Evidence For Vitamin K2
Evidence For the Health Benefits Of Vitamin K2
We can have varying degrees of confidence in different health benefits attributed to vitamin K. In this section, I refer generally to vitamin K. I discuss the difference between vitamins K1 and K2 below. This section is meant to be readable on its own, but if you don’t have a background in the biochemistry of vitamin K, it will be helpful to read the biochemistry section first.
Evidence for the Role of Vitamin K in Blood Clotting
The only incontrovertible effect of vitamin K is to support blood clotting (Suttie, 2014). On this basis, vitamin K is used to prevent hemorrhage in infants and inhibitors of vitamin K recycling such as warfarin and other 4-hydroxycoumarins are used as the principle anticoagulant therapy. Genetic deficiencies in vitamin K-dependent clotting factors lead to well characterized coagulation disorders, and otherwise fatal cases of bleeding can be rescued with fully carboxylated clotting factors. Thus, there is no room for a reasonable person to doubt this role of vitamin K.
Evidence for the Role of Vitamin K in Controlling Calcium Distribution
Vitamin K supports the carboxylation of matrix Gla protein (MGP), which controls the distribution of calcium in the body and thereby supports the mineralization of bones and teeth, prevents the pathological calcification of soft tissues such as the heart, blood vessels, and kidneys, and supports growth during early development by preventing the premature calcification of growth plates.
These roles are most clearly demonstrated in the MGP knockout mouse (Luo, 1997). It has short stature because of calcified growth plates, suffers from osteopenia and spontaneous fractures, and dies within two months due to the rupture of heavily calcified blood vessels. In other words, calcium fails to go into the right places (bone) and instead goes into all the wrong places (blood vessels and the growth plate cartilage). The evidence that MGP plays the same role in humans is extensive, and the sections below discuss that evidence in the context of each specific health benefit.
Evidence for the Role of Vitamin K in Heart Health
The evidence for the importance of vitamin K in heart health is compelling. Uncarboxylated MGP accumulates in atherosclerotic plaque in proportion to the amount of calcium deposited in the plaque (Roijers, 2011) and circulates in plasma in proportion to the severity of vascular calcification (Schurgers, 2010; Dalmeijer, 2013). Inhibitors of vitamin K recycling such as warfarin and other 4-hydroxycoumarins worsen blood vessel calcification in patients at risk for heart disease (Zhang, 2014). People who consume more vitamin K2 in the diet have a lower risk of heart disease (Geleijnse, 2004; Gast, 2009; Buelens, 2009; Zwakenberg, 2016). Two different randomized controlled trials lasting three years support the role of vitamin K in heart health: one showed that vitamin K1 prevents the worsening of arterial calcification (Shea, 2009) and the other showed that vitamin K2 reduces arterial stiffness (Knapen, 2015). The first randomized controlled trial using vitamin K2 to prevent or reverse arterial calcification is currently underway and will likely be finished by 2018 (Vossen, 2015). Thus, a wide array of observational and experimental evidence in humans agrees that dietary vitamin K supports heart health.
Evidence for the Role of Vitamin K in Bone Health
A number of randomized controlled trials from Japan have shown that a very high pharmacological dose (45 mg/day) of vitamin K2 as MK-4 exerts powerful protection against fracture risk in women with osteoporosis (Iwamoto, 2013). However, this pharmacological dose is far higher than what anyone could obtain from food, so its effects cannot be generalized to K2-rich foods or supplements using nutritionally relevant doses.
The question is whether nutritional doses, which I would define as those under one milligram per day, offer meaningful support to bone health. Observational studies have associated the use of vitamin K antagonists as anticoagulants with lower bone mineral density (Caraballo, 1999) and have associated self-reported vitamin K intake with higher bone mineral density (Macdonald, 2008; Kim, 2015) and a lower risk of hip fracture (Apalset, 2011). Similarly, intake of natto, the richest source of vitamin K2, is associated with less bone loss over time in postmenopasual women (Ikeda, 2006).
There are several randomized controlled trials (RCTs) using nutritional (100-200 μg/d) or borderline nutritional (1.5 mg/d) doses of vitamin K that suggest improvements in bone health, but they are not consistently convincing. Some show the improvement only in the lumbar spine (lower back) (Inoue, 2001; Moschonis, 2011; Kanellakis, 2012), and others only in the forearm (Koitaya, 2014; Bolton-Smith, 2007); one claims a benefit on the basis that bone health got worse in the control group or better in the vitamin K group without any difference between the two groups at the end of the study (Koitaya, 2014); and none of them report an improvement in whole body BMD or a decrease in the risk of fracture.
Among all of the RCTs, the most convincing one showed that three years of 180 μg/d vitamin K2 as MK-7 improved several measures of bone health in postmenopausal women when compared to a placebo (Knapen, 2013). Bone mineral density and bone mineral content both increased at the lumbar spine (lower back) and femoral neck (the “ball” that fits into the hip “socket”), although not at the hip itself. Estimates of bone strength improved, and less shrinkage occurred in the height of the thoracic spine (mid-back). Although the number of fractures was too small for statistical tests, six subjects in the placebo group but only one subject in the vitamin K group suffered vertebral fractures. This latter finding hints at a possibly very large reduction in the risk of fracture, but a larger study with sufficient numbers of fractures for statistical tests would be needed to confirm it.
The benefits to bone health in this study did not occur until the third year. Most other trials have only been one year long. Thus, while the RCTs are not in perfect agreement, the data are consistent with a powerful effect of vitamin K that takes several years to manifest. Future studies should be larger, at least three years long, and compare different doses and forms of vitamin K in different contexts to improve our understanding of how to best take advantage of this vitamin for bone health. For now, the principle is sufficiently compelling to consider it likely over time that optimizing vitamin K intake is likely to provide meaningful benefits to bone health.
Evidence for the Role of Vitamin K in Dental Health
Vitamin K is centrally important to oral health. The salivary glands contain the second highest concentration of vitamin K2 within the body (Thijssen, 1994), and both vitamin K2 (Glavind, 1948) and vitamin K-dependent proteins (Zacharski, 1979) are secreted into saliva. Dentin, the tissue underneath the enamel, produces both osteocalcin and MGP (Trueb, 2007).
Between 1945 and 1946, two studies tested the ability of menadione-laced chewing gum to protect against dental cavities in humans (Burrill, 1945; Mäkilä, 1968). Menadione is a precursor to the MK-4 form of vitamin K2, but it also has direct antibacterial effects. One study showed it was effective but the second failed to replicate the findings and the topic was largely forgotten thereafter. At the time, researchers thought any effect of menadione would be a result of its antibacterial activity. A study published in the 1950s, however, found that menadione prevented tooth decay in hamsters more effectively when injected into their abdominal cavities than when given orally (Gebauer, 1955). While it’s possible that some of the abdominally injected menadione made it into the saliva where it would have direct antibacterial activity, a more likely interpretation is that the abdominally injected menadione protected against tooth decay through its conversion to vitamin K2. This conversion is variable between and even within species, and variation in the ability of humans to make the conversion could have contributed to the conflicting findings with menadione-laced chewing gum.
While no studies have yet clearly shown dietary or supplemental vitamin K to improve dental health, this is most likely a result of the dental field largely ignoring any role for nutrition in the prevention of tooth decay beyond the role of carbohydrates in promoting bacterial acid production. The ubiquity of vitamin K and its proteins in the tissues of the mouth makes its importance clear, and what we need to move forward are clinical studies that take its role seriously.
Evidence for the Role of Vitamin K in Kidney Health
Human kidneys contain high concentrations of vitamin K2 (Thijssen, 1996) and use it to activate MGP . By the mid-1980s, we knew that a vitamin K-dependent protein isolated from patients with kidney stones, presumably MGP, was between four and twenty times less effective at preventing the growth of calcium oxalate crystals compared to the same protein isolated from healthy patients (Vermeer, 1986). Patients on renal dialysis have very high circulating levels of inactive MGP, and vitamin K2 supplementation dose-dependently improves its activation (Caluwé, 2014). Observational studies show that patients who consume more than the recommended intake of vitamin K spend less time on dialysis (Boxma, 2012) and have improved survival (Cheung, 2015).
These results suggest that patients with kidney disease have very high needs for vitamin K, but it is unclear whether vitamin K deficiency is a primary contributor to the initial development of kidney disease and so far no clinical trials have shown that vitamin K supplementation can prevent, treat, or reverse the disease. Still, it seems promising that optimizing vitamin K status could be a valuable prophylactic and seems advisable for renal patients to, with medical supervision, supplement with doses shown to improve MGP activation.
Evidence for the Role of Vitamin K in Growth
When used during pregnancy, vitamin K antagonists interfere with the growth of bone and cartilage in the fetus, especially the maxilla and nose, leading to underdevelopment of the middle third of the face (Howe, 1997). Growing children and adolescents likely have a high demand for vitamin K. In boys and girls between the ages of 10-14, fracture risk increases to such an extent that a 14-year-old boy has the same risk as a 53-year-old woman (Saggese, 2002). This is accompanied by very high levels of undercarboxylated osteocalcin, ranging from 11 to 83 percent of total osteocalcin (O’Connor, 2007; van Summeren, 2007; van Summeren, 2008). Whether improved intake of vitamin K can reverse the fracture risk or improve the rate of growth remains to be seen, but should be regarded as plausible.
Evidence for the Role of Vitamin K in Metabolic and Hormonal Health
Vitamin K plays two known roles in metabolic and hormonal health: one is to support the function of osteocalcin, an endocrine hormone produced by bone tissue, and the other is to support the production of sex hormones through the regulation of gene expression. The role of osteocalcin is most clearly supported by osteocalcin knockout mice: they are obese and have low metabolic rates, high blood sugar, poor insulin sensitivity, deficient levels of insulin and males have low testosterone and infertility (Lee, 2007; Oury, 2011). The role of gene expression is most clearly supported by cellular experiments that have characterized the related mechanisms and by a study showing that vitamin K increases the expression of the enzyme that converts cholesterol to pregnenolone in rats (Ito, 2011). Pregnenolone is the precursor to all of the steroid hormones, including all of the sex hormones, and vitamin K’s support of pregnenolone synthesis increases testosterone in male rats. To date, the targets of vitamin K’s regulation of gene expression are poorly characterized and they may impact sex hormones beyond simply increasing pregnenolone synthesis.
Direct evidence that vitamin K supports these roles in humans is limited, but there are key reasons to believe that it does. The sections below discuss the human evidence in the context of each specific health benefit.
Evidence for the Role of Vitamin K in Metabolic Health
A rare genetic defect in what appears to be the osteocalcin receptor results in fasting hyperinsulinemia and postprandial glucose intolerance, suggesting that osteocalcin plays the same role in metabolic health in humans as it does in mice (Oury, 2013). As noted below, this genetic defect also results in low testosterone.
Several randomized controlled trials have shown that 1 milligram of vitamin K1 (Rasehki, 2015 a; Rasehki, 2015 b) or 30-90 mg of vitamin K2 as MK-4 (Choi, 2011; Sakamoto, 2000) given daily for one to four weeks improves a variety of markers of glucose and insulin metabolism. From among these, the trial most relevant to nutritional doses of vitamin K (Rasheki, 2015 a; Rasheki, 2015 b) compared 1 mg/day of K1 to a placebo over four weeks and found that it lowered glucose and insulin levels postprandially (after a glucose tolerance test) but not in the fasting state. It also increased adiponectin, supporting the mechanism outlined in animal experiments whereby osteocalcin is released from bone and acts on adipose tissue to increase adiponectin, which then acts on other tissues such as muscle and liver to increase insulin sensitivity.
As described in the section on different vitamin K forms below, while certain forms of vitamin K2 may more effectively reach bone than K1, K1 does reach bone in substantial amounts, and the dose used in the Rasehki study was high. No one has yet compared nutritional doses of K1 to other forms of vitamin K, but we could predict that the forms that reach bone most effectively, such as MK-7, could prove even more effective.
The authors of these studies have generally argued that their results contradict the animal experiments rather than supporting them. The animal experiments show that osteocalcin has to be in its undercarboxylated state to improve metabolic and hormonal health, and these supplementation trials have shown what has already been well established, that vitamin K increases the carboxylated form and decreases the undercarboxylated form. However, the animal experiments provide a view that is much more nuanced than “undercarboxylated good, carboxylated bad.” Vitamin K is needed to “prime” osteocalcin by allowing it to accumulate in bone matrix; bone decarboxylates it and releases it in response to specific stimuli, one of which is exercise. Vitamin K deficiency causes a continuous, slow, unregulated leak of undercarboxylated osteocalcin into the blood. Supplying vitamin K to bone allows bone to properly store the hormone and release it at the right time.
While we need to learn more about osteocalcin physiology to completely reconcile all of these findings, the evidence that both vitamin K and osteocalcin are critical to metabolic health is strong.
Evidence for the Role of Vitamin K in Sex Hormone Optimization
A rare genetic defect in what appears to be the osteocalcin receptor results in low testosterone in men, suggesting that osteocalcin plays the same role in sex hormone production in humans as it does in mice (Oury, 2013). As noted above, this genetic defect also results in poor metabolic health.
Evidence that vitamin K optimizes sex hormones in humans is limited, but a recent randomized controlled trial in women with polycystic ovarian syndrome (PCOS) provides intriguing results (Razavi, 2016). PCOS is a condition involving insulin resistance and high levels of androgens (hormones that should be high in males and low in females). Compared to a placebo, a cocktail of vitamin D (400 IU), calcium, (1000 mg), and vitamin K2 (180 μg, as MK-7) taken over the course of nine weeks cut the levels of androgens in half. This could have been a result of osteocalcin-mediated improvements in insulin sensitivity, gene expression-mediated improvements in sex hormone production, or some combination of these mechanisms. The use of a nutritional cocktail precludes a definitive conclusion about the effect of vitamin K itself or how it would act alone, but the possibility that vitamin K has such a powerful effect on sex hormone optimization is promising.
Evidence for the Role of Vitamin K in Cancer
Cell experiments suggest that the MK-4 subform of vitamin K2 protects against cancer through its regulation of gene expression (Shearer, 2014). In 2004, a randomized controlled trial provided an incredible demonstration of this effect in humans: in women with viral cirrhosis, supplementation with 45 miligrams per day of MK-4 reduced the risk of liver cancer by over 80 percent over the course of 8 years (Habu, 2004).
Other trials have looked at the ability of the same exact treatment regimen to reduce the recurrence of liver cancer in people who had already recovered from it once. A meta-analysis examined five of these trials and found that vitamin K2 reduced the recurrence of liver cancer by 29-34% at two and three years (Riaz, 2012). These results are less dramatic than those of the 2004 paper, but the trials were much shorter. Even in the 2004 paper, the effect of K2 at 2-3 years was small and only became large in years four through eight of the study. Thus, it may be that this treatment is highly protective against liver cancer when carried out over a long enough duration.
The dose of MK-4 used in these studies is hundreds of times what any of us could expect to get from food. Unfortunately, we don’t know if such a high dose was actually needed. In other words, perhaps the first 200 micrograms of that dose (the first 0.44%) got rid of 80 percent of the cancer and the rest of the dose did nothing. Alternatively, it could be that such high doses have pharmacological effects that amounts of MK-4 found in food do not have. In that case, obtaining vitamin K2 from food could be irrelevant to cancer.
Observational studies offer some limited support for the importance of K2 from foods: the EPIC-Heidelberg study found that German men who consumed more than 46 micrograms per day of K2 were almost two-thirds less likely to develop advanced prostate cancer and lung cancer as those consuming less than 26 micrograms per day (Nimptsch, 2008; Nimptsch, 2010).
Thus, data from cell experiments, observational studies, and randomized controlled trials agree that vitamin K2 protects against cancer, but differences in the doses used and the types of cancer investigated leaves many open questions to be investigated by future research.