Bone health is critically important to the overall health and quality of life. Healthy bones provide the body with a frame that allows for mobility and for protection against injury. Bones stores minerals such as calcium that are vital to the functioning of for instance muscle contraction, nerve signaling and many enzymatic reactions. Production of blood cells takes place in the bone marrow.
Bone consists of a hard compact outer layer (cortical) bone and an interior part called trabecular bone (spongy matrix). The major constituents of bone are calcium and phosphate (approx. 70% hydroxyapatite) and proteins (30%) such as collagen and osteocalcin.
Figure 1: Bone mass as a function of age
Bones are a living substance and throughout life it is important to secure sufficient calcium to maintain a healthy skeleton. Bone mass increases until early twenties when it starts to decline. After 30-40 years of age, bone mass is decreasing in both men and women (Figure 1).
This implies that sufficient calcium intake is important in childhood/puberty and from age 35 onwards. It is well known that in addition to calcium, vitamin D3 is needed for incorporation of calcium into bone. However, just as important is sufficient availability of vitamin K. Recent scientific studies have demonstrated that vitamin K is key for optimal utilization of calcium (Booth 2009, Cheung et al. 2008, Illich et al. 2000, Kidd 2010, Knapen et al. 2007, Yaegashi et al. 2008).
A major health challenge today is the increasing number of people with weak bones caused by osteopenia (lower than normal bone mineral density, BMD) and osteoporosis (disease state with low BMD) (figure 2). Osteoporosis is estimated to affect 200 million women worldwide (Kanis 2007, WHO 2003). An estimated 75 million people in Europe, USA and Japan are affected. In 2000, approximately 9 million new osteoporotic fractures occurred, of which 1,6 million were at the hip, 1,7 million were at the forearm and 1,4 million were clinical vertebral fractures (Kanis 2007). Subjects in Europe and the Americas accounted for 51% of all these fractures, while most of the remainder occurred in the Western Pacific region and Southeast Asia (Kanis 2007).
It is estimated that in Europe, 179 000 men and 611 000 women will suffer a hip fracture each year and that the cost of all osteoporotic fractures in Europe is provisionally €25 billion (Melton et al. 2003).
Figure 2: Structure of healthy bones, left, and osteoporotic bones, right
Bone remodelling and Bone Strength
Bone is a dynamic organ. While performing its basic functions to support body stature and movements and to maintain body mineral balance, our bone continually undergoes remodelling to serve these fundamental functions. Bone remodelling is simply a process in which old bone is renewed and replaced by newly formed bones. It is a natural and life-long process.
In the cycle of bone remodelling, bone cells called osteoclasts removes old or damaged bone (this removal is called bone resorption) and the removed bone constituents (including calcium) are released into circulating blood. At the same time, another type of bone cells called osteoblasts produce new bone (bone formation). Osteoblasts produce a.o. osteocalcin, OC, a protein that, in its activated form, can bind calcium (Crokett et al. 2011, review).
Important parameters for measuring bone health are computerized scanning analysis of bone mineral density (BMD), bone mineral content (BMC) and calculated bone strength as well as score for bone fractures. The BMD is measured with a dual energy x-ray absorptiometric procedure (referred to as a DXA scan).
Osteocalcin – A Biomarker For Bone Health and Vitamin K Status
The most widely used supplements to support bone health are calcium and vitamin D3. However, several clinical studies show that the combination of vitamin D3 and vitamin K is needed for optimal bone health (Iwamoto et al. 2003, Lanham-New et al. 2008, Shearer 1997, Ushiroyama et al. 2002, Yaegashi et al. 2008).
The beneficial effects of vitamin D3 for bone health are well documented; it increases the intestinal uptake of calcium and has multiple other effects such as induction of the synthesis of OC in osteoblast (Shearer 1997, Harvard School of Public Health, 2012). OC needs to be carboxylated (activated) in order to bind calcium. Vitamin K is a co-factor for the enzyme that activates OC resulting in calcium-binding (figure 3). Activated OC is important for bone mineralization and calcium homeostasis, although the exact mechanism of action is not established (Booth 2009).
From bone, OC partly goes into the circulation and the fraction of undercaboxylated-osteocalcin (ucOC, inactive) and carboxylated osteocalcin (cOC) can be measured in serum. High serum con-centration of ucOC is correlated with low BMD and is a predictor of hip fracture risk (Szulc et al. 1994, 1993, Shearer 1997). Since high ucOC concentration also is a marker of vitamin K deficiency, the studies show the pivotal role of sufficient intake of vitamin K for optimal bone health.
Figure 3: Osteocalcin activated by a vitamin K dependent step can bind calcium and bring it to bone
As may be expected, low serum concentration of vitamin K, in particular vitamin K2, menaquinone- 7, MK-7, is correlated with increased fracture incidences (Hodges et al. 1993, Arunakul et al. 2009, Yaegashi et al.2008).
Conclusion: ucOC is a biomarker for vitamin K status. Vitamin K deficiency gives a high ucOC in blood. High concentration of ucOC (and low vitamin K) is related to low BMD and risk of hip fractures.
Vitamin K – Crucial For Maintaining Optimal Bone Health
Vitamin K is a fat-soluble vitamin. Vitamin K comprises a number of structurally related compounds including phylloquinone (vitamin K1) and menaquinones (vitamin K2s). Menaquinones are classified according to the length of their side chains; menaquinone-4 (MK-4) and menaquinone-7 (MK-7) are the most important.
Vitamin K1, phylloquinone is found in green plants and menaquinones in some dairy products. Vitamin K2, MK-7 is the most potent of the K vitamins due to a very good uptake and very long half-life in the body (Shearer and Newman 2008, Schurgers et al. 2007). A unique source of MK-7 is natto, a traditional Japanese dish. Natto is produced by Bacillus subtilis natto by fermentation of soybeans.
Studies on vitamin K and bone health have mainly focused on effects of vitamin K1 and vitamin K2, MK-4. Both vitamins have a very short half-life in the body.
In elderly people, high dietary intake of vitamin K1 (median intake 254 μg/d) was inversely associated with hip fractures while no such effects could be demonstrated for BMD (Booth et al. 2000). In an intervention study in elderly men and women, when given 500 μg/d vitamin K1 (plus vitamin D3 and calcium) for three years, no effects were found on bone turnover and bone turnover parameters (Booth et al. 2008). However, Braam and coworkers found that in postmenopausal women intake of 1mg/d of vitamin K1 (plus vitamin D3) for 3 years, resulted in reduced bone loss at the femoral hip (Braam et al. 2003). Supplementation with 5 mg of vitamin K1 for two years in postmenopausal women did not prevent decline in BMD, but did result in fewer fractures compared to placebo group (Cheung et al. 2008). Intake of 45mg/d MK-4 improved bone geometry in postmenopausal women (Knapen et al. 2007) and prevented fractures in patients with osteoporosis (Shiraki et al. 2000).
Summary of the literature indicates that high doses of Vitamin K1 (1-5 mg) or MK-4 (45mg) result in improved bone strength and reduce the fracture incidences in postmenopausal women. (Reviews by Iwamoto et al. 2009, Cocayne 2006). The dosages applied, however, are much higher than nutritional vitamin doses, but are allowed as “vitamin doses” in some countries. Dietary supplement tablets typically contain 50 -100 μg (vitamin K1) and vitamins in mg doses are often classified as drugs (MK-4 is registered as a drug for treatment of osteoporosis in Japan).
The importance of combining vitamin K2, MK- 7 with vitamin D3 and calcium for optimal bone health is gradually becoming acknowledged (Yaegashi et al. 2008, Kidd 2010).
Studies on effects of vitamin K2, MK-7 are mainly based on intake of natto (the traditional dish in Japan based on fermented soybeans). Numerous studies demonstrate a link between intake of natto and improvement of bone health; reduction of fractures, improved bone strength and improved BMD both in men and women (Kaneki et al. 2001, Fujita et al.2012). Intake of natto, equivalent to between 50-200 μg MK- 7/day for three years, demonstrated a statistically significant effect on BMD at the femoral neck in elderly women (Ikeda et al. 2006).
Effects of MK-7 on bone mineral content in transplantation patients was been demonstrated after 12 months treatment (Førli et al. 2010). In contrast, no effects on BMD were observed after 12 months intervention with a daily dose of 360 μg MK-7 in early postmenopausal women, although a reduction in ucOC and an increase in OC were observed (Emaus et al. 2009).
The first long term double-blind placebo controlled clinical trial on effects of K2, MK-7 in postmenopausal women has just been completed (244 women) in the Netherlands. The results of this three years trial are not published yet, but it is stated that the primary end-points are met, indicating a positive effect of 180 μg/day of MK-7 on BMD, bone mineral content and bone geometry in postmenopausal women (www.Clin.trial.gov , NCT00642551).
Beneficial effects on bone health has been observed after prolonged intake (three years at least) of low doses of K2, MK-7 (incl. natto). Substantially higher concentrations of vitamin K1 (and MK-4) are necessary to obtain similar effects on bone health (mg vs. μg).
Vitamin K and The Calcium Paradox
Calcification of vessels is a process resembling the bone formation process. Many recent publications indicate a link between CVD and osteoporosis (Adams and Pepping 2005, Booth 2009, Fujita et al. 2000, Kidd 2010, Szulc et al. 2009, Hamerman, 2005 review). The link is mainly based on epidemiological studies showing that people with arterial calcification also had increased bone loss (Jørgensen et al. 2004, Wiklund et al. 2011, Hofbauer et al. 2007). Excess calcium intake may for instance have negative effects on the cardiovascular system; a met-analysis of women on calcium and vitamin D showed that calcium supplements with or without vitamin D modestly increased the risk of cardiovascular events (Bolland et al. 2011).
The findings are supported by studies looking at mechanisms, which may explain why risk for vascular disease may be related to risk of osteoporosis (Hamerman 2005, Eastell et al. 2010, Vattikuti and Towler 2004). The mineral composition of bone (hydroxyapatite) is chemically very similar to that observed in calcific deposits in atherosclerotic arteries (Duer et al. 2008, Doherty et al. 2004). Vitamin K is involved in both processes; as a cofactor for activating osteocalcin associated with building calcium into bone and for activating MGP, inhibiting calcium accumulation in vessel (Shearer and Newman 2008, Rishavy et al. 2004, Schurgers et al. 2008). This is the so-called “the Calcium Paradox” (Adams and Pepping 2005).
Conclusion: Research points to an important link between bone and cardiovascular health and the regulation of calcium in body tissue. Vitamin K2 is one pivotal key; activating Gla-proteins such as osteocalcin and matrix Gla-protein, for calcium binding and regulation.
Vitamin K Deficiency – Result of Anti-Coagulant Treatment – Negative Effects On Bone
Oral anti-coagulants or vitamin K-antagonists like the coumarins are used globally for primary and secondary prevention of both arterial and venous thrombosis (Ansell et al. 2004). Coumarins interfere with the activation of vitamin K-dependent coagulation factors, thereby lowering the ability for blood clotting. While warfarin may prevent stroke and pulmonary embolism, it may possibly contribute to complications associated with low vitamin K activity, such as osteoporosis, bone fractures, and calcification of arteries (Barnes et al. 2005, Caraballo et al. 1999, Cranenburg et al. 2007, Hylek et al. 2003, Lerner et al. 2009, Rennenberg et al. 2010, Weijs et al. 2010.)
Danzinger has given a review of the effects of warfarin treatment on Gla-proteins. The coagulation factors are activated in the liver, while for instance MGP is activated in the vasculature. As warfarin inhibits the vitamin K-cycle, both processes are inhibited (Danziger 2008). This implies that warfarin may lead to vitamin K deficiency in peripheral tissue, potentially giving serious side-effects (Figure 5).
In a cross sectional study in middle aged long term coumarin users and a matched control group, it was demonstrated that chronic coumarin therapy is associated with enhanced vascular calcification (Rennenberg et al. 2010). It was suggested that the effects were related to the inhibition of MGP activation, resembling vitamin K2 deficiency.
In a recent study, use of anti-coagulants was associated with negative effects on cardiovascular health (Weijs et al. 2011). The authors suggest that chronic use of vitamin K antagonist may enhance potentially harmful coronary calcification in elderly low-risk atrial fibrillation patients.
Figure 5: Hepatic carboxylation: vitamin K1 and peripheral carboxylation: vitamin K2. In the presence of warfarin little or no carboxylation takes place in the periphery. Modified from Danziger 2008.
Conclusion: Current research indicates that long term therapy with anti-coagulants, i.e. blocking vitamin K2 activity, may result in negative effects on bone health and calcification of blood vessels.
Vitamin K – Importance of The Three Dimensional Structure
The biological activity of K vitamins are dependent on the three-dimensional configuration; the molecule can be in a cis or a trans form. Studies have demonstrated that only the all-trans form is biological active (Lowenthal et al. 1979)¸ probably due to the requirement for the trans structure to interact with the enzymes in the vitamin-K cycle (Li et al. 2010).
Vitamin K2 – Is There A Deficiency In The General Population?
Phylloquinone is found in green/leafy vegetables such as green salads, broccoli and spinach. Vitamin K1 is considered as the major dietary source of vitamin K, accounting for approximately 90% of total vitamin K intake (Schurgers and Vermeer 2000). Based on different studies of content in food worldwide, Suttie has given a summary of estimated intake of vitamin K in the general population to be between 70 to 250 mcg/day (Suttie, ed.“Vitamin K”, 2009). However the bioavailability of vitamin K1 from food is low; less than 20% is absorbed (Garber et al. 1999).
Vitamin K2, menaquinones, are found in animal products, meat, dairy, eggs (mainly MK-4) and fermented food, e.g. cheese, yoghurt, and fermented soybean products, natto (mainly MK- 7), (Schurgers and Vermeer 2000). In the Netherlands the average intake of MK-4 and long-chain menaquinones from eggs and cheese has been reported to be in the range 7μg/day and 22μg/day, respectively (Schurgers et al. 1999). Limited knowledge exists regarding intake of longer menaquinones from US and European food; fermented cheeses may be the most important source (Drevon et al. 2004, Schurgers and Vermeer 2000).
The intake of K vitamins is sufficient for 100% activation of the clotting factors in the liver in the healthy population (Schurgers et al. 2007, Theuwissen et al. 2012)
In contrast to the coagulation factors, several studies have demonstrated that both OC and MGP in serum are not fully activated in the general population (10-40%) and that supplementation increases the degree of activation (Booth 2009, Sokoll et al. 1997, Shea et al. 2011, Westenfeld et al. 2012, Theuwissen et al. 2012). Potentially, extra-hepatic carboxylation of Gla-proteins contributes to better health (Theuwissen et al. 2012).
A unique source of MK-7 is the Japanese dish, natto, that contain up to 940 μg MK-7/100g (Kamao et al. 2007). There are major local differences in intake of natto in Japan, resulting in large variations in serum MK-7 levels in the population. Kaneki and co-workers studied the geographic differences in serum level of MK-7 related to intake of natto and the incidence of hip fractures; the mean MK-7 concentration was approx. 5 ng/ml in women in Tokyo (frequent natto eaters), approx. 1ng/ml in women in Hiroshima (low intake of natto), compared to approx. 0,4 ng/ml in British women (Kaneki et al. 2001). They further demonstrated an inverse relationship between intake of natto and hip fractures in Japan. Other studies show that level of MK-7 in serum in the general population in the Netherlands and Norway was between 0-0,5 ng/ml, corresponding to intake of less than 10 μg/day (Theuwissen et al. 2012, own data, unpublished).
In studies in healthy volunteers, only intake of doses of 90μg and above would significantly improve the carboxylation of OC or MGP (Tsukamoto et al., Brugè et al. 2011, Theuwissen et al. 2012, own data, unpublished). In a small study in 12 volunteers, only by intake of the highest dose, 90 μg a significant biological effect, measured as increase of the cOC:ucOC ratio (well-recognized index of the functionality of OC) was observed (Brugè et al.2011). In a small study giving placebo or 45- 180 μg MK-7 for six weeks, a significant linear dose response relationship for plasma level of MK-7 was found. Only at the highest dose given, 180 μg, a significant increase in cOC and decrease in ucOC from baseline were found (own data, unpublished). Similar results have been reported by others (Schurgers et al. 2007) In a recent publication, Theuwissen et al. (Theuwissen et al. 2012) demonstrated that after intake of MK-7 in doses close to Recommended Daily Intake, RDA, (i.e. 90 μg and above) a significant increased circulation of carboxylated OC and MGP was observed .
In a “vitamin setting”, using vitamin doses, the extra-hepatic tissue level of MK-7 will be much higher than for vitamin K1, resulting in a higher fraction of activated Gla-proteins. Vitamin K2, MK-7, with a long half-life can be given only once daily in low dosages to establish a steady state serum level.
Vitamin K2 deficiencies seem to be quite common in the general population, except for people eating natto and a few other foods. Recent published data indicates that doses of vitamin K2 in the range of RDA (90μg/day) are needed for sufficient activation of osteocalcin, OC, and MGP.
Important: Patients on oral anticoagulant therapy using coumarins/warfarin should always consult their physician before taking vitamin K2
Source and permission: Kappa Bioscience AS, Oslo, Norway
Adams J, Pepping J; Vitamin K in the treatment and prevention of osteoporosis and arterial calcification. Am J Health Sys Pharm. 2005 Aug 1; 62(15):1574-81.
Ansell J, Hirsh J, Poller L ,et al., The pharmacology and management of the vitamin K antagonists: the Seventh ACCP Conference on Antithrombotic and Thrombolytic TherapyChest. 2004 Sep; 126(3 Suppl):204S-233S
Arunakul M, Niempoog S, Arunakul P, Bunyaratavej N. Level of undercarboxylated osteocalcin in hip fracture Thai female patients. J Med Assoc Thai. 2009 Sep;92 Suppl5:S7-11.
Barnes C, Newall F, Ignjatovic V, et al.; Monagle P; Reduced bone density in children on long-term warfarin. Pediatr Res. 2005;57(4):578-81.
Bolland MJ, Grey A, Avenell A, Gamble GD, Reid IR; Calcium supplements with or without vitamin D and risk of cardiovascular events: reanalysis of the Women`s Health Initiative limited access dataset and meta-analysis, BMJ. 2011; 342:d2040
Booth SL. Roles for vitamin K beyond coagulation. Annu Rev Nutr. 2009; 29:89-110
Booth SL, Tucker KL, Chen H,et al.; Dietary vitamin K intakes are associated with hip fracture but not with bone mineral density in elderly men and women, Am J Clin Nutr. 2000 May;71(5):1201-8
Braam LA, Knapen MH, Geusens P et al.; Vitamin K1 supplementation retards bone loss in postmenopausal women between 50 and 60 years of age, Calcif Tissue Int. 2003 Jul;73(1):21-6
Brugè F, Bacchetti T, Principi F et al.; Olive oil supplemented with menaquinone-7 significantly affects osteocalcin carboxylation, Br J Nutr. 2011;106(7):1058-62
Caraballo PJ, Heit JA, Atkinson E, et al, Long-term Use of Oral Anticoagulants and the Risk of Fracture,
Arch. Intern. Med., 1999, 159, 1750-1756
Cheung AM, Tile L, Lee Y, et al.; Vitamin K supplementation in postmenopausal women with osteopenia (ECKO trial): a randomized controlled trial, PLoS Med. 2008 Oct 14;5(10):e196
Cockayne S, Adamson J, Lanham-New S, et al.; Vitamin K and the prevention of fractures: systematic review and meta-analysis of randomized controlled trials, Arch Intern Med. 2006;166(12):1256-61
Cranenburg EC, Schurgers LJ, Vermeer C: Vitamin K: The coagulation vitamin that became omnipotent. Thromb Haemost. 2007; 98:120-5
Crockett JC, Rogers MJ, Coxon FP, Hocking LJ, Helfrich MH; Bone remodelling at a glance, J Cell Sci. 2011;124(Pt 7):991-8
Danziger J; Vitamin K-dependent proteins, warfarin, and vascular calcification; Clin J Am Soc Nephrol. 2008;3(5):1504- 10
Doherty TM, Fitzpatrick LA, Inoue D, Qiao JH, Fishbein MC, Detrano RC, Shah PK, Rajavashisth TB; Molecular, endocrine, and genetic mechanisms of arterial calcification; Endocr Rev. 2004; 25(4):629-72
Duer MJ, Friscić T, Proudfoot D, Reid DG, Schoppet M, Shanahan CM, Skepper JN, Wise ER; Mineral surface in calcified plaque is like that of bone: further evidence for regulated mineralization; Arterioscler Thromb Vasc Biol. 2008;28(11):2030-4
Drevon CA, Henriksen HB, Sanderud M, Gundersen TE, Blomhoff R: Biological effects of vitamin K and concentration of vitamin K in Norwegian food Tidsskr Nor Laegeforen. 2004; 124:1650-4
Eastell R, Newman C, Crossman DC. Cardiovascular disease and bone; Arch Biochem Biophys. 2010;503(1):78-83
Emaus N, Gjesdal CG, Almås B, et al.; Vitamin K2 supplementation does not influence bone loss in early menopausal women: a randomised double-blind placebo-controlled trial, Osteoporos Int. 2010; 21(10):1731-40
Fujita Y, Iki M, Tamaki J et al.; Association between vitamin K intake from fermented soybeans, natto, and bone mineral density in elderly Japanese men: the Fujiwara-kyo Osteoporosis Risk in Men (FORMEN) study, Osteoporos Int. 2012;23:705-14
Førli L, Bollerslev J, Simonsen S et al. ; Dietary vitamin K2 supplement improves bone status after lung and heart transplantation, Transplantation. 2010; 89(4):458-64
Garber AK, Binkley NC, Krueger DC and Suttie JW, Comparison of Phylloquinone Bioavailability from Food Sources or a Supplement in Human Subjects, J Nutr , 1999, 129,1201-1203
Hamerman D; Osteoporosis and atherosclerosis: biological linkages and the emergence of dual-purpose therapies; QJM. 2005;98(7):467-84
Hanslik T and Prinseau J. The use of vitamin K in patients on anticoagulant therapy: a practical guide. Am J Cardiovasc Drugs. 2004; 4(1):43-55.
Harvard School of Public Health,2012, The Nutrition Source
Vitamin D and Health, homepage
Hofbauer LC, Brueck CC, Shanahan CM, Schoppet M, Dobnig H; Vascular calcification and osteoporosis–from clinical observation towards molecular understanding; Osteoporos Int. 2007;18(3):251-9
Hodges SJ, Akesson K, Vergnaud P, Obrant K, Delmas PD; Circulating levels of vitamins K1 and K2 decreased in elderly women with hip fracture. J Bone Miner Res, 1993;8:1241-5
Hylek EM, Go AS, Chang Y, et al, Effect of intensity of oral anticoagulation on stroke severity
and mortality in atrial fibrillation. N Engl J Med 2003; 349: 1019–26
Ikeda Y, Iki M, Morita A, Kajita E, Kagamimori S, Kagawa Y, Yoneshima H; Intake of Fermented Soybeans, Natto, Is Associated wiht Reduced Bone Loss in Potmneopausal Women: Japanese Population-Based Osteoporosis (JPOS) Study, J. Nutr., 2006, 136, 1323-1328
Ilich JZ, Kerstetter JE; Nutrition in bone health revisited: a story beyond calcium; J Am Coll Nutr. 2000; 19(6):715-37
Iwamoto J, Sato Y, Takeda T, Matsumoto H; High-dose vitamin K supplementation reduces fracture incidence in postmenopausal women: a review of the literature, Nutr Res. 2009;29(4):221
Iwamoto J, Takeda T, Ichimura S; Treatment with vitamin D3 and/or vitamin K2 for postmenopausal osteoporosis, Keio J Med. 2003; 52(3):147-50
Jørgensen L, Joakimsen O, Mathiesen EB, et al ; Carotid plaque echogenicity and risk of nonvertebral fractures in women: a longitudinal population-based study. Calcif Tissue Int. 2006; 79(4):207-13.
Kaneki M, Hodges SJ, Hosoi T, et al., Japanese fermented soybean food as the major determinant of the large geographic difference in circulating levels of vitamin K2: possible implications for hip-fracture risk, Nutrition. 2001; 17:315-21
Kamao M, Suhara Y, Tsugawa N et al.; Vitamin K content of foods and dietary vitamin K intake in Japanese young women, J Nutr Sci Vitaminol (Tokyo). 2007; 53(6):464-70
Kanis JA; WHO Technical Report, University of Sheffield, 2007, UK: 66.
Kidd PM. Vitamins D and K as pleiotropic nutrients: clinical importance to the skeletal and cardiovascular systems and preliminary evidence for synergy. Altern Med Rev. 2010; 15(3):199-222
Lanham-New SA; Importance of calcium, vitamin D and vitamin K for osteoporosis prevention and treatment, Proc Nutr Soc. 2008; 67(2):163-76
Lerner RG, Aronow WS, Sekhri A, et al. Warfarin use and the risk of valvular calcification. J Thromb Haemost. 2009; 7(12):2023-2027.
Li W, Schulman S, Dutton RJ, Boyd D, Beckwith J, Rapoport TA; Structure of a bacterial homologue of vitamin K epoxide reductase; Nature, 2010,463(7280):507-12
Lowenthal J, Vergel Rivera GM, Comparison of the activity of the cis and trans isomer of vitamin K1 in vitamin K-deficient and coumarin anticoagulant-pretreated rats; J Pharmacol Exp Ther. 1979 209:330-3
Melton LJ, 3rd, Gabriel SE, Crowson CS, et al. (2003) Cost-equivalence of different osteoporotic fractures. Osteoporos Int 14:383
Nakano T, Tsugawa N, Kuwabara A, Kamao M, Tanaka K, Okano T; High prevalence of hypovitaminosis D and K in patients with hip fracture; Asia Pac J Clin Nutr. 2011;20(1):56-61
Rennenberg RJ, van Varik BJ, Schurgers LJ, et al, Chronic coumarin treatment is associated with increased extra-coronary atrial calcification in humans, Blood 2010; 115, 5121 -5123
Rishavy MA, Pudota BN, Hallgren KW, et al.; A new model for vitamin K-dependent carboxylation: the catalytic base that deprotonates vitamin K hydroquinone is not Cys but an activated amine; Proc Natl Acad Sci U S A. 2004 21; 10113732-7
Shearer MJ; The roles of vitamins D and K in bone health and osteoporosis prevention, Proc Nutr Soc. 1997; 56(3):915-37
Shearer MJ and Newman P: Metabolism and cell biology of vitamin K. Throm Haemost. 2008; 100:530-47
Schurgers LJ, Cranenburg EC, Vermeer C; Matrix Gla-protein: the calcification inhibitor in need of vitamin K, Thromb Haemost. 2008; 100:593-603
Schurgers LJ, Geleijnse JM, Grobbee DE et al.; Nutritional intake of vitamins K1 (Phylloquinone) and K2 (Menaquinone) in the Netherlands; J Nutr. & Environ. Med. 1999, 9, 115-122
Schurgers LJ, Teunissen KJ, Hamulyák K, Knapen MH, Vik H Vermeer C; Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7, Blood, 2007b; 109:3279-83
Schurgers LJ, Vermeer C; Determination of phylloquinone and menaquinones in food; Effect of food matrix on circulating vitamin K concentrations. Haemostasis. 2000; 30(6):298-307
Shiraki M, Shiraki Y, Aoki C, Miura M; Vitamin K2 (menatetrenone) effectively prevents fractures and sustains lumbar bone mineral density in osteoporosis, J Bone Miner Res. 2000; 15(3):515-21
Sokoll LJ, Booth SL, O`Brien ME et al.; Changes in serum osteocalcin, serum phylloquinone, and urinary gamma-carboxyglutamic acid in response to altered intakes of dietary phylloquinone in human subjects; Am J Clin Nutr. 1997; 65:779-84
Szulc P, Chapuy MC, Meunier PJ, Delmas PD; Serum undercarboxylated osteocalcin is a marker of the risk of hip fracture in elderly women, J Clin Invest. 1993; 91(4):1769-74
Szulc P, Arlot M, Chapuy MC, Duboeuf F, Meunier PJ, Delmas PD; Serum undercarboxylated osteocalcin correlates with hip bone mineral density in elderly women, J Bone Miner Res. 1994; 9(10):1591-5
Szulc P, Samelson EJ, Kiel DP, Delmas PD; Increased bone resorption is associated with increased risk of cardiovascular events in men: the MINOS study J Bone Miner Res. 2009;24(12):2023-31
Suttie JS ed, Vitamin K in health and disease, CRC Press, 2009
Theuwissen E, Cranenburg EC, Knapen MH, Magdeleyns EJ, Teunissen KJ, Schurgers LJ, Smit E, Vermeer C; Low-dose menaquinone-7 supplementation improved extra-hepatic vitamin K status, but had no effect on thrombin generation in healthy subjects; Br J Nutr. 2012 Jan 31:1-6
Ushiroyama T, Ikeda A, Ueki M. Effect of continuous combined therapy with vitamin K(2) and vitamin D(3) on bone mineral density and coagulofibrinolysis function in postmenopausal women. Maturitas. 2002 25; 41:211-21
Vattikuti R, Towler DA; Osteogenic regulation of vascular calcification: an early perspective; Am J Physiol Endocrinol Metab. 2004;286(5):E686-96
Weijs B, Blaauw Y, Rennenberg JMW, et al, Patients using vitamin K antagonists show increased levels of coronary calcification: an observational study in low-risk atrial fibrillation patients, Eur. Heart J, 2011, 32, 2555-2562
Westenfeld R, Krueger T, Schlieper G, Cranenburg EC, et al; Effect of vitamin k(2) supplementation on functional vitamin k deficiency in hemodialysis patients: a randomized trial; Am J Kidney Dis. 2012 Feb;59(2):186-95
WHO: Prevention and Management of Osteopososis, 2003, report of a WHO scientific group, Geneva, WHO technical report series; 921
Wiklund P, Nordström A, Jansson JH, Weinehall L, Nordström P; Low bone mineral density is associated with increased risk for myocardial infarction in men and women; Osteoporos Int. 2011 Apr 20. [Epub ahead of print]
Yaegashi Y, Onoda T, Tanno K, Kuribayashi T, Sakata K, Orimo H; Association of hip fracture incidence and intake of calcium, magnesium, vitamin D, and vitamin K; Eur J Epidemiol. 2008,23:219-25