If you’ve researched the vitamins you need for healthy bones, you’ve probably come across vitamin K. And you might agree that this is not a simple vitamin.
I see it debated on social media. How much vitamin K do you really need for bone health, and what’s the best way to get it? And so many different forms. Do we need to get them all? We’ll explore these things as we follow the vitamin K roadmap.
Table of Contents
It’s not just one vitamin: All the forms of vitamin K
You might say vitamin K has an identity crisis. Instead of being just one vitamin, there are 3 forms of vitamin K. And one of those forms has about 10 subtypes. Let’s dig in a little deeper and discover what they are, and what they do in your body.
K1
Phylloquinone, also known as K1, is the form we are most familiar with. The US RDA of 90-120 mcg refers to K1. Plants produce vitamin K1. You can get large amounts from green vegetables.
K2
Menaquinone is another form of vitamin K, also called K2. And here’s the catch – there are at least 14 different types of menaquinones. K2 is found in animal tissues (meat, milk and eggs), and in some fermented foods.
The different forms of menaquinone are named the following way: M (for menaquinone), K (for vitamin K) and a number from 4-13. The number tells how many isoprene units are attached to the main body of the vitamin (1).
To simplify it, just know that as the numbers get higher, the MK molecules are getting longer and longer.
The forms of MK with the most research behind them are MK4 and MK7. We’ll talk more about these in a sec, as they are important for bone health.
K3
Menadione, also known as MK3, is the third form of vitamin K. It is an intermediate unit formed in the body when vitamin K is metabolized. Menadione is sometimes used in animal feed. Humans don’t need to consume K3, so we won’t worry about that one.
Vitamin K’s One Job
Vitamin K has one job – to carboxylate certain proteins in your body. Let me explain what that means.
There are 18 or 19 important proteins (we know about so far) that are missing a piece of their puzzle (2). In order to get busy and do their job, they need a tiny chemical piece called a carboxyl group. Vitamin K comes in and sticks that carboxyl group onto those proteins.
Think of the proteins as construction workers waiting around for a special piece of equipment that they need – the carboxyl generator! Vitamin K rushes in to save the day and deliver that generator.
What do these construction workers (proteins) do in your body? One of their crucial jobs is blood clotting. After an injury, they complete the clotting process to stop the bleeding.
Another function is bone building. Osteocalcin (also called Bone Gla Protein, or BGP), is in charge of attaching minerals, like calcium, to your bone matrix. Vitamin K activates osteocalcin.
Yet another protein that needs vitamin K is Matrix Gla Protein, or MGP. This one prevents calcium from accumulating in your arteries, and directs it towards your bones.
All the forms of vitamin K can do the job of delivering a carboxyl group and sticking it onto the necessary proteins (3). But there are still some key differences between the forms of vitamin K.
Metabolism and Storage of Vitamin K
Let’s look at how the different forms of vitamin K are metabolized, and where they end up in your body.
Digestion and Absorption of Vitamin K
All forms of vitamin K that you consume, through either food or supplements, are absorbed in your small intestine. Since vitamin K is a fat soluble vitamin, this process requires bile. You secrete bile from your gall bladder to break down fats and package them up into a form that can be absorbed (3).
Vitamin K is packaged into a little droplet called a micelle. Once inside your intestinal cells, the micelles are packed up into a larger package called a chylomicron. These pass through the gut lining into the lymphatic system, along with other fats that you eat. Eventually vitamin K makes its way into your bloodstream.
This is where the different forms of vitamin K part ways.
K1
The liver rapidly clears K1 from the bloodstream. The liver produces blood clotting proteins. Since this process is so important for your survival, and it needs vitamin K, the liver soaks up most of the K1. About 80-90% of K1 in your bloodstream goes to the liver (4).
After the liver is satisfied, any leftover K1 is taken up by tissues and converted to MK4. We’ll talk more about that process in a sec.
MK4
The liver takes up a small amount of MK4. However, most of MK4 goes directly to your tissues. Your brain, testes, kidneys, and pancreas contain high concentrations of MK4. MK4 is cleared from the bloodstream and into various tissues at a fairly rapid rate (3).
MK7
MK7 takes longer to clear from your bloodstream. Because of its longer side chain, MK-7 travels around your bloodstream incorporated into low density lipoproteins (LDL) and very low density lipoproteins (VLDL).
MK7 can circulate in your blood for 3 or 4 days, giving it plenty of time to reach tissues such as your bone. Your cells probably absorb it through an LDL receptor (4)(5).
How Vitamin K is Stored as MK4
Any form of vitamin K can be converted and stored as MK4 in your tissues. Certain parts of the body like the brain, pancreas, arteries and bones, prefer the structure of MK4 to other forms of vitamin K (6).
Here’s how the conversion happens (6).
- The tail of either K1 or an MK form is cleaved off, leaving K3, or menadione.
- Menadione is rebuilt into MK4 by adding on a molecule called Geranylgeranyl Pyrophosphate (GGPP).
- This reaction requires either magnesium or manganese, and vitamin B3 (niacin).
This doesn’t apply to every single vitamin K molecule. The liver still retains plenty of K1, and MK7. Other long-chain menaquinones are present in some tissues (6).
Since a fair amount of MK7 is converted to MK4, research suggests that eating MK7 or taking it as a supplement is a more efficient way to raise your MK4 levels than actually taking MK4 (7).
A key takeaway here is that you need magnesium (or manganese) and vitamin B3 to make the MK4 conversion happen.
Food Sources of Vitamin K1 and K2
Plants produce vitamin K1. The best way to get this vitamin is by eating lots of green vegetables, especially leafy greens.
Animals make MK4, so you can get it from meat. Chicken, beef and other meats all contain MK4, but not in very high amounts. Eggs can be a good source of MK4 if the chickens are free range, or their feed contains menadione.
It’s hard to get a lot of MK4 from foods, and maybe not as necessary, since the other forms can be converted to MK4 in your cells.
Fermented dairy products, such as cheese and yogurt, contain MK7 and other long chain menaquinones. You get small amounts of MK7 in fermented vegetables like sauerkraut.
The best way to get MK7 from foods is by eating a fermented soybean dish called natto.
Although MK4 and MK7 get the most airtime, there are many other menaquinones present in fermented food, often in much higher amounts. For instance, MK8 and 9 are the dominant forms in fermented dairy. MK10-MK12 are also present in cheese, milk, yogurt and kefir (8)(9).
Read more about the foods that contain different forms of vitamin K.
Research on Vitamin K and Bone Health
Evidence for K1
In 2008, a study was done to determine whether a high dose of vitamin K1 could safely reduce bone loss, bone turnover, and fractures. The subjects were postmenopausal women who already had osteopenia. The study ran for two years with another two years of followup. They found that 5 mg of K1/day did not protect against age-related decline in bone mineral density, but it did result in a significantly lower fracture rate in postmenopausal women (10).
In 2017, Hao et al conducted a meta-analysis that looked exclusively at phylloquinone (K1) intake and risk of fracture. The study represented 80,982 total subjects and 1114 fracture cases. The subjects with the highest intake of dietary vitamin K were found to have a 22% reduction in the risk of fractures. when compared with those who had the lowest intake. The highest level of intake in the included studies was 200-254 mcg/day.
The study established that the risk of fracture was reduced by 3% for every 50 mcg increase in dietary vitamin K. This is true up to about 200 mcg/day. After that amount they didn’t see an extra benefit for reducing fracture risk (11).
Takeaway: Vitamin K1 may strengthen bones and reduce fracture risk without necessarily increasing bone density. The optimal amount looks like around 200 mcg/day.
Find out how to get 200 mcg K1 from foods.
Evidence for MK4
Of all the forms of vitamin K, MK4 has shown the strongest evidence for increasing bone mineral density.
There have been numerous human studies done on MK4, mostly in Japan. A study to determine proper dosage found that 45 mg was the minimum effective dose for improving bone mass in postmenopausal women with osteoporosis (12).
Consequently, 45 mg of menatetrenone (MK4) has been used as a pharmaceutical treatment for osteoporosis in Japan since 1995.
Can you still get a benefit from MK4 at lower doses? The research suggests that you can.
Several studies have demonstrated that a dose of 1.5-5 mg of MK4/day can quickly improve the activation of osteocalcin, sometimes in just a few weeks. A dose of 1.5 mg/day has also been shown to improve bone mineral density in the forearm (13)(14)(15).
This is still a high-ish dose, since we measure vitamin K in micrograms (mcg) and the dose they studied is in milligrams (mg), which is 1000x greater.
Takeaway: MK-4 has strong evidence for supporting bone health, with higher (pharmacological) doses improving bone mass and lower doses shown to activate osteocalcin and, in some studies, improve BMD.
Evidence for MK7
In a randomized controlled trial published in 2013, 244 healthy postmenopausal women received a low dose of MK7 (180 mcg) vs placebo for 3 years. MK7 significantly decreased the rate of decline in bone mineral content and bone mineral density at the lumbar spine and femoral neck. MK7 also decreased the loss in vertebral height compared to the placebo (16).
To be clear, MK7 didn’t improve bone density or even stop decline of bone density. It just slowed how quickly the decline happened.
The decreased loss of vertebral height, may indicate a stronger spine. This suggests MK7 may improve bone strength independently of bone mineral density.
A 2016 randomized controlled trial gave a higher dosage, 375 mcg MK7, to women with osteopenia. The study lasted 12 months. Results showed that trabecular bone structure at the tibia was preserved in the group that received MK7, but not in the placebo group (17). Once again there was no improvement in bone mineral density, but there was a preservation of bone already there.
A 3 year trial conducted in 2021 also used a higher dose of 375 mcg MK7. In postmenopausal women with osteopenia, 375 μg daily as an add-on to calcium and vitamin D increased the carboxylation of osteocalcin, the protein that adds minerals to your bone matrix.
Aside from that, the study showed no added benefit for the increased dosage. There was no significant improvement in biochemical markers of bone turnover, bone mineral density, or bone microarchitecture (18).
Takeaway: MK-7 may help preserve bone quality and slow age-related bone loss, even when it doesn’t increase bone mineral density—suggesting potential benefits for bone strength and structure beyond what a DEXA scan can measure.
Assessing Vitamin K Status
How do you know whether you are getting enough vitamin K? Just looking at the amount in your bloodstream is not a good indicator. It mainly reflects what you ate in your last meal, and not how much is available in your cells.
There are ways to measure the amount of vitamin K that is available for blood-clotting proteins. However, this doesn’t tell you whether you have an optimal amount for your bones.
Doctors can also look at under-carboxylated osteocalcin as a measure of vitamin K deficiency. Blood tests can measure serum undercarboxylated osteocalcin (ucOC), and the ucOC:total OC ratio. However this test is not standardized very well. There are many variables that can affect results. As a stand alone test it isn’t a practical way to assess vitamin K status (19).
As a Functional Nutritionist, I assess your vitamin K status by looking at how much vitamin K you are getting from foods and supplements. You can combine this information with labs such as ucOC, bone turnover markers and other metabolic testing to get an overall picture of where you stand with your bone health. Remember, vitamin K is just one small part of an overall picture.
Make a plan to get your vitamin K
The RDA for vitamin K for an adult female is 90 mcg, and for a male is 120 mcg. However, this is the amount needed for normal blood coagulation, not necessarily to optimize your bone formation (20). And it doesn’t take into consideration all the different forms of vitamin K.
How much K1 do you need?
Research suggests that we need about 200 mcg of K1 to support bone health. You can easily get this from eating one or two servings of leafy green vegetables every day. Find out which foods are best for K1.
How much MK4 do you need?
The lowest effective dose of MK4 to activate osteocalcin is 1.5-5 mg. Since we usually measure vitamin K in micrograms, even 1.5 milligrams is a substantially higher dose, which you aren’t likely to get in foods.
Do you need to supplement with this amount?
Since all the other forms of vitamin K are ultimately converted to MK4 in your body, I don’t feel it’s as important to take this supplement. But if you want to cover your bases, MK4 is a safe supplement even at very large doses.
How much MK7 do you need?
Evidence is not as robust for MK7 improving bone density, but it may still help preserve bone quality and slow bone loss. MK7 stays in circulation longer, and has a better chance of reaching bone tissue.
Because MK7 can be converted to MK4, research even suggests that eating MK7 or taking it as a supplement may be a more efficient way to raise your MK4 levels than actually taking MK4 (7).
The lowest effective dose of MK7 is 180 mcg, so that is a good target for supplementation. Keep in mind though that if you eat some other foods containing MK7, you may not need to supplement as much. Link to blog.
Possible side effects of MK7
We generally consider vitamin K to be a safe vitamin. The US guidelines do not set a Tolerable Upper limit for any form of vitamin K. Neither do the Institute of Medicine’s (IOM), the European Commission or the WHO/FAO.
The research has tested high doses of MK7 and found no toxic effects. However, so far this is all done with animal research, not humans.
Nevertheless, some people do report significant side effects from MK7. These include anxiety, sleeplessness, a thumping heart beat, and increased blood pressure. Others experience headaches, rash, fatigue, nausea and diarrhea. Symptoms can build up slowly over time, since MK7 has a 3 day half-life.
This may be due to the fact that MK7 is metabolized and transported through the body differently from the other forms of vitamin K.
If you are taking MK7 and experience any of these symptoms, stop taking it for at least 3-4 days to give the vitamin time to be cleared from your blood. Then you can try a small amount, and gradually build it up to see if symptoms return.
If you take anticoagulant medications like Warfarin or Coumadin, don’t start taking MK7, or any form of vitamin K for that matter, without checking with your doctor. Amounts as low as 10–20 μg MK7 can rapidly destabilize the control of therapeutic anticoagulant treatments (22).
Should you take vitamin K with or without foods?
Take all forms of vitamin K with food, specifically with a meal that contains some fat. This is because fat stimulates the release of bile acids from your gallbladder into your gut.
Vitamin K is absorbed in tiny micelles along with fats from your meal, with the help of bile acids (2).
How much fat do you need for your body to release bile? A meal with 10 grams of fat gets a very efficient response from your gallbladder (22). One tablespoon of oil contains about 14 grams of fat, which is more than enough.
A Wrap Up of Vitamin K
There are 3 forms of vitamin K: phylloquinone (K1), menaquinone (K2) and menadione (K3). Menaquinone has at least 10 different variations in its structure.
All the forms of vitamin K have one basic job – to carboxylate various proteins. One of these proteins is osteocalcin, which is responsible for attaching calcium to the hydroxyapatite matrix of your bones.
After vitamin K is digested and absorbed into your bloodstream, the different forms follow their own separate paths. K1 mostly ends up in the liver. Different organs and tissues quickly absorb MK4 from your bloodstream. MK7 hitches a ride on LDL particles and travels around your body for a longer time.
Your cells can covert all the forms of vitamin K to MK4.
MK4 has the strongest evidence that it can increase bone mineral density. K1 and MK7, have also been shown to slow bone loss, help preserve bone quality, and decrease fracture risk.
Some people experience side effects from MK7, so it’s best to be cautious if you are taking it for the first time. Also, if you are on blood thinning medications, don’t start a vitamin K supplement or increase your vitamin K without talking to your doctor first.
Are you currently taking supplements of vitamin K, or trying to get it in your diet? Let me know in the comments.