What is Vitamin K?

Vitamin K is a fat-soluble vitamin involved in blood clotting, maintenance of bone health, and various metabolic reactions in the body.

 

What are the different forms of Vitamin K?

Rather than being one specific molecule, the term ‘Vitamin K’ refers to a family of closely-related molecules. There are therefore different forms (or ‘vitamers’) of Vitamin K, with the two main naturally-occurring forms being:

• Vitamin K1 (phylloquinone) – the main dietary form of Vitamin K, which is made by plants. Green, leafy vegetables are rich sources of Vitamin K1.
  
  
• Vitamin K2 (menaquinones) – forms of Vitamin K that are produced by gut bacteria and are found in some fermented foods and animal products.

There are several different molecules, called menaquinones, which fall under the banner of Vitamin K2. These are denoted MK-4 to MK-13.

Bacteria in our intestines mainly produce MK-10 and MK-11. MK-4 can be produced by the body from phylloquinone, without the need for gut bacteria. MK-4 also found in small quantities in meat, dairy, and eggs. Natto, a Japanese food made from fermented soybeans, is rich in MK-7.

Vitamin K3 (menadione) is a synthetic form of Vitamin K mainly used in animal feed.  

KEY POINTS

• K1 (phylloquinone) and K2 (menaquinones) are the two naturally-occurring forms of Vitamin K.
• Vitamin K1 is the main form of Vitamin K in our diet.
• Vitamin K1 is found in plant sources such as green, leafy vegetables.
• Vitamin K2 is produced by gut bacteria and found in fermented foods and some animal products.
• Vitamin K2 is the collective name for the menaquinones, MK-4 to MK-13.
• Natto (Japanese fermented soybeans) is rich in MK-7.

 

Why is Vitamin K important?

Vitamin K is an important cofactor. Cofactors are molecules that help enzymes to effectively carry out chemical reactions in the body.

More specifically, Vitamin K is a cofactor for a key enzyme called γ-glutamylcarboxylase (GGCX). This enzyme activates various proteins known collectively as Vitamin K-dependent proteins.

Vitamin K-dependent proteins play important roles in blood clotting (coagulation) and bone health.   

 

KEY POINTS

• Vitamin K is important for blood clotting and maintaining bone health. 
• Vitamin K acts as a cofactor for the GGCX enzyme. 
• The GGCX enzyme activates various Vitamin K-dependent proteins, including those involved in blood clotting and bone metabolism. 

 

How is Vitamin K involved in blood clotting (coagulation)?

If you were to cut yourself shaving, you may initially start bleeding. After a while, however, the bleeding will hopefully stop. Why is this?

After you cut/injure a blood vessel, platelets in your bloodstream quickly come and plug the injury site. Next, proteins circulating in your bloodstream called clotting factors activate each other in a complex cascade (called the coagulation cascade), ultimately leading to the production of a tough, fibrous protein known as fibrin.

Long strands of fibrin form a net that traps more platelets and red blood cells to form a tough, durable blood clot that prevents further bleeding.

 

So, how does Vitamin K fit in?

Many of the clotting factors circulating in your bloodstream are Vitamin K-dependent proteins. This means we require Vitamin K to make and activate these clotting factors, allowing us to form a blood clot.

Of the various clotting factors involved the coagulation cascade, factors II (prothrombin), VII, IX, and X are Vitamin K-dependent proteins.

Vitamin K is also needed to activate certain anticoagulant proteins that inhibit blood clotting. These proteins include Protein C, S and Z. The role of these anticoagulant proteins is to limit and prevent excess blood clotting.

Highlighting the importance of Vitamin K in blood clotting, people with low levels of Vitamin K have blood that takes a longer time to clot. This is reflected in a longer prothrombin time – a test used by clinicians that measures how long it takes blood to clot. More severe cases of Vitamin K deficiency can lead to excessive bleeding and haemorrhage.

Similarly, drugs that oppose the function of Vitamin K, such as warfarin, can be used to inhibit the activation of clotting factors to help prevent the formation of blood clots (or ‘thin the blood’).

 

KEY POINTS

• Platelets and proteins called clotting factors form a clot when blood vessels are injured. 
• Clotting factors are involved in a sequence of chemical reactions that result in the formation of a fibrin net that prevents bleeding. 
• Several clotting factors are Vitamin K-dependent proteins. This means they require Vitamin K in order to be activated and form a clot. 
• Low levels of Vitamin K are associated with longer blood clotting times and a risk of bleeding.
• Warfarin is an anticoagulant drug that prevents blood clotting by inhibiting the function of Vitamin K.  

 

How is Vitamin K involved in bone health?

Several bone-related proteins are also Vitamin K-dependent proteins that must be activated using Vitamin K. One of these proteins is osteocalcin.

Osteocalcin binds calcium and helps to deposit it within bone – a process known as bone mineralisation. This helps to strengthen bones and makes them less susceptible to fracture.

Another Vitamin K-dependent protein involved in bone health is matrix GLA protein (MGP). MGP stimulates bone cells called osteoblasts, which lay down new bone.

Given the role of Vitamin K in strengthening and maintaining healthy bones, it is thought that supplementing with Vitamin K could reduce the risk of osteoporosis. Osteoporosis is a condition characterised by a loss of bone mineral density, which makes bones fragile and more susceptible to fracture.

A 2006 meta-analysis of 7 trials, largely involving Japanese post-menopausal women, found that supplementation with 45 mg/day of MK-4 (a form of Vitamin K2) was associated with a significantly reduced risk of hip and spine fractures.

By contrast, a more recent 2019 meta-analysis of 36 studies concluded that “for post-menopausal or osteoporotic patients, there is no evidence that vitamin K affects bone mineral density or vertebral fractures.”

The same 2019 meta-analysis found that Vitamin K supplementation was associated with a 24% lower risk of non-vertebral fractures, but the authors commented that more evidence is needed before we can conclude that Vitamin K prevents fractures.

 

KEY POINTS

• Vitamin K is needed to activate Vitamin K-dependent proteins that deposit calcium, strengthen, and lay done new bone tissue.
• There is mixed evidence that Vitamin K supplementation can help reduce the risk of osteoporosis and bone fractures.

 

How is Vitamin K absorbed and metabolised?

As it a fat-soluble vitamin, Vitamin K must be absorbed along with fat in the intestines. Vitamin K is then packaged with fat into lipoprotein particles and transported in the bloodstream to the liver and other tissues.

Vitamin K from our diet is in a form (known as its quinone form) that is inactive. In order to act as an enzyme cofactor and activate various Vitamin K-dependent proteins (such as clotting factors and bone proteins), dietary Vitamin K must be converted into its active form (known as its hydroquinone form). This process occurs in a series of chemical reactions known as the Vitamin K cycle.

 

Vitamin K cycle

The Vitamin K cycle is a series of metabolic reactions that allows us to convert inactive Vitamin K from our diet into active Vitamin K. In turn, active Vitamin K is used by the GGCX enzyme to activate Vitamin K-dependent proteins for blood clotting and maintaining bone health.

The Vitamin K cycle also allows us to recycle and reuse active Vitamin K. This is useful as tissues have a limited ability to store Vitamin K.

 

There are three main stages of the Vitamin K cycle:

1) Inactive dietary Vitamin K (quinone) is converted into active Vitamin K (hydroquinone). This reaction is carried out by an enzyme known as VKOR (Vitamin K oxidoreductase) as well as by other reductase enzymes.

2) Active Vitamin K hydroquinone is used as a cofactor by the GGCX enzyme. The GGCX enzyme activates Vitamin K-dependent proteins such as clotting factors. As a result of this process, Vitamin K hydroquinone is converted into Vitamin K epoxide.

3) Vitamin K epoxide is recycled back into inactive Vitamin K (quinone). This reaction is carried out by the VKOR enzyme. Vitamin K quinone can then re-enter stage 1 and be converted into active Vitamin K hydroquinone.

 

Some anticoagulant drugs, such as warfarin, act by inhibiting stages 1) and 3) of the Vitamin K cycle. This prevents active Vitamin K hydroquinone from being used to activate blood clotting factors.

 

KEY POINTS

• Vitamin K must be absorbed with fat in our diet. 
• Dietary Vitamin K is inactive and must be converted into active Vitamin K in order to carry out metabolic reactions in the body. 
• Vitamin K is activated and recycled in a series of reactions known as the Vitamin K cycle. 
• Warfarin acts by inhibiting stages of the Vitamin K cycle, meaning blood clotting factors cannot be activated.  

 

What is warfarin and how does it work?

Warfarin is an anticoagulant or ‘blood thinning’ drug that makes blood less likely to clot. This prevents both the formation of new blood clots and the enlargement of existing blood clots.

It is used in the treatment of people with:

• a history of previous blood clots e.g. deep vein thrombosis (DVT) or pulmonary embolism (PE).
• a high risk of developing blood clots e.g. heart rhythm abnormalities, blood clotting disorders (thrombophilia).

In the first few weeks of treatment, doctors often adjust the dose of warfarin to achieve the correct clotting ability of blood. If the warfarin dose is too high, blood is less able to clot and this increases the risk of excessive bleeding. By contrast, if the warfarin dose is too low, then blood clots are highly likely to form and pose health risks.

 

How does warfarin work?

Warfarin belongs to a class of drugs known as Vitamin K antagonists. It works by preventing the body from using Vitamin K to make clotting factors.

As mentioned previously, our blood contains proteins called clotting factors that become activated in a series of a reactions known as the coagulation cascade. The activation of these clotting factors leads to the formation of fibrin, which helps to form a durable blood clot. Some of these clotting factors are Vitamin K-dependent proteins, meaning they require Vitamin K in order to be activated and go on to form a clot.

Warfarin works by inhibiting the activation of Vitamin K-dependent clotting factors (II [Prothrombin] VII, IX, and X) thereby preventing blood clot formation. This is illustrated in the diagram below.   

 

In terms of Vitamin K metabolism, warfarin inhibits key stages in the Vitamin K cycle that allow the conversion of inactive Vitamin K into its active form.

Specifically, warfarin inhibits the VKOR enzyme that converts inactive Vitamin K quinone from our diet into active Vitamin K hydroquinone that is used to activate blood clotting factors.

Warfarin also inhibits the recycling of Vitamin K epoxide back into Vitamin K quinone to be reused.

 

Given that it blocks the usage of Vitamin K, the amount of Vitamin K in your diet can significantly affect the efficacy of warfarin. For this reason, people using warfarin are advised to keep their diet stable and avoid sudden changes in intake of Vitamin K-rich foods.

 

KEY POINTS

• Warfarin is an anticoagulant drug used to prevent blood clots.
• Warfarin works by preventing the liver from using Vitamin K to make and activate clotting factors.
• The amount of Vitamin K in the diet can affect how well warfarin works.

 

What is CYP4F2?

As well as being recycled in the Vitamin K cycle (described previously), Vitamin K can also be inactivated and removed by the liver.

This is carried out by your CYP4F2 enzyme. (Incidentally, this is the same enzyme that breaks down Vitamin E, which we encountered before in your Vitamin E Breakdown trait.)

 

Your CYP4F2 enzyme acts to remove Vitamin K quinone from the Vitamin K cycle, thereby inactivating it. It can then be further broken down and excreted in urine.  

By removing and breaking down Vitamin K, your CYP4F2 acts to ‘siphon off’ and prevent excessive Vitamin K accumulation in the liver.

 

KEY POINTS

• Your CYP4F2 enzyme inactivates and breaks down Vitamin K.
• Your CYP4F2 enzyme prevents Vitamin K levels from becoming excessively high.

 

How do CYP4F2 gene variants affect Vitamin K breakdown?

Your CYP4F2 enzyme is coded for by your CYP4F2 gene.

A SNP (Single Nucleotide Polymorphism) within this gene (designated rs2108622) creates two different CYP4F2 gene variants or ‘alleles’:

• the ‘C’ allele
• the ‘T’ allele.

The ‘T’ allele leads to reduced enzyme levels and therefore reduced breakdown and inactivation of Vitamin K.

Furthermore, the ‘T’ allele has an ‘additive effect.’ This means people who inherit two copies of the ‘T’ allele (i.e. TT genotype) will have lower CYP4F2 levels and more markedly reduced Vitamin K breakdown than those who inherit just one ‘T’ allele (the CT genotype).

 

KEY POINTS

• There are two variants of the CYP4F2 gene: the ‘C’ and ‘T’ variants.
• The ‘T’ variant is associated with lower CYP4F2 enzyme levels and reduced breakdown of Vitamin K.
• In order of decreasing Vitamin K breakdown, your possible CYP4F2 genotypes are: CC, CT, TT.

 

How do CYP4F2 gene variants affect Vitamin K levels?

Due to reduced Vitamin K breakdown, people with the ‘T’ allele of the CYP4F2 gene have higher tissue and blood levels of Vitamin K.

On this note, a study of 217 Japanese patients found that those with the TT genotype (i.e. those carrying two copies of the ‘T’ allele) had significantly higher plasma levels of Vitamin K1 and MK-4 (a form of Vitamin K2) compared to those with the CT and CC genotypes.

It is important to note, however, that variants of the CYP4F2 are only associated with small variations in tissue and blood Vitamin K levels. Furthermore, CYP4F2 gene variants are not associated with Vitamin K deficiency or excessively high Vitamin K levels.

 

KEY POINTS

• The ‘T’ allele of the CYP4F2 gene is associated with slightly higher Vitamin K levels.
• Higher Vitamin K levels likely result from reduced vitamin breakdown.

 

How do CYP4F2 gene variants affect warfarin dose?

Several studies suggest that the ‘T’ allele of the CYP4F2 gene is associated with the need for a higher warfarin dose.

For example, a systematic review of 30 studies, encompassing 9,740 subjects, found that, compared to those with the CC genotype, people with the ‘T’ allele required an 8.3% higher dose of warfarin in the initial phases of treatment.

The need for a higher warfarin dose likely results from increased Vitamin K levels in people with the ‘T’ allele. As described earlier, warfarin works by preventing the liver from using Vitamin K to make blood clotting factors. Higher tissue levels of Vitamin K would therefore necessitate higher dosages of warfarin to overcome this.

One potential risk of warfarin treatment is the risk of major bleeding as the ability of blood to clot is reduced. Some evidence suggests that people with the ‘T’ allele of the CYP4F2 gene are at a lower bleeding risk when using warfarin. For example, a study of 570 patients in the community setting found that those with the ‘T’ allele had a 38% lower risk of major bleeding.

Again, this effect may be due to higher levels of Vitamin K in ‘T’ allele carriers, which is used to make and activate blood clotting factors.  

 

KEY POINTS

• The ‘T’ allele of the CYP4F2 gene is associated with a higher warfarin dose in the initial stages of treatment.
• The ‘T’ allele of the CYP4F2 gene is linked to a lower risk of major bleeding when using warfarin.

 

Your Vitamin K breakdown trait

Your Vitamin K breakdown trait analyses your CYP4F2 genotype and classifies your CYP4F2 enzyme levels and rate of Vitamin K breakdown. You will fall into one of three categories:

 

• Reduced Vitamin K breakdown (associated with highest Vitamin K levels and higher warfarin dosage)
  
• Moderately reduced Vitamin K breakdown (associated with intermediate Vitamin K levels and higher warfarin dosage)
  
• Normal Vitamin K breakdown (associated with lowest Vitamin K levels)

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