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Vitamin K2 (the menaquinones) is a group name for a family of related compounds, generally subdivided into short-chain menaquinones (with MK-4 as the most important member) and the long-chain menaquinones, of which MK-7, MK-8 and MK-9 are nutritionally the most recognized.

Forms
All K vitamins are similar in structure: they share a “quinone” ring, but differ in the saturation and number of the attached carbon and hydrogen atoms (called “side chains”). The length is indicated in the name of the particular menaquinone chain (e.g., MK-4 means that four molecular units - called isoprene units - are attached to the main ring) and influences their transport to different target tissues.



Mechanism of action
The mechanism of action of vitamin K2 is similar to vitamin K1. Traditionally, K vitamins were recognized as the factor required for coagulation, but the functions performed by this vitamin group were revealed to be much more complex. K vitamins play an essential role as cofactor for the enzyme γ-glutamyl carboxylase, which is involved in carboxylation of the vitamin K-dependent proteins [i.e., : conversion of peptide-bound glutamic acid (Glu) to γ-carboxy glutamic acid (Gla)].



Carboxylation of vitamin K-dependent proteins, known as Gla-proteins, is important and serves as a recycling pathway to recover vitamin K from its epoxide metabolite (KO) for reuse in carboxylation. Several human Gla-containing proteins synthesized in several different types of tissues have been discovered:
 * Coagulation factors (II, VII, IX, X), as well as anticoagulation proteins (C, S, Z). These Gla-proteins are synthesized in the liver and play an important role in blood homeo-stasis.
 * Osteocalcin. This non-collagenous protein is secreted by osteoblasts and plays an essential role in the formation of mineral in bone.
 * Matrix-Gla-protein (MGP). This calcification inhibitory protein is found in numerous body tissues, but its role is most pronounced in cartilage and in arterial vessel walls.
 * Growth arrest-specific protein 6 (Gas6. Gas6 is secreted by leucocytes and endothelial cells in response to injury and helps in cell survival, proliferation, migration, and adhesion.
 * Proline-rich Gla-proteins (PRGP), transmembrane Gla-proteins (TMG), Gla-rich protein (GRP) and periostin. Their precise functions are still unexplored.

Bone health
It has been suggested that vitamin K2 may play an important role in human bone’s health. However, data on the subject is inconclusive - some clinical trials show no improvement of BMD after vitamin K supplementation. First indications came from patients with femoral neck fractures, who demonstrated an extremely low level of circulating vitamin K. The strong association between vitamin K2 deficiency and impaired bone health was later proved by both laboratory and clinical studies. It has been found that vitamin K deficiency results in a decreased level of active osteocalcin, which in turn increases the risk for fragile bones. Research also showed that vitamin K2, but not K1 in combination with calcium and vitamin D can decrease bone turnover. Moreover, a study performed by Knapen et al. clearly demonstrated that vitamin K2 is essential for the maintenance of bone strength in postmenopausal women, and was the factor for improving bone mineral content and femoral neck width.

More arguments supporting the unique function of vitamin K2 came from Japan. Unexpectedly, the Japanese population seems to be at lower risk for bone fractures compared to European and American citizens. This curious finding has been connected with the high intake of natto food, a traditional breakfast dish made of fermented soybeans. Scientific research performed by Ikeda and Yaegashi confirmed the hypothesis of natto's beneficial effects on bone. It has been shown that increased intake of MK-7 from natto results in higher levels of activated osteocalcin and a significant reduction in fracture risk.

Heart health
Patients suffering from osteoporosis were shown to have extensive calcium plaques, which impaired blood flow in the arteries. This simultaneous excess of calcium in one part of the body (arteries), and lack in another (bones) –which may occur even in spite of calcium supplementation - is known as the Calcium Paradox. The underlying reason is vitamin K deficiency, which leads to significant impairment in biological function of MGP, the most potent inhibitor of vascular calcification presently known. Fortunately, animal research showed that vascular calcification might not only be prevented, but even reversed by increasing the daily intake of vitamin K2. The strongly protective effect of K2 and not vitamin K1 on cardiovascular health was confirmed by, among others, Geleijnse et al. in the Rotterdam Study (2004, see Figure 3) performed on a group of 4,800 subjects. Results of more than 10 years of follow-up were verified, also by Gast et al., who demonstrated that among K vitamins, the long-chain types of K2 (MK-7 through MK-9) are the most important for efficiently preventing excessive calcium accumulation in the arteries.

Vitamin K2 and children’s health
Laboratory experiments, population-based studies, and clinical trials tightly link better vitamin K status to the attainment of strong and healthy bones. The beneficial role of vitamin K in children was confirmed by van Summeren et al. that revealed a strong positive association between vitamin K status and bone mineral content. Findings from previous studies indicated also that additional vitamin K intake may improve bone geometry and positively influence the gain in bone mass. In a study of 223 healthy girls (11-12 years), O’Connor et al. found a positive relation between vitamin K status and bone mineral density.

Children have much higher bone metabolism than adults, so they need K vitamins in significantly larger quantities. Results from a number of studies suggest however a pronounced vitamin K deficiency in bone. In the majority of examined children, a marked elevation of undercarboxylated osteocalcin was observed, indicative for a poor K vitamin status. A similar observation was made by Kalkwarf et al. showing the interdependence between vitamin K status and bone turnover. This research underlined that the requirement for K vitamins may be higher than the current recommendation, which was set in accordance only with coagulation needs.

Absorption profile of different K vitamins
Vitamin K is absorbed along with dietary fat from the small intestine and transported by chylomi-crons in the circulation. Most of vitamin K1 is carried by triacylglycerol-rich lipoproteins (TRL) and rapidly cleared by the liver; only a small amount is released into the circulation and carried by LDL and HDL. MK-4 is carried by the same lipoproteins (TRL, LDL, and HDL) and cleared fast as well. The long-chain menaquinones are absorbed in the same way as vitamin K1 and MK-4, but are efficiently redistributed by the liver in predominantly LDL (VLDL). Since LDL has a long half life in the circulation, these menaquinones can circulate for extended times resulting in higher bioavailability for extra-hepatic tissues as compared to vitamin K1 and MK-4. Accumulation of vitamin K in extra-hepatic tissues has direct relevance to vitamin K functions not related to haemostasis.

Dietary sources and adequate intake
Presently, the Recommended Daily Allowance (RDA) for K vitamins (range of 80-120 mcg) might be too low, as it is based on hepatic (i.e. related to the liver) requirements only. This hypothesis is supported by the fact that the majority of the Western population exhibits a substantial fraction of undercarboxylated extra-hepatic proteins. Thus, complete activation of coagulation factors is satisfied, but there doesn’t seem to be enough vitamin K2 for the carboxylation of osteocalcin in bone and MGP in the vascular system. Highest concentrations of vitamin K1 are found in green leafy vegetables, but significant concentra-tions are also present in non-leafy green vegetables, several vegetable oils, fruits, grains and dairy. In Europe and the USA 60%, or more, of total vitamin K1 intake is provided by vegetables, the majority by green leafy vegetables. National surveys reveal that K1 intakes are very wide. Intakes determined by weighed-dietary Intakes are similar in mainland Britain to the USA with average intakes of around 70–80 μg/d, which is less than the adequate intake for vitamin K. Apart from animal livers, the richest dietary source of long-chain menaquinones are fermented foods (from bacteria not moulds or yeasts) typically represented by cheeses (MK-8, MK-9) in Western diets and natto (MK-7) in Japan. Food frequency questionnaire-derived estimates of relative intakes in the Netherlands suggest that ~90% of total vitamin K intakes are provided by K1, ~7.5 % by MK-5 through to MK-9 and ~ 2.5 % by MK-4. Most food assays measure only fully unsaturated menaquinones; accordingly cheeses have been found to contain MK-8 at 10–20 μg/100g and MK-9 at 35–55 μg/100 g.

Dietary intake sources
Vitamin K2 is preferred by the extra-hepatic tissues (bone, cartilage and vasculature) and is of bacterial origin. Scientific discussions are ongoing as to what extent K2 produced by our intestinal bacteria contributes to our daily vitamin K2 needs. If, however, intestinal bacterial supply was enough to supplement all tissues needing K2, we would not find high fractions of undercarboxylated Gla-proteins in human studies. Natural K2 is also found in bacterial fermented foods, like mature cheeses and curd. The MK-4 form of K2 is often found in relatively small quantities in meat and eggs. However, the richest source of Natural K2 is the traditional Japanese dish natto made of fermented soybeans, which provides an unusually rich source of Natural K2 as long-chain MK-7: its consumption in Japan has been linked to significant improvement in K vitamin's status and bone health in many studies. Unfortunately, the intensive smell and “controversial” taste make this soyfood a less attractive source of Natural K2 for Westerners' taste buds.

Other sources
Accumulating evidence suggests that Western society is affected by subclinical deficiency of vitamin K. Moreover, it has been scientifically proven that for optimal bone and vascular health, relatively high in-takes of vitamin K are required. The synthetic (and less effective) short-chain vitamin K1 is commonly used in food supplements. In case of vitamin K2, the most popular forms are MK-4 and MK-7.

Vitamin K deficiency
There are two kinds of vitamin K deficiency: acute and chronic. Widely recognized, acute deficiency is characterized by unusual bleeding from gums, nose, or the gastrointestinal tract. Consequences can be severe, including internal clogging, strokes, lung damage, and death caused by immoderate blood loss. Newborn infants are at increased risk for acute vitamin K deficiency, because vitamin K is not transported sufficiently across the placenta, and the newborn gut is sterile at the beginning. Thus, there are no bacteria to produce the required amount of vitamin K. Vitamin K deficiency may also occur with the use anticoagulant drugs (i.e., warfarin or other coumarins), prolonged use of antibiotics, gallbladder disease, and Crohn’s disease. Chronic vitamin K deficiency is less obvious than acute deficiency. It is actually more dangerous because there are no alarming symptoms and the results - impairments in bone, cardiovascular health, and other disease of aging – might be severe. It had been long believed that vitamin K deficiency is rare. Requirements could be easily met via diet and microbial biosynthesis by bacteria living in the gut. However, recent scientific data show that the amount of vitamin K is not as abundant in the diet as once thought. Even a well-balanced diet might not provide vitamin K in the amounts sufficient for satisfying the body’s needs. This is especially concerning given that, according to researcher CJ Prynne, mean dietary intake of vitamin K is currently significantly lower than it was 50 years ago, while the daily consumption of vitamin K has gradually decreased since 1950. This shortage can be partly explained by alterations in food composition (people eat much less green-leafy vegetables, which are rich in vitamin K1) and different preparation practices. Food used to be made in the presence of various bacteria species (synthesizing vitamin K2). Now, sterile conditions introduced by international standards of food manufacturing stop microorganisms, including beneficial flora, from multiplying and penetrating the human body. Dietary patterns have also changed over decades. For example, children in 1950 derived around 15% of their vitamin K intake from fats and oil sources and 55% from vegetables (excluding potatoes). In the 1990s, 35% came from fats and oils, and just 30% from vegetables. Moreover, it was shown that all K vitamins are absorbed from the gastrointestinal tract in the small intestine. Bacterial colonies producing menaquinones are located in the lower segment of the intestine, where the bile salts required for vitamin K uptake are not present. As stated earlier, the efficacy of intestinal vitamin K absorption might be questionable. Further, the presently used Recommended Dietary Intake for vitamin K might be too low The need for complete activation of coagulation factors is satisfied, but it's not enough to fulfill all of vitamin K's benefits.

Vitamin K status
Measurement techniques:
 * Food frequency questionnaires to determine vitamin K intake. Disadvantage = Rough estimates
 * Circulating vitamin K levels
 * Biomarker to reflect vitamin K sufficiency

High-performance liquid chromatography is the technique to measure the level of vitamin K in the blood. Disadvantage = However, circulating vitamin K levels correspond to the daily intake of green-leafy vegetables, cheeses, etc. The best way to evaluate vitamin K efficiency is to determine the decrease in the circulating undercarboxylated form of the vitamin K-dependent proteins in the blood (e.g., the level of carboxylated osteocalcin or MGP).

Anticoagulants and K2 supplementation
Recent studies found a clear association between long-term anticoagulant treatment (OAC) and reduced bone quality due to reduction of active osteocalcin. OAC might lead to an increased incidence of fractures, reduced bone mineral density/bone mineral content, osteopenia, and increased serum levels of undercarboxylated osteocalcin. Bone mineral density was significantly lower in stroke patients with long-term warfarin treatment compared to untreated patients and osteopenia was probably an effect of warfarin-interference with vitamin K recycling. Furthermore, OAC is often linked to an undesired soft-tissue calcification in both children and adults. This process has been shown to be dependent upon the action of K vitamins. Vitamin K deficiency results in undercarboxylation of MGP. Vascular calcification was shown to appear in warfarin-treated experimental animals within two weeks. Also in humans on OAC treatment, two-fold more arterial calcification was found as compared to patients not receiving vitamin K antagonists. Among consequences of anticoagulant treatment: increased aortic wall stiffness, coronary insufficiency, ischemia, and even heart failure. Arterial calcification might also contribute to systolic hypertension and ventricular hypertrophy. Coumarins, by interfering with vitamin K metabolism, might also lead to an excessive calcification of cartilage and tracheobronchial arteries.

Anticoagulant therapy is usually instituted to avoid life-threatening diseases and a high vitamin K intake interferes with the anticoagulant effect. Patients on coumarin treatment are therefore advised not to consume diets rich in K vitamins. However, the latest research proposed to combine vitamins K with OAC to stabilize the INR.

An overdose problem – does it exist?
There is no known toxicity associated with high doses of menaquinones (vitamin K2). Individuals taking anticoagulant medications, such as warfarin (coumarins) should consult their doctor before taking Vitamin K2. Unlike the other fat-soluble vitamins, vitamin K is not stored in any significant quantity in the liver; therefore toxic level is not a described problem. Taking too much of any vitamin or nutritional sup-plement is not recommended. All data available at this time demonstrate that vitamin K has no adverse effects in healthy subjects. The recommendations for the daily intake of vitamin K, as issued recently by the Institute of Medi-cine, also acknowledge the wide safety margin of vitamin K: “A search of the literature revealed no evidence of toxicity associated with the intake of either K1 or K2”. A point of concern is however the potential interference of K vitamins with OAC treatment. Individuals taking anticoagulant medications, such as warfarin (coumarins) should consult their doctor before taking Vitamin K2.