Vitamin D and Vitamin K Team Up to Lower CVD Risk: Part II

Abstract

Strong correlations have been noted between cardiovascular diseases and low bone density / osteoporosis—connections so strong that the presence of one is considered a likely predictor of the other. This relationship has led to the hypothesis that these conditions share core pathophysiological mechanisms. Recent advances in our understanding of the complimentary roles played by vitamin D3 and vitamin K2 in vascular and bone health provide support for this hypothesis, along with insight into key metabolic dysfunctions underlying cardiovascular disease and osteoporosis.

Part I of this review, Vitamin D Deficiency – a Non-Traditional Risk Factor for Cardiovascular Disease — summarized current research linking vitamin D deficiency to cardiovascular disease, the physiological mechanisms underlying vitamin D’s cardiovascular effects, and leading vitamin D researchers’ recommendations for significantly higher supplemental doses of the pro-hormone. Read Part I: Vitamin D Deficiency – a Non-Traditional Risk Factor for Cardiovascular Disease

Part II, The Vitamin K Connection to Cardiovascular Health, reviews the ways in which vitamin K regulates calcium utlization, preventing vascular and soft tissue calcification while complimenting the bone-building actions of vitamin D, and also discusses vitamin K safety and dosage issues, and the necessity of providing vitamin K and vitamin A along with vitamin D to preclude adverse effects associated with hypervitaminosis D.

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

Underlying Mechanism of Action: Gamma-carboxylation

Vitamin K is the cofactor for the enzyme, γ-glutamyl carboxylase, which converts specific glutamic acid residues in a number of substrate proteins to γ-carboxyglutamic acid (Gla) residues, which then serve to form calcium-binding groups in these proteins and are essential for their biologic activity.

Carboxylation thus activates this family of Gla-proteins, which are involved in numerous essential activities throughout the body, including blood coagulation, bone metabolism, vascular repair, prevention of vascular calcification, regulation of cell proliferation, and signal transduction.^{15 16}

K1 is preferentially utilized in the carboxylation of clotting factors in the liver. K2 is preferentially used in the rest of the body to carboxylate the other vitamin K-dependent Gla-proteins, including osteocalcin (which is essential for bone health and primarily synthesized in bone), and matrix-Gla protein (MGP) (which prevents calcification of soft tissue, [e.g., the vasculature, myocardium, breasts and kidneys], and is primarily synthesized in cartilage and the vessel wall. Vitamin K2 is also found in high concentrations in the brain, where it contributes to the production of myelin and other important compounds.^{17 18}

Vitamin K Recycling

The body very efficiently utilizes vitamin K by recycling this nutrient in a cyclic interconversion called the vitamin K cycle.^{19 20} In this cycle, the vitamin K quinone form is reduced by the FAD-containing enzyme DT-diaphorase (a.k.a. NAD(P)H:quinone oxidoreductase ) into the vitamin K hydroquinone (KH2), which then serves as the cofactor for vitamin K carboxylation of Gla-proteins and, in so doing, is oxidized to vitamin K epoxide. Vitamin K epoxide is then recycled back to the quinone form by the enzyme vitamin K epoxide reductase (VKOR), completing the cycle. On a molecular level, vitamin K expoxide is reduced in two steps: first to the quinone form by VKOR, then to vitamin K hydroquinone (KH2) by DT-diaphorase.

Besides being a cofactor in the vitamin K-dependent carboxylation, KH2 also possesses antioxidant activity and is highly sensitive to free radicals, which may oxidize (and thus inactivate) KH2 before it can take part in the carboxylation reaction. KH2’s reactivity to free radicals may increase need for K2 in arteries burdened by atherosclerotic plaque, where high levels of oxidized LDL can contribute to a local vitamin K deficiency, further exacerbating the atherosclerotic process.²¹

As noted, VKOR is a crucial enzyme in vitamin K metabolism, enabling its re-utilization after it has been oxidized in the carboxylase reaction through which it activates Gla-proteins. Because of VKOR recycling, the human requirement for vitamin K is extremely low—just 45 mcg/day is suggested to be all that is needed of its most potent form, MK-7. VKOR is also the target for warfarin and related coumarin derivatives, which block the recycling of vitamin K by inhibiting this enzyme, thereby decreasing vitamin K available for the activation of Gla-proteins. The gene for VKOR has recently been identified, and it appears that most of the variability observed in patients’ response to warfarin is attributable to variability in the VKOR gene.^{21 22 23}

K2 Regulates Calcium Deposition: Mineralizing Bone, Preventing Vascular Calcification Mineralizing Bone

Mechanisms

Only after its carboxylation by vitamin K is osteocalcin, the major non-collagenous protein responsible for inducing bone mineralization in human osteoblasts, able to attract calcium ions and incorporate them into hydroxyapatite crystals forming the bone matrix. When vitamin K2 levels are insufficient, osteocalcin remains uncarboxylated with the result that bone mineralization is impaired.²⁴

Not only is vitamin K2 a key inducer of bone mineralization in human osteoblasts, but this form of vitamin K also inhibits osteoclast differentiation and is necessary to bring to fruition the bone-building effects of vitamin D3’s upregulation of osteoblast’s expression of osteocalcin.^{24 25 26 27 28}

Research Evidence

Numerous epidemiologic and intervention studies have shown that vitamin K insufficiency, with associated high levels of undercarboxylated osteocalcin, causes reductions in bone mineral density (BMD) and increases fracture risk. Conversely, supplementation with vitamin K2 has been shown to increase osteocalcin activation(carboxylation), promote bone mineralization and lessen risk of fracture.^{1 29 30 31 32}

A further consideration is that a number of clinical trials have demonstrated that the combination of K2 and vitamin D3 is more effective in preventing bone loss than either nutrient alone.^{33 34} In a study of 173 osteoporotic/osteopenic women, those given both K2 and D3 experienced an average 4.92% increase in bone mineral density (BMD), while average BMD increase was just 0.13 in those receiving K2 alone.35 In another study evaluating the effects of vitamin D or K singly or in combination, 92 postmenopausal women were assigned to one of four groups: K2 (45 mg/day), D3 (0.75 mcg/day [1 mcg D3 = 40 IU, so this was a 3,000 IU dose), both K2 and D3 at the aforementioned dosages, or calcium lactate (2 g/day). In the women receiving only calcium, lumbar BMD decreased. Those given either D3 or K2 experienced a slight increase in BMD. In those taking both, K2 and D3, lumbar BMD increased an average of 1.35%.³⁵

K2 has also been shown to work with D3 to lessen the risk of osteoporosis in Parkinson’s disease, which is thought to be related in part to immobilization as well as a deficiency of vitamin D caused, not by a lack of vitamin D, but rather to suppression of D3 by the high blood levels of calcium seen in Parkinson’s. When K2 (MK-4) (45 mg/day for 12 months), was given to 54 female Parkinson’s patients with osteoporosis, only one hip fracture occurred, compared to 10 fractures in a control group of 54 women with Parkinson’s who were not treated with K2. Average bone loss in the untreated group was 4.3% compared to 1.3% in those given K2.⁸

Preventing Arterial Calcification

Mechanisms

Matrix Gla-protein (MGP) is the strongest inhibitor of tissue calcification presently known. Its importance for vascular health was first demonstrated in animals bred to be MGP-deficient, all of which died of massive arterial calcification within 6–8 weeks after birth.⁷¹

MGP is produced by small muscle cells in the vasculature where—once carboxylated by vitamin K2—it protects against calcification through several mechanisms, including inhibiting bone morphological protein-2 (BMP-2), upregulating the gene for DT-diaphorase, and downregulating the gene for osteoprotegerin:

Bone Morphological Protein-2 (BMP-2)

MGP inhibits calcification by binding to and inhibiting the activity of BMP-2, a potent bone morphogen whose expression triggers the induction of an osteogenic gene expression profile in vascular smooth muscle cells (VSMC), which causes them to transform into osteoblast-like cells, a transformation known to precede arterial calcification. BMP-2 is expressed by cells in atherosclerotic lesions, and its expression can be induced by oxidative stress, inflammation or hyperglycemia.^{36 37 67} Overexpression of non γ-carboxylated MGP, as is seen in calcified lesions in the aorta, results in unopposed BMP-2 activity, which promotes osteoblastic differentiation of VSMC and the laying down of a calcified matrix.³⁸

DT-diaphorase

DT-diaphorase (a.k.a. NAD(P)H:quinone oxidoreductase) is a FAD-containing enzyme (i.e., incorporates riboflavin as its cofactor) that plays a key role in vitamin K recycling by reducing vitamin K to vitamin K hydroquinone, which then serves as the cofactor for vitamin K carboxylation of Gla-proteins. Specifically, K2’s effects on the gene expression of DT-diaphorase increase the enzyme’s activity in the vasculature 4.8-fold, greatly increasing levels of activated MGP.^{19 38}

Osteoprotegerin

Osteoclast-like cells have been identified in calcified human aortic plaques.38 Their activation is inhibited by osteoprotegerin but upregulated by activated MGP. Specifically, osteoprotegerin promotes vascular calcification by acting as a RANKL (RANK/receptor activator of NF-kappa B ligand) decoy receptor, thus preventing RANKL from binding to the transmembrane receptor RANK on osteoclast precursors, where it induces the differentiation and activation of osteoclasts.^{39 40} By lessening the production of osteoprotegerin in the vessel wall, activated MGP increases RANKL concentrations, thus increasing osteoclastic activity and the removal of calcified areas from the vasculature.

Safeguarding Elasticity

While oxidized cholesterol’s contribution to atherosclerosis has been treated as the primary issue in cardiovascular disease, arteriosclerosis, the calcification of the arterial intima, is just as lethal. The elasticity characteristic of a healthy artery is what enables it to accommodate increases in blood flow. Enough calcium deposition and that pliability is lost: blood pressure rises, damaging the vasculature and contributing to atherosclerosis. The two pathologies—arteriosclerosis and atherosclerosis—are synergistic. By preventing arterial calcification, vitamin K2 also provides protection against atherosclerosis.

In addition, K2 directly promotes blood vessel elasticity by safeguarding elastin, the core protein in the muscle fibers primarily responsible for the elasticity of the arterial wall.  Calcium deposition not only damages existing elastin, but inhibits new elastin production.⁴¹

Clinical Evidence

The question of whether high vitamin K-intake is protective against arterial calcification was first addressed in the Rotterdam Study, a massive European clinical trial following 4,807 subjects aged ≥55 over a 7-10 year period. Dietary intake of vitamin K2 (but not K1) was inversely correlated with cardiovascular calcification and cardiovascular death. Elderly people in the highest tertile of vitamin K2 intake had 52% reduction in severe aortic calcification, a 57% reduced risk of cardiovascular disease, and a 26% decreased risk for all-cause mortality. K1 intake correlated with none of these beneficial outcomes.⁴¹

Sudden death from heart attack is even more highly correlated with calcification of the aorta than cholesterol. In Framingham study research, aortic calcification was associated with double the risk of death from cardiovascular disease in men and women younger than 65, even after other risk factors (e.g., cholesterol) were taken into account. In men younger than 35, aortic calcification increased risk of sudden coronary death 7-fold.^{42 43}

Coronary Artery Calcification – A Key Biomarker of Functional Age

Research involving more than 100,000 men and women in California revealed that aortic calcification increased risk of coronary heart disease 127% in men and 122% in women. Among women, risk of stroke also increased concurrently by 146%.⁴⁴

A high coronary artery calcium score on electron beam tomography has been found to be a better predictor of mortality than age. A calcium score of less than 10 confers a reduction in functional age by 10 years in subjects older than 70, while a calcium score of >400 adds as much as 30 years of functional aging to younger patients.^{45 46 47}

Vitamin K-dependent Gla-proteins have been shown to inhibit calcification in the heart and arteries; in the kidneys, where K2 prevents the calcification that typically accompanies dialysis and diabetes; and in the breast. Women whose diets provide the most vitamin K2 have significantly less breast calcification compared to those whose diets provide the least.⁴⁸ In women, calcification of breast tissue (which several studies have correlated with vitamin K2 insufficiency^{49 50}) is associated with a 132% increased risk of cardiovascular disease, a 141% increased risk of stroke, and a 152% increased risk of heart failure.⁵¹

Uncarboxylated MGP has also been identified as a key player in the increased calcification seen in the development of varicosis, as well as in other vascular diseases. Researchers compared healthy veins from 36 male patients (aged 30 to 83) and varicose veins from 50 male patients (aged 40 to 81). In the men with varicose veins, levels of uncarboxylated MGP, were high, indicating the local vascular vitamin K status in varicose veins is insufficient to mediate full carboxylation of all newly formed MGP. Vitamin K supplementation inhibited the mineralization process in varicose small muscle cell cultures, suggesting that in vitro, carboxylation of MGP could be induced and that its inhibitory effect on varicosis could be restored.⁵²

In a clinical intervention study in which 78 women between 55 and 65 years of age received either vitamin K2 (1 mg/day) or placebo for three years, vascular characteristics were assessed (elasticity and distensibility). In subjects in the placebo group, vascular elasticity had decreased by 10–13%, which is consistent with what has been considered a “normal” decrease during a three year time period for women in this age group; in the vitamin K group, however, vascular characteristics remained unchanged, suggesting that the process of vascular aging can be retarded by increased vitamin K intake.⁵³

Intra-cranial atherosclerosis, a newly identified risk factor for ischemic stroke⁵⁴, has been shown to be an age-independent risk factor for cerebral atrophy.⁵⁵ Given the protective effects of carboxylated MGP against calcification the heart, vasculature, kidneys, and breasts, and the fact that K2 is concentrated in the brain where it has been shown to completely block free radical accumulation and cell death in cell cultures of developing fetal cortical neurons} ⁵⁶ it does not seem unreasonable to hypothesize that K2 may also play a protective role against calcification in the brain.

Vitamin K: Key to the Osteoporosis – Atherosclerosis Connection

Osteoporosis and arterial calcification have been thought to be unrelated conditions, but a number of recent studies suggest a connection.31 In the U.S., ~75–95% of men and women have some degree of coronary artery calcification on autopsy; 54% of postmenopausal women have osteopenia, and 30% have osteoporosis.31 It has been noted that patients with low bone mass, osteopenia or osteoporosis frequently also exhibit vascular calcification, which has been shown to predict both cardiovascular morbidity/mortality and osteoporotic fractures.⁵⁷

Aortic calcifications, specifically, have been positively associated with osteoporotic fractures, and the progression of aortic calcification has been positively associated with the rate of decline in lumbar spine BMD.⁵⁸ In a study of 195 postmenopausal women, the association between echogenic carotid artery plaques, low bone mass and vertebral fractures was so strong that researchers suggested it could partly explain why osteoporotic vertebral fractures are linked to increased mortality.⁵⁹ Similar associations have been found in men. In a 10 year prospective study of 781 men ≥ 50, calcifications in the abdominal aorta increased fracture risk 2 to 3-fold, regardless of subjects’ BMI, comorbidities and medications.⁶⁰

An explanation for this correlation between osteoporosis and atherosclerosis is being developed in studies analyzing the two conditions’ underlying pathophysiological mechanisms, which appear to coincide in one common factor: vitamin K deficiency.

It is becoming apparent that the development of arterial calcification resembles the process of osteogenesis.⁶¹

• Both involve the same cell types, proteins and cytokines that lead to tissue mineralization.
• More than 90% of atherosclerotic plaques undergo calcification. Ectopic bone tissue has been identified in calcified plaques, and bone-specific cells have been found in the arterial wall, with evidence that endothelial cells have transdifferentiated into osteoblasts.
• Calcified arteries have also been shown to contain osteoclast-like cells.
• Local and serum lymphocytes, monocytes and macrophages are involved in both osteoporosis and vascular calcification.
• Chemical mediators of bone metabolism including osteocalcin, bone morphogenetic protein (BMP), osteopontin (OPN), osteonectin, osteoprotegerin (OPG), receptor activator of nuclear factor kappa B ligand (RANKL), and inflammatory cytokines are also present in atherosclerotic arteries.
• The vitamin K-dependent Gla proteins, osteocalcin and MGP, are mainly expressed in bone and vascular cells, and are mediators and inhibitors of osteoid formation.
• Although osteocalcin does not appear to play a significant role in the process of vascular calcification, MGP (if carboxylated) is a key protective factor. Not only do MGP-knockout mice form extensive and lethal arterial calcifications, they also present with osteopenia, fractures, short stature, and erratic mineralization of the growth plates.
• As noted earlier in this review, carboxylated MGP protein inhibits mesenchymal differentiation into osteogenic cell lines by blocking the action of bone morphogenic protein (BMP), a potent factor of bone maturation that initiates the differentiation of vascular mesenchyme into bone cells, thus increasing calcification.
• MGP isolated from calcified atherosclerotic plaques of mice shows incomplete carboxylation.

The Calcification Paradox – Another Iteration of the Same Theme

Upon entering menopause, women simultaneously lose calcium from bone and increase its deposition in arteries—a negative double whammy called the “calcification paradox,” which greatly increases their risk of both osteoporosis and cardiovascular disease.³¹ The drop in estrogen causes both problems; vitamin K2 can help rectify them.

Estrogen impacts calcium regulation metabolism through several different pathways. Estrogen is involved in the conversion of vitamin D to its active bone-building form (1,25-dihydroxycholecalciferol [1,25(OH)2D] or calcitriol). When estrogen levels drop, osteoclasts become more sensitive to parathyroid hormone, which signals them to increase their activity. Plus, the decline in estrogen allows production of the cytokine, interleukin-6, to increase, and IL-6 stimulates the production of even more osteoclasts.^{27 62}

Among postmenopausal women not using estrogen replacement, low levels of vitamin K or high levels of uncarboxylated osteocalcin are associated with low spine BMD.⁶³ Supplementation with vitamin K2, however, has been shown to prevent bone loss associated with estrogen decline. In a 3-year study, 325 postmenopausal women were given either K2 (in the form of MK-4 or menatetrenone, for which the dosage is 45 mg/day, specifically 15 mg/tid) or placebo. In those receiving K2, bone mineral content increased, and hip and bone strength remained unchanged, whereas in the placebo group, both bone mineral content and bone strength decreased significantly.⁶⁴

Estrogen also protects premenopausal women from cardiovascular disease by increasing endothelial production of prostacyclin, PG12, which inhibits platelet aggregation and promotes vasodilation. When estrogen levels drop in menopause, these protective effects are lost.⁶⁵

Fortunately, MGP (if carboxylated) both inhibits vascular calcification and, as noted above, helps maintain blood vessel elasticity. In a 3-year study of 181 postmenopausal women, one-third were given a supplement containing vitamin D, one-third got a supplement providing both vitamin K and D, and one-third were given a placebo. In both the vitamin D and the placebo group, elasticity of the common carotid artery decreased; in those receiving K along with D, elasticity was maintained.⁶⁹

Vitamins K and A: Essential for the Prevention of Vitamin D Toxicity

Vitamin D upregulates the expression of Gla-proteins, whose activation depends on vitamin K-mediated carboxylation. Vitamin D thus increases both the demand for vitamin K and the potential for benefit from K-dependent proteins, including osteocalcin in bone and MGP in blood vessels.²⁵

Another way of looking at this, however, is that by increasing the need for vitamin K2, increased levels of vitamin D may actually induce a functional vitamin K2 deficiency, with the result that levels of uncarboxylated osteocalcin and matrix-Gla protein rise in the circulation and vasculature. In this case, not only is calcium not delivered to the bones, which become porous, but it is deposited in the arteries, which become calcified.^{31 66 11 67 68 69 70 71}

It has recently been proposed that vitamin D toxicity is the result of precisely such induction of vitamin K2 deficiency.²⁵. As vitamin D induces levels of Gla proteins to rise, the pool of available vitamin K available to carboxylate them becomes depleted, so vitamin K-dependent processes that retain minerals in the bone matrix, protect the soft tissues from calcification, and support the nervous system can no longer be performed.

In support of this hypothesis, warfarin, a coumadin derivative that induces a functional vitamin K deficiency by inhibiting the recycling of the vitamin, has definitively been shown to produce extensive hypervitaminosis D-like calcification of the soft tissues and to exert toxicity synergistically with vitamin D when the two are combined.^{72 73 74 75}

Uncarboxylated MGP is abundant in calcified arterial plaque, where its presence is thought to reflect a reactive attempt by the local tissue to protect itself from calcification—an attempt rendered futile by inadequate supplies of vitamin K2. In addition, vitamin K alone has been shown to fully reverse the calcification induced by warfarin} ⁷⁶ both confirming that the drug’s inhibition of vitamin K is directly responsible for its induction of calcification, and also adding to the likelihood that vitamin D toxicity is due to the same or a similar mechanism.²⁵³¹

Vitamin A — Balancing the Actions of Vitamin D

The hypothesis further proposes, and a number of recent studies suggest, that vitamin A protects against possible vitamin D toxicity by downregulating the expression of MGP, thus exerting a vitamin K-sparing effect, which counteracts the depletion of vitamin K potentially induced by increased levels of vitamin D.^{77 78 79}

A number of animal experiments have shown that high doses of vitamin A protect against the growth retardation, soft tissue calcification and bone resorption induced in rats by dietary vitamin D3, and that vitamin A completely protects against renal calcification induced by dietary vitamin D3 in turkeys. Vitamin A has also been shown to decrease MGP expression in human cells.25 Retinoic acid and 1,25(OH)2D3 compete for the same nuclear partners; both the retinoic acid receptor and the VDR must form heterodimers with retinoid X receptors (RXRs) to binding to response elements and initiate transcription. For this reason, 1,25(OH)2D3 and retinoic acid naturally balance one another’s effects.⁸⁰

Also, in relation to the efficacy of vitamin D at potentially lower doses or in individuals carrying VDR SNPs with impaired binding efficacy, recent research has shown that 9-cis-retinoic acid, a derivative of vitamin A, increases the affinity of VDR/RXR to its DNA recognition site, induces recruitment of coactivators by the DNA-bound heterodimer and potentiates vitamin D-dependent transcriptional responses.⁸¹

Thus, the proposed model suggests vitamin D toxicity is actually due, not to higher supplemental doses of vitamin D, but results from an imbalance among vitamins D, A and K. Proper consideration of the synergistic relationship among these vitamins could allow vitamin D to be therapeutically effective at lower doses or to be administered in higher therapeutic doses without incurring the risks associated with hypervitaminosis D.

As noted in Part I of this review, the body’s ability to utilize cholecalciferol in the numerous roles played by the vitamin D endocrine system is not optimized until blood levels of 25(OH)D are ≥40 ng/ml (98 nmol/L). Not until this level is the Vmax, of the 25-hydroxylase enzyme achieved (i.e., are all enzyme sites saturated). Below this level, chronic substrate deficiency prevents full actualization of the myriad benefits of vitamin D.⁸² For some individuals, supplementation of vitamin D3 in the range of 5,000 – 10,000 IU/day may be necessary to reach and maintain these blood levels, which underscores the concomitant need for adequate supplies of vitamin A as well as vitamin K. The National Institutes of Health has set the RDI for vitamin A at 3,000 IU for males ≥ 14 years and 2,310 IU for females ≥ 14 years, and the tolerable upper limits for retinols in both men and women at 10,000 IU.⁸³

Factors Affecting Vitamin K Deficiency

Assuming that normal, healthy levels of beneficial bacteria are present in the intestines, these bacteria produce about 75% of the vitamin K2 the body absorbs each day. Thus, even a diet quite rich in leafy greens when consumed by an individual with healthy gut flora supplies less than half the vitamin K2 needed for this nutrient’s calcium-regulating activities.

Unlike the other fat-soluble nutrients (vitamins A, D and E), vitamin K1 is cleared from the body within 8 hours, and even the MK-7 form of vitamin K2 is not stored in the body for more than 72 hours, thus this nutrient is best provided daily. Despite the production of vitamin K2 (specifically MK-4) by healthy intestinal bacteria, humans can develop a deficiency of the vitamin in as few as 7 days on a vitamin K-deficient diet.⁸⁴

Absorption of vitamin K, like that of other fat-soluble nutrients (A, D and E), is dependent upon healthy liver and gallbladder function. Digestive health is also a factor. Deficiency is more likely in people with digestive problems such as celiac disease, irritable bowel disease, or who have had intestinal bypass surgery, all of which increase the likelihood of fat malabsorption.

Vitamin K recycling is dependent upon DT-diaphorase (a.k.a. NAD(P)H:quinone oxidoreductase), a FAD-containing enzyme that reduces vitamin K to vitamin K hydroquinone, which then serves as the cofactor for vitamin K carboxylation of Gla-proteins. FAD is derived from riboflavin (B2), thus vitamin K recycling is dependent upon adequate supplies of riboflavin.

Vitamin K needs increase with age. Older individuals (>70) require higher levels of vitamin K1 or K2 to maintain low levels of uncarboxylated vitamin-K dependent proteins.⁸⁵

Bile acid sequestrants (e.g., Cholestyramine, Colestipol), which bind to bile acids, forming large compounds that are poorly reabsorbed from the gut and eliminated in the feces, also bind and remove fat-soluble vitamins, including vitamin K.

Canola and soybean oils are the primary source of vitamin K in the American diet. Hydrogenation changes the vitamin K1 (phylloquinone) in these oils into dihydrophylloquinone, a form that does not carboxylate osteocalcin and other vitamin-K dependent proteins. In 2,544 men and women (average age 58.5) who participated in the Framingham Offspring Study, those with the highest intake of vitamin K from hydrogenated oils had the lowest BMD at the neck, hip and spine.⁸⁶ If your patient eats a fair amount of processed or fast foods that contain hydrogenated oils, risk of functional vitamin K deficiency is greatly increased.⁸⁷

While levels of vitamin K (K1, specifically) are rarely insufficient to meet clotting needs, levels of vitamin K necessary for clotting are much lower than those needed (in the form of K2) for bone and arterial protection. Studies of healthy adults have found high levels of uncarboxylated osteocalcin and matrix Gla-protein (MGP) in all subjects tested.⁷¹

Laboratory Assessment of Vitamin K Status

A normal prothrombin time is not an indication that sufficient vitamin K is present to maintain carboxylation of osteocalcin or MGP.^{24 68 71}

To check vitamin K levels, request an osteocalcin test; this measures how much uncarboxylated osteocalcin is present in the blood. High levels of uncarboxylated osteocalin (ucOC) indicate insufficient vitamin K for bone health and indirectly indicate that MGP is insufficiently carboxylated.⁷¹

Safety and Efficacy

Even in high doses, neither K1 nor K2 has produced adverse effects in individuals not on coumadin derivatives. For this reason, the Institute of Medicine at the National Academy of Sciences chose not to set a Tolerable Upper Limit (UL) for vitamin K when it revised its public health recommendations for this vitamin in 2000.

Drug Interactions

Anticoagulant Medications

In patients on warfarin or other coumadin derivatives, vitamin K1 can interfere with these drugs’ anti-clotting activity in amounts as small as 1 mg.

As noted above, oral anticoagulant medications, e.g., warfarin and other coumadin derivatives, promote arterial calcification by preventing vitamin K from activating matrix Gla-protein.^{15 88}

These medications decrease clotting by blocking vitamin K epoxide reductase (VKOR), thus preventing vitamin K recycling and greatly increasing risk of vitamin K deficiency, and have also been shown to block the conversion of K1 to K2.⁸⁹

A case report recommended physicians prescribing warfarin consider arterial calcification as a potential consequence after routine examination of a healthy man on long-term warfarin treatment found his coronary arteries were highly calcified.⁹⁰ Other case reports have noted pathologic tracheobronchial calcification with long-term warfarin therapy in children, an 18-year-old male, and an elderly woman.^{91 92 93} Two recent studies involving more than 100 subjects have shown that patients treated with oral anticoagulants have double the calcification of patients not on these vitamin K-blocking drugs.⁸⁸

When improving vitamin K status, however, patients on these medications must be closely monitored. A dose of just 1-2.5 mg of oral vitamin K1 reduces the range of the international normalized ratio (INR) from 5.0-9.0 to 2.0-5.0 within 24-48 hours; even eating a vitamin K-rich diet can make anticoagulant medications less effective.⁹⁴

On the other hand, recent studies have shown that the INR is more sensitive to vitamin K changes in patients with a low vitamin K status than in those with a normal or high vitamin K status and that dietary vitamin K intake in unstable patients is considerably lower than in stable patients.^{95 96 97}

Research conducted by Schurgers et al., sugggests that MK-7 supplements supplying <50 mcg/day are not likely to affect the INR value; however, doses of >50 mcg/day may interfere with oral anticoagulant treatment in a clinically relevant way. A 50 mcg dose is comparable to the menaquinone content of 75 to 100 grams (2.6 to 3.5 ounces) of cheese, an amount that should lead to a disturbance of the INR value of no more than 10%. In addition, the long half-life of MK-7 suggests that regular intake in combination with properly adapted coumarin doses may result in more stable INR values.5

Other Interactions

K3, the synthetic form of vitamin K, promotes ROS production and glutathione depletion. High doses of K3 have been used in cancer research precisely for its ability to promote oxidative stress and cell death. Even in lower doses, K3 has produced jaundice and hemolytic anemia in human infants. For these reasons, the U.S. Food and Drug Administration banned the use of K3 in nutritional supplements.

Considerations when Choosing a Vitamin K Supplement

In animal studies, at very high intakes of K1, (200-fold the daily requirement of the liver), vitamin K1 is converted to K2 (MK-4) in amounts that may be sufficient to help decrease arterial calcification.⁹⁸

It is important to differentiate between the two commercially available forms of K2 (the MK-4 and MK-7 menaquinones) since they differ in clinically significant ways.^{5 99 100} MK-4 is a short-chain menaquinone available as a synthetic compound (menatetrenone), while MK-7, a long chain menaquinone, is a natural menaquinone derived from natto fermentation.

The vast majority of studies evaluating the effectiveness of vitamin K for the prevention of both osteoporosis and arterial calcification have used K2 (MK-4) at a dosage of 45 mg/day (specifically, 15 mg/tid). Not only has the majority of the research been done using MK-4, but MK-4 is the predominant form of K2 into which the body converts K1. MK-4 appears quickly in the blood but has a half-life of only 1-2 hours, for which reason, high pharmacological doses (typically 45 mg/day given as 15 mg tid) are necessary. Such large doses necessitate medical supervision in patients on blood-thinning medications (e.g., warfarin).

MK-7 is not only highly bioavailable and bioactive—45 mcg/day was sufficient to activate osteocalcin in the Rotterdam study—but has a much longer serum half life of 3 days, which enables the body to build up a buffer that can supply vitamin K2 to all tissues 24 hours a day. At 45 mcg/day (a dose 1,000 times less than that typically used in the research for MK-4), natto-derived MK-7 is less likely to interact negatively with blood-thinning medications.

Conclusion

As research documenting the widespread and significant beneficial actions of vitamin D continues to appear in the peer-reviewed medical literature accompanied by reports that the majority of the U.S. population is deficient in this nutrient, more clinicians are evaluating their patients’ vitamin D levels and prescribing supplementation, often in amounts as high as 5,000 to 10,000 IU/day, without awareness of the risk of provoking an imbalance among vitamins D, K and A. Consideration of the synergistic relationship among these vitamins could allow vitamin D to be administered in doses of greater therapeutic value without incurring the risks of osteoporosis and vascular calcification associated with hypervitaminosis D.

Read Part I: Vitamin D and Vitamin K Team Up to Lower CVD Risk:
Vitamin D Deficiency – a Non-Traditional Risk Factor for Cardiovascular Disease

References