Vitamin K is a fat-soluble vitamin required as a coenzyme during the synthesis of many proteins involved in blood clotting and bone formation. Good food sources include brussels sprouts, avocado, liver, and dark leafy greens (spinach, kale, broccoli, parsley). One cup of spinach provides enough vitamin K to meet the current AI levels (90 mg/day for adult women and 120 Mg/day for adult men). No adverse effects have been associated with vitamin K consumption from food or supplements, but patients on anticoagulant therapy should monitor their vitamin K intake to ensure a consistent daily intake.

Vitamin K is involved in both blood clotting (viaprothrombin synthesis) and bone metabolism (via carboxylation of osteocalcin). Vitamin K is typically added in small amounts to most multivitamins and is frequently found as part of many bone formulas. As a dietary supplement for promoting bone health, vitamin K is needed as a cofactor in the production of osteocalcin, a key protein required for bone formation, and for the adequate deposition of calcium within the bone matrix. Vitamin K is required for full activity of the osteocalcin protein, and elderly subjects with low vitamin K intake have been shown to have inadequate carboxylated osteocalcin, suboptimal bone density, and increased risk of osteoporotic fractures.

Vitamin K’s primary function is to regulate normal blood clotting (owing to its role in the synthesis of prothrombin). The newest DRI report from the National Academy of Sciences establishes an AI level of vitamin K at 90 mg/day for adult women and 120 mg/day for adult men. The vitamin K content of most foods is very low (10 mg/100 g), and the bulk of dietary vitamin K intake is provided by a few leafy green vegetables (spinach, kale, parsley, broccoli) and vegetable oils (soybean, cottonseed, canola, olive). For the two types of naturally occurring vitamin K, absorption of phylloquinone from plant foods is poor and the small amounts of menaquinone produced by intestinal bacteria provide only a minor portion of daily requirements.

In bones, vitamin K mediates the X carboxylation of glutamyl residues on several bone proteins, most notably, the bone formation protein osteocalcin. High serum concentrations of undercarboxylated osteocalcin and low serum concentrations of vitamin K are associated with lower bone mineral density and increased risk of hip fracture. Women with higher vitamin K intakes have a significantly lower relative risk of hip fracture (about 30% lower) than do women with lower vitamin K intake (less than 70 Kg/day; Booth et al., 2003). In a similar study of 838 elderly men and women, those with the highest vitamin K intake (median of 254 day) had a significantly lower adjusted relative risk (0.35) of hip fracture than did those with lower vitamin K intake (median of 56 Mg/day; Booth et al., 2000). Feskanich et al. (1999) have shown that the risk of hip fracture in postmenopausal women is reduced by almost half for those women with the highest lettuce consumption. Those women who consumed one or more servings of lettuce per day were 45% less likely to have low bone density compared with women eating one or fewer servings of lettuce each week. Studies in postmenopausal women and older men have shown that an increased intake of vitamin K results in an increase in bone formation and osteocalcin levels and a slowing of bone loss (McKeown et al., 2002). Another study, conducted in female athletes, also showed that 1 month of vitamin K supplementation increased the body’s ability to bind calcium in die bones and resulted in a 15-20% increase in bone formation and a 20-25% decrease in bone breakdown (Craciun et al., 1998). These findings suggest that low dietary intake of vitamin K may increase the risk of hip fracture in women.

High intake of vitamin K from either foods or supplements is not recommended for patients taking anticoagulant medications such as warfarin (Coumadin) is widely assumed that a dietary vitamin K-warfarin interaction exists, and patients taking the drug arc instructed to consume a constant dietary intake of vitamin K (to avoid fluctuations in the activity of the blood-thinning medication). In most cases, a constant dietary intake of vitamin K from dietary supplements containing AI levels (90-120 mg/day) of vitamin K is the most acceptable practice for patients on warfarin. As with any fat-soluble vitamin, chronic consumption of doses above Al levels is not recommended because of concerns regarding buildup and toxicity.

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