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Part II: The Vitamin K Connection to Cardiovascular Health
Introduction
First recognized by German researchers as a nutrient required for normal blood “koagulation,” vitamin K is actually a family of structurally similar, fat-soluble compounds, some of which (the K2 forms) play essential roles in cardiovascular health, primarily through regulating the body’s use of calcium – both promoting its integration into bone and preventing of its deposition within blood vessels -- and also by exerting anti-inflammatory and insulin-sensitizing actions.1
In nature, vitamin K appears primarily in two forms: K1 (phylloquinone [phyllo – relating to a leaf] and K2 (the menaquinones [mena – in reference to their methylated napthoquinone ring structure]). While all forms of vitamin K share 2-methyl-1,4-naphthoqinone as their common ring structure, individual forms differ in the length and degree of saturation of a variable aliphatic side chain attached to the 3-position.
K1, a single compound that contains a monounsaturated side chain of four isoprenoid residues, is found primarily in plants and algae in association with chlorophyll. Dietary sources of K1 include green leafy vegetables, such as broccoli, kale and Swiss chard, and unhydrogenated plant oils, including canola and soybean oil.
K2, the menaquinones (MKs) are classified based on the length of their unsaturated side chains into 15 different types denominated as MK-n, with “n” denoting the number of isoprenyl residues in the side chain. The most common MKs in humans are the short-chain menaquinone, MK-4, which is now thought to be primarily produced via the systemic conversion of K1 to K2 in the body} 2 3 4 and the long-chain menaquinones, MK-7 through MK-10, which are exclusively synthesized by bacteria and gut microflora in all mammals, including humans. K2 (primarily its long-chain forms, MK-7, MK-8 and MK-9) is found in fermented foods, notably cheese and natto (fermented soybean); the latter is the richest dietary source of vitamin K presently known, almost all of which occurs in the form of MK-7.45
Vitamin K1, MK-4 and MK-7 are available as supplements: MK-4 as a synthetic version called menatetrenone, and MK-7, as the natural compound extracted from natto. MK-7 has a much longer half-life than either K1 or MK-4, which share similar molecular structures (both contain 4 isoprenoid residues, 3 of which are saturated in K1 but contain a double bond in MK-4) and therefore similar physiokinetics. In contrast, the longer-chain menaquinones, including MK-7, are much more hydrophobic and are handled differently by the body. In vivo, they have longer half-lives and are incorporated into low-density lipoproteins in the circulation, resulting in much more stable serum levels and accumulation to 7- to 8-fold higher levels during prolonged intake.5
K3 (menadione), a third, much simpler form of the vitamin, is considered a synthetic analogue, although intestinal bacteria can produce minute amounts from K1.6 K3 has been utilized in research on vitamin K's anti-cancer effects because it potentiates the cytotoxic activity of chemotherapeutic agents and vitamin C (when acting as an antioxidant, vitamin C is oxidized to dehydroascorbate, a potent free radical that is spontaneously reduced by glutathione as well as in reactions using glutathione or NADPH7; however, because of its toxicity, the FDA has banned its use in nutritional supplements.8
Although, following intestinal absorption, both K1 and K2 are taken up in the triglyceride fraction from which they are rapidly cleared by the liver, only the K2 forms are also taken up and systemically redistributed by low-density lipoproteins.910 Compared to K1, whose primary activity is the carboxylation of blood coagulation factors (II [prothrombin], VII, IX, and X, the anticoagulant proteins C, S and Z), which are synthesized in the liver, K2 has a much wider range of action, playing a significant role in bone formation and protection against bone loss, arterial calcification, and oxidation of LDL cholesterol.11 12 In addition, K2 is a 15-fold more powerful antioxidant than K1 and is the predominant form of vitamin K in all tissues, except the liver.13 Finally, K2 is better absorbed than K1 and remains biologically active far longer; K1 is cleared by the liver within 8 hours, while measurable levels of the MK-7 form of K2 have been detected up to 72 hours after ingestion.14
