Writing in the journal Thrombosis and Haemostasis, scientists from University of Maastricht's VitaK & Cardiovascular Research Institute CARIM state that it is questionable if present recommendations are sufficient to cover the requirements of [non-liver] tissues.
Indeed, tests looking at levels of under-carboxylated species of osteocalcin and matrix Gla protein (MGP) - generated in vitamin K deficiency - were substantial in "apparently healthy subjects", said the reviewers.
"This raises the intriguing question of whether all (or: most) apparently healthy adults are subclinically vitamin K deficient," wrote lead author Ellen Cranenburg.
The vitamin is less well known than vitamins A to E, but this increasing body of research, as well as increased marketing and advertising from supplement makers, is raising public awareness of vitamin K.
"As compared with other vitamins, the dietary intake of vitamin K is very low. The recommended dietary allowance (RDA) is 1 micrograms per kilogram of body weight per day," added Cranenburg.
"Also, its biological half-life time is relatively short: upon deprivation experimental animals develop symptoms of vitamin K-deficiency within a few days."
There are two main forms of vitamin K: phylloquinone, also known as phytonadione, (vitamin K1) which is found in green leafy vegetables such as lettuce, broccoli and spinach, and makes up about 90 per cent of the vitamin K in a typical Western diet; and menaquinones (vitamins K2), which make up about 10 per cent of Western vitamin K consumption and can be synthesised in the gut by microflora.
Menaquinones (MK-n: with the n determined by the number of prenyl side chains) can also be found in the diet; MK-4 can be found in animal meat, MK-7, MK-8, and MK-9 are found in fermented food products like cheese, and natto is a rich source of MK-7.
MK-4 is distinct from other MKs because it is not a major constituent of the spectrum of MKs produced by gut microflora, but can be derived from K1 in vivo.
A synthetic form of vitamin K, known as K3, does exist but is not recommended for human consumption.
"Poor vitamin K status must be regarded as a serious risk factor for increased postmenopausal bone loss and for artery calcification, notably in diabetes, endstage renal disease and aging," wrote the authors.
Cranenburg and co-workers added that several clinical trials have reported the benefits of supplementary vitamin K for decreasing bone loss, as well as in maintenance of bone strength and carotid artery elasticity.
"For K-1 and MK-4 these effects have been reported at pharmacological doses ranging between 1 and 45 milligrams per day," they said.
"On the basis of its longer half-life time and extra-hepatic tissue distribution it is to be expected that similar effects will be obtained with MK-7 at nutritional doses (i.e. below the RDA for vitamin K). Therefore, MK-7 is the obvious choice for enrichment of dietary supplements and functional foods to be used for disease prevention in healthy subjects."
Source: Thrombosis and Haemostasis
July 2007, Volume 98, Issue 1, Pages 120-125
"Vitamin K: The coagulation vitamin that became omnipotent"
Authors: E.C.M. Cranenburg, L.J. Schurgers, C. Vermeer