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Dietary Sources and Usual Intakes

The vitamin K content of commonly consumed foods is not known with precision and therefore is not given in food composition tables. Early data, mainly from bioassays, were summarized by Olson (1988). In more recent studies in which high pressure liquid chromatography was used, the phylloquinone content of common vegetables often differed by as much as threefold (higher or lower) from values found using chick bioassays (Shearer et al., 1980). Green leafy vegetables, which provide 50 to 800 µg of vitamin K per 100 g of food, are clearly the best dietary sources. Small but significant amounts of vitamin K (1 to 50 µg/100 g) are also present in milk and dairy products, meats, eggs, cereals, fruits, and vegetables.

Human milk is relatively low in vitamin K (approximately 2 µg/ liter). Thus, breastfed infants may ingest only about 1 µg/day, which amounts to only 20% of the presumed requirement of 5 µg/day, or to an even smaller portion of the rather generous recommended content of 4 µg/100 kcal in infant formulas (AAP, 1976). Cow's milk contains 4 to 18 µg of vitamin K per liter (Haroon et al., 1982; Shearer et al., 1980).

Another potentially important source of vitamin K is the bacterial flora in the jejunum and ileum. The extent of utilization of menaquinones synthesized by gut microorganisms is not clear, however.

A normal mixed diet consumed daily by a healthy adult in the United States has been estimated to contain an average of 300 to 500 µg of vitamin K (Olson, 1988), although more recent studies suggest that these estimates may be too high (Suttie et al., 1988). Green leafy vegetables were consumed by only 1 of 12 persons in the United States on a specific day in 1977 (USDA, 1980); however, the average daily intake of vitamin K by surveyed individuals still seems to be adequate. The vitamin K intake in a single day is not a reliable indicator of its average intake by an individual over an extended period, and diets largely free of green leafy vegetables may still contain adequate amounts of vitamin K.

Recommended Allowances

Adults

The major criterion for assessing the adequacy of vitamin K status in adult humans is the maintenance of plasma prothrombin concentrations in the normal range, i.e., from 80 to 120 µg/ml (Blanchard et al., 1981). Although prothrombin levels are commonly based on assays that determine clotting time, both normal and