorbed and utilized (Section 5.1). Prolonged use of antibiotics leads to impaired blood clotting, but simple dietary restriction of vitamin K results in prolonged prothrombin time and increased circulating preprothrombin so it is apparent that bacterial synthesis is inadequate to meet requirements in the absence of a dietary intake of phylloquinone. Preprothrombin is elevated at intakes between 40 to 60 /xg per day, but not at intakes above 80 /rg per day (Suttie etal., 1988). 

6 /rmol per mol of creatinine in adults. Children excrete more, presumably reflecting greater turnover of osteoctdcin. In patients receiving emticoagulants, the urinary excretion of y-ctuboxygluttunate falls to htdf as the prothrombin time incretises two- to three-fold (Suttie et al., 1988). 

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Reference Intakes for Infants and Children For obvious ethical reasons, there have been almost no experimental studies of the vitamin requirements of infants and children. For infants, it is conventional to use the nutrient yield of breast milk and assume that this is equal to or greater than requirements. Although this is termed an RNI in U.K. tables (Table 1.5), in the U.S./Canadian tables (Table 1.7), it is more correctly referred to as an acceptable intake. 

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