Boning Up on Vitamin K

The importance of vitamin K, phylloquinone (K1) and menaquinones (MKn), extends beyond the antihemorrhagic factor identified in the late 1930s. Vitamin K, in either form, participates as a cofactor for a specific unique enzyme, γ-glutamyl carboxylase. The reduced form of the vitamin in the presence of CO₂ converts the glutamate moieties of several coagulation proteins to γ-carboxyglutamate amino acids. The resulting modified proteins act as calcium-binding proteins, which stimulate the coagulation protein cascade.

The recognition of the integral role of vitamin K in skeletal health, bone repair, regulation of serum calcium levels, and calcification of soft tissues has enhanced its nutritional significance. Upon further examination of vitamin K metabolism, some investigators suggest that different forms of vitamin K may activate different signaling pathways, processes that may vary in different tissues (Shearer and Newman, 2008). In addition, recent research indicates that vitamin K may affect vascular disease, cancer, and diabetes risk. Interestingly, each of these diseases seems to be affected by calcium-binding activities expressed through γ-carboxyglutamate (Gla) proteins, e.g. MGP (matrix Gla protein) or osteocalcin (Cranenberg et al., 2008; Binkley et al., 2007).

With respect to bone health, some evidence in primates suggests that the calcium-binding proteins are not adequately carboxylated, as observed in vitamin K insufficiency induced by the anti-coagulant medication warfarin. In these short-term studies, the level of uncarboxylated proteins increase, yet bone mineral density (BMD) is not altered (Binkley et al., 2007). On the other hand, data from long-term population studies suggest direct association with low dietary vitamin K intake and low BMD in women, but not in men, even when corrected for vitamin D and calcium intake (Booth, S.L. et al., 2003; Feskanich, D. et al., 1999). Others noted that while phylloquinone fortification of a food may improve vitamin K status during a 16-week study, an improvement in BMD was not observed (Kruger, M.C. et al., 2003). More study is needed to better understand the mechanisms and clinical relevancy of the vitamin and bone health relationship.

Differences in the various forms of vitamin K—from plant sources (K1) or animal products and bacterial synthesis (K2 or MKn)—are now being reported. MK4 can be synthesized in animals from K1. MK7 has a greater efficiency than K1 in γ-carboxylation of both liver and bone Gla proteins and comprises the greater hepatic storage form. The longer side chain appears to extend the half-life of the MK form. Conversely, exposure to warfarin inhibits the vitamin K cycle and decreases the availability of the requisite reduced form of vitamin K. This prevents γ-carboxylation and results in an elevation of uncarboxylated Gla protein markers. Of interest, the blockage of the vitamin K cycle in the liver can inhibit Gla synthesis in the liver, but still allow vitamin K regulation of Gla proteins in the bone or vessel wall (Shearer and Newman, 2008). This apparent metabolic priority to preserve bone structure and vasculature emphasizes the importance of vitamin K as a critical cofactor for the γ-glutamyl carboxylation to maintain skeletal integrity and cardiovascular patency.

The physiological role vitamin K plays in bone health appears to be intertwined with osteocalcin, a bone-building hormone. The genetic expression of this protein is regulated by calcitriol, the 1,25-di-hydroxy form of vitamin D. Its activity is dependent upon the γ-carboxylation of glutamate within osteocalcin that enables calcium binding to the bone matrix (Bügel, S., 2008). Human research indicates that Vitamin K1 stimulates osteocalcin carboxylation, but preliminary data suggest more than 1,000 μ/day are required to maximize the γ-carboxylation. This amount of vitamin K is 10 times greater than the normal Western or Eastern dietary intakes of about 80 μg/day, which is less than the adequate intake for adults (IOM, 2000).

Human studies indicate the microbial menaquinone, or MK7, is more effective than phylloquinone (K1) in the carboxylation of vitamin K–dependent matrix Gla (MGP) proteins in liver and bone, whereas the plant-derived source may be more effective for improving thrombotic functions.

It appears that MGP may be the sole protein that inhibits arterial calcification and thus has a critical role in preserving vascular health (Schurgers, L.J. et al., 2008). In fact, arterial calcification is induced by warfarin administration, which inhibits MGP activation through incomplete carboxylation (Danziger, J., 2008). The result of increased calcification in the arterial wall is a loss of vascular elasticity (hypertension) and formation of sites where cholesterol deposits can produce atherosclerosis and cardiovascular disease.

Observational and clinical studies of 6–48 mo duration suggest bone loss may be affected by phylloquinone intake. This observation among populations such as elderly and post-menopausal women suggests vitamin K intake is directly associated with bone health, yet the observational data are not consistently supported by the clinical evidence (Shea and Booth, 2008; Volpe, S.L. et al., 2008). Binkley et al. (2008) noted that various vitamin K forms (K1 and K2) may differ in their effects on skeletal health, particularly in post-menopausal women, independent of calcium and vitamin D supplementation. In addition to their respective impact on the post-translational carboxylation of vitamin K–dependent proteins, single nucleotide polymorphisms (SN Ps) in the vitamin K epoxide reductase gene (VKORC 1) also influence the uncarboxylated and intact forms of osteocalcin (Nimptsch et al., 2008). This same gene influences a person’s warfarin sensitivity as evidenced by differences in the level of uncarboxylated osteocalcin (Garcia and Reitsma, 2008) and their risk of a cardiovascular event (Wang, Y. et al., 2006).

As noted earlier, vitamin K deficiency can be induced by long-term warfarin (coumarin) administration to prevent clotting, blocking conversion of the vitamin K intermediate to its active reduced form. In rat studies, the warfarin experiment led to increased uncarboxylated MGP (Matrix Gla Protein) and accumulation of calcium in the arterial wall and aortic arch (Shurgers et al., 2007). Replenishment of K1 or MK4-7 resulted in increased γ-carboxylation and unexpected calcium removal from arterial wall deposition. These results were also demonstrated in murine pro-myoblast cell cultures (Wallin et al., 2008). Warfarin produced a similar elevation in uncarboxylated osteocalcin in an intervention study with male Rhesus monkeys. However, warfarin treatment did not alter typical biomarkers of bone turnover or decrease BMD (Binkley et al., 2007). We are reminded thatMGP is synthesized in many tissues, whereas osteocalcin is exclusively a product of osteoblasts. While each is a vitamin K–dependent protein, their activation appears to differ and appears to be tissue-specific(O’Donnell et al., 2006).

It appears that—based on recent research developments—additional criteria might be incorporated into future dietary recommendations regarding vitamin K.

References for the studies cited in this article are available from the authors.

Roger Clemens, Dr.P.H.,
Contributing Editor
[email protected]

Wayne Bidlack, Ph.D.,
Contributing Editor
[email protected]