by Emily Finnan, RD
10 years ago, vitamin K2 was largely unheard of. Today, it’s a top Google search term, the subject of numerous books, and over 500 supplements are sold on Amazon. In part, due to a growing number of vitamin K2 supporters who champion it as a necessity for bone and heart health. However, 76 years after its discovery, it seems we still have more questions than answers about this important nutrient.
What is it?
Vitamin K2 isn’t a new nutrient; it’s simply a form of vitamin K. Vitamin K is a term for a group of essential
compounds that all contain the chemical structure methyl-1,4-napthoquinone. This group can be further divided into vitamin K1, K2 and K3. Vitamin K2, or menaquinones, is a term for several compounds named MK-4 through MK-13.
Where is it?
Vitamin K2 is predominantly made by bacteria. It’s found in fermented foods and animal products.
MK-7 and MK-4 are the two most talked about and studied forms of vitamin K2. MK-7 is the form found in Natto, a Japanese fermented soy product. MK-4 is the form found in animal products. Additionally, your body likely makes MK-4 from vitamin K1 eaten. The other “MKs” are made by different strains of bacteria found in fermented foods or in your gastrointestinal tracts. It’s debated, but likely a minimal amount of vitamin K from the gut is actually absorbed and used by your body.
Vitamin K1, or phylloquinone, is made by plants. It’s found in a variety of vegetables, some fruits, and vegetable oils. Leafy greens are an especially good source. 90% of the vitamin K we eat is in this form.
Vitamin K3, or menadione, is a synthetic precursor of vitamin K. It isn’t recommend for humans, but it is used in animal feed.
What about grass-fed?
Blogs that tout the benefits of vitamin K2 likely recommend grass-fed animal products as the premier source. Grass does contain vitamin K1. But a cow’s primary source of vitamin K comes from large colonies of K2-producing bacteria that live in their ruminant stomachs. Conventionally raised livestock are frequently given antibiotics, which can diminish gut bacteria. However, livestock feed is typically fortified with vitamin K3, which the animal directly converts o MK-4.
MK-4 is present in conventionally-raised dairy, beef, poultry, and other animal-based foods. A study conducted in the Netherlands, found no substantial difference in MK-4 content between wild, free-range, and “intensively raised” meat, dairy, and eggs. Currently, there isn’t evidence to support grass-fed animals as a superior source of MK-4.
What does it do?
All forms of vitamin K help carboxylate (add extra acid groups) to certain proteins, which helps the proteins’ function. Un-carboxylated vitamin K-dependent proteins are those that vitamin K has not acted on.
Vitamin K & blood clotting
This is vitamin K’s most studied role. Vitamin K is essential for proper blood clotting. A person with a severe vitamin K deficiency, which is rare, will have clotting problems.
Vitamin K & bone
Vitamin K carboxylates the bone protein, osteocalcin, allowing it to act on bone. This has led to the hypothesis that a high level of un-carboxylated osteocalcin is an indicator of vitamin K insufficiency and poor bone health. Vitamin K2 and K1 have been shown to increase osteocalcin carboxylation. Additionally, researchers have found both inside bone.
Two large Japanese observational studies, totaling almost 3,000 people, found positive associations between dietary MK-7 and increased bone mineral density. However, observational trials can’t determine causation. People who eat more vitamin K, might have a healthier diet and lifestyle; especially because vitamin K is found in typically healthful foods.
Randomized controlled trials (RCT) can help determine causation. 11 RCTs have been conducted with 15 to 45 milligram (or 15,000 to 45,000 micrograms) MK-4 supplements. The majority do report that the MK-4 supplement group had a positive result in at least one marker of bone health. In Japan, where most of these trials were conducted, MK-4 supplements are routinely used as part of osteoporosis treatment. Of note, these doses of vitamin K are much higher than you can obtain from food. Vitamin K is therefore being used as a medication, not as a dietary factor.
RCTs and observational trials conducted using vitamin K1 are inconclusive.
Vitamin K & vascular calcification
Vitamin K may have a role in preventing vascular calcification, a major risk factor for heart disease. This is through vitamin K’s carboxylation of matrix Gla-protein (MGP). It’s not fully understood, but un-carboxylated MGP may increase vascular calcification. Vitamin K1 and MK-4 both reduce un-carboxylated MGP.
Only one observational, cohort study has shown a positive association between total dietary vitamin K2 intake and reduced vascular calcification. Observational studies using vitamin K1 intake show no effect. An RCT, conducted at the Jean Mayer USDA Human Nutrition Research Center on Aging (HNRCA), found that vitamin K1 supplementation did slow progression of calcification in those with pre-existing coronary artery calcification.
Do I need it?
The optimum level of vitamin K in the diet is unclear. The adequate intake (AI) of vitamin K, for men 19 years and older, is 120 micrograms (mcg). This was based simply on the amount of vitamin K healthy people eat. The AI doesn’t specify targets of vitamin K1 versus K2. It’s been suggested that the amount of vitamin K needed to prevent clotting problems is less than 10 mcg per day; but at least 1,000 mcg per is needed for optimum bone density.
Below is a table of vitamin K content in various foods. The Vitamin K1 data is predominately from the USDA Nutrient Database. Vitamin K2 data was obtained from three individual studies: here, here, and here.
Many books and health blogs (here, here, here, and here) claim that the US population is widely deficient in vitamin K2, which they report is specifically essential for bone and vascular health. However, there is a lot more we need learn about vitamin K2. Do vitamin K2 and K1 actually have different functions in our body? If we can make vitamin K2 from K1, does it even matter how much K2 we eat? We don’t know what a sufficient level of vitamin K2 is, let alone a deficient level, or even the best biomarker of K2 status. Furthermore, if 1,000 mcg is the true optimum intake then it seems it would be much easier to reach this level by focusing on vitamin K1 sources rather than K2- you’d need to eat 7 pounds of blue cheese or 300 eggs a day to reach 1,000 mcg!
The good news is that a varied diet that includes variety of vegetables, leafy greens, as well as meats and dairy can supply a person with well over the AI of vitamin K. There is also no known harm of taking high-dose vitamin K supplements. My advice: eat a varied diet that includes servings of vitamin K-rich vegetables and fermented foods. These foods are great for other reasons too– high in other important micronutrients and fermented foods contain beneficial probiotics. If you’re thinking about taking a vitamin K2 supplement, talk to your doctor as vitamin K does interact with some medications.
Emily Finnan is a dietitian and finishing her first year in the Biochemical and Molecular Nutrition master’s program. She’ll be getting acquainted with vitamin K this summer, completing a practicum in the HNRCA’s vitamin K laboratory.