Vitamin K2: Bone Health, Arterial Calcification, and MK-7 vs MK-4

Vitamin K was discovered in 1929 as a clotting factor, and for most of the past century that is what it remained: a nutrient important for blood coagulation, found in leafy greens, with a recommended intake set at about 120 micrograms per day for men and 90 for women. In the last 20 years, however, a separate role has emerged that the original RDA does not capture. Vitamin K, particularly in the K2 form, is required to activate a family of proteins that move calcium out of soft tissues and into bone and teeth. The two most cited trials on this are Gast and colleagues' 2009 study on bone health in postmenopausal women, published in the British Journal of Nutrition, and Beulens and colleagues' 2009 study on arterial calcification in healthy women, published in Atherosclerosis. Both used menaquinone-7 (MK-7), a long-chain form of vitamin K2, and both found meaningful effects. The picture is now clear enough that the question is no longer whether K2 matters but how much, in what form, and for whom.

Vitamin K1 vs K2 vs MK-4 vs MK-7

Vitamin K is a family of compounds with a shared naphthoquinone ring and different side chains. The two main natural forms are:

• Vitamin K1 (phylloquinone) is found in green leafy vegetables (kale, spinach, collards, broccoli). It is the dominant dietary form in the typical Western diet (typically 75 to 90 percent of total K intake) and is preferentially taken up by the liver for use in clotting factor activation. K1 has a short half-life (about 1 to 2 hours) and is relatively inefficient at reaching extrahepatic tissues.
• Vitamin K2 (menaquinones) is found in fermented foods and some animal products. The menaquinones are subdivided by the length of their side chain, abbreviated MK-4 through MK-13. The two that matter for supplementation are MK-4 (short chain, found in pastured dairy, egg yolks, liver, and produced endogenously from K1) and MK-7 (long chain, found in natto, the Japanese fermented soybean dish, and in most K2 supplements).

The pharmacokinetics differ in important ways. K1 is efficiently absorbed from vegetables only when consumed with fat (the vitamin is fat-soluble), and it is mostly delivered to the liver. MK-4 has a very short half-life (about 1 hour) and requires multiple daily doses to maintain blood levels. MK-7 has a half-life of about 72 hours, reaches steady state with once-daily dosing, and is efficiently delivered to extrahepatic tissues including bone and arteries. This is why most modern K2 supplements use MK-7 rather than MK-4.

The calcium paradox and matrix Gla protein

The "calcium paradox" is the observation that women with the highest calcium intake often have the most arterial calcification, despite calcium being prescribed for bone health. The paradox is resolved by vitamin K2. Two vitamin K-dependent proteins, osteocalcin (in bone) and matrix Gla protein (MGP, in arteries), are produced in inactive forms and must be carboxylated by vitamin K to bind calcium. Osteocalcin, when carboxylated, draws calcium into bone mineral. MGP, when carboxylated, scavenges calcium from arterial walls and prevents calcification.

Without adequate vitamin K2, both proteins remain in their undercarboxylated (inactive) forms. Osteocalcin cannot deposit calcium in bone, so the calcium stays in the blood. MGP cannot remove calcium from arteries, so the calcium deposits in arterial walls. The result is the paradox: calcium builds up where it should not (arteries) and is depleted where it should be (bone). The Rotterdam Study, a large prospective cohort in the Netherlands, found that men in the highest tertile of menaquinone intake (mostly from Gouda and Edam cheese) had 51 percent lower risk of severe arterial calcification and 26 percent lower risk of all-cause mortality over 10 years compared to the lowest tertile. K1 intake had no association with these outcomes.

The Gast 2009 bone trial

Gast and colleagues published one of the most cited K2 trials in 2009 in British Journal of Nutrition. They randomized 325 healthy postmenopausal Dutch women to 180 micrograms per day of MK-7 (as menaquinone-7) or placebo for 3 years. The primary outcomes were bone mineral density at the lumbar spine and hip, and the secondary outcomes included bone mineral content, bone strength indices, and measures of undercarboxylated osteocalcin.

The findings:

• Bone mineral content at the femoral neck declined less in the MK-7 group than placebo, with a significant between-group difference.
• Bone strength indices at the femoral neck declined less in the MK-7 group.
• Undercarboxylated osteocalcin dropped by about 50 percent in the MK-7 group versus placebo, indicating that the mechanism (carboxylation of osteocalcin) was engaged.
• Bone mineral density at the lumbar spine did not differ significantly between groups, though the trend favored MK-7.

The Gast trial is important because it shows a clinically meaningful effect of MK-7 at a moderate dose (180 micrograms per day) over a meaningful timeframe (3 years). The effect on bone strength indices at the femoral neck is particularly relevant because femoral neck fracture is the most devastating consequence of osteoporosis. The 50 percent reduction in undercarboxylated osteocalcin confirms that the dose is sufficient to engage the mechanism, which is the strongest evidence we have that bone benefit is causal rather than coincidental.

The Beulens 2009 arterial calcification trial

Beulens and colleagues published a complementary trial in 2009 in Atherosclerosis, focusing on the arterial side of the calcium paradox. They randomized 564 healthy postmenopausal Dutch women to 180 micrograms per day of MK-7 or placebo for 3 years. The primary outcome was arterial stiffness, measured by carotid-femoral pulse wave velocity (a gold standard measure of arterial stiffness), and the secondary outcomes included measures of vascular calcification.

The findings:

• Arterial stiffness (pulse wave velocity) increased less in the MK-7 group than placebo over 3 years, with a significant between-group difference.
• Vascular calcification measures trended in the favorable direction but did not all reach statistical significance.
• Undercarboxylated MGP dropped in the MK-7 group, confirming the mechanism.

The Beulens trial was modestly powered but consistent with the Rotterdam cohort data and with the broader K2 literature. The effect on pulse wave velocity is meaningful because arterial stiffness is an independent predictor of cardiovascular events, comparable in magnitude to systolic blood pressure. A 1 meter per second reduction in pulse wave velocity is associated with about a 10 percent reduction in cardiovascular events in meta-analyses.

Food sources

Dietary vitamin K2 is concentrated in a small number of foods, most of which are uncommon in the typical Western diet:

• Natto (fermented soybeans) is by far the richest source, with about 1,000 micrograms of MK-7 per 100 g. It is a daily staple in parts of Japan but a rarity elsewhere, mostly because of its strong flavor and slimy texture.
• Pastured egg yolks contain about 30 to 50 micrograms of MK-4 per yolk. Conventionally raised chickens produce eggs with much less K2 because the diet is lower in K1 to convert.
• Goose liver and other organ meats from pastured animals contain substantial MK-4.
• Aged hard cheeses (Gouda, Edam, Brie) contain 50 to 100 micrograms of MK-7 and MK-8 per 100 g, the second-best dietary source after natto.
• Butter and full-fat dairy from pastured animals contain modest amounts of MK-4.
• Sauerkraut and other fermented vegetables contain some K2, mostly as MK-7.

The NIH Office of Dietary Supplements notes that K1 intake in the United States averages about 100 micrograms per day (mostly from vegetables), and K2 intake is much lower, typically 20 to 40 micrograms per day, mostly from cheese, eggs, and meat. Most Americans are not eating natto, and most modern dairy is not pastured. Use the Calcium Calculator alongside any K2 protocol, because calcium without K2 may be the worst of both worlds.

Dosing

The dosing in trials:

• For bone and arterial health: 90 to 180 micrograms per day of MK-7. The Gast and Beulens trials used 180 micrograms per day for 3 years. A common commercial dose is 100 micrograms per day, which is sufficient to fully carboxylate osteocalcin and MGP in most people.
• For osteoporosis support (under medical supervision): The Japanese osteoporosis treatment protocol uses 45 mg per day of MK-4 (not micrograms), which is 500 times the MK-7 dose. MK-4 at pharmacological doses has trial support for fracture reduction in Japanese populations, but the doses are impractical and MK-7 is the better choice for routine supplementation.
• For general support: 100 to 180 micrograms per day of MK-7, taken with a meal containing fat. K2 is fat-soluble and absorption improves dramatically with fat.

Pair K2 with vitamin D and calcium for the strongest bone effect. Vitamin D increases intestinal calcium absorption, K2 directs the absorbed calcium to bone, and calcium provides the raw material. The combination is more effective than any alone. Use the Vitamin D Calculator to make sure you are getting enough vitamin D, and the Calcium Calculator for calcium intake.

Safety and drug interactions

Vitamin K2 at typical supplemental doses (under 200 micrograms per day of MK-7) is safe and well tolerated. The main cautions are:

• Warfarin (Coumadin). Warfarin works by inhibiting vitamin K-dependent clotting factor activation. Adding vitamin K2 (or K1) can reduce warfarin's anticoagulant effect and increase clot risk. People on warfarin should not take K2 supplements without consulting their cardiologist or anticoagulation clinic. The newer direct oral anticoagulants (apixaban, rivaroxaban, dabigatran) do not interact with vitamin K and are not a concern.
• High-dose vitamin A or E. These vitamins can interfere with K2 absorption at high doses. Keep vitamin A under 10,000 IU and vitamin E under 400 IU per day if supplementing K2.
• Bile acid sequestrants and orlistat. These reduce fat absorption and can reduce K2 absorption.

K2 does not appear to cause excessive clotting in people with normal clotting function. The clotting system is rate-limited by other factors, and adding more K2 above what is needed for clotting factor carboxylation simply activates more osteocalcin and MGP. This is why the recommended dose is in the microgram range, not the milligram range.

Who should consider K2 supplementation

The strongest case for K2 supplementation is for:

• Postmenopausal women at risk of osteoporosis or arterial calcification. The Gast and Beulens trial populations are the template.
• People on long-term vitamin D and calcium supplementation. The calcium paradox is most relevant here: taking calcium without K2 may accelerate arterial calcification.
• People who do not eat fermented foods, aged cheese, eggs, or natto. K2 intake is low in most Western diets.
• People with established osteoporosis or low bone density, in conjunction with a bone health program (weight-bearing exercise, adequate protein, vitamin D, calcium, and medications if prescribed).
• People with elevated coronary artery calcium scores, in conjunction with comprehensive cardiovascular risk reduction.

K2 is not a stand-alone treatment for osteoporosis or cardiovascular disease. It is an adjunct that addresses one specific mechanism (calcium misallocation) on top of the foundational interventions. Discuss with your doctor before adding K2 if you have cardiovascular disease, kidney disease, or are on warfarin.

Common pitfalls

Pitfall 1: Confusing K1 and K2

Most multivitamins list "vitamin K" and provide only K1 (phylloquinone). K1 is sufficient for clotting but does not effectively activate the extrahepatic proteins (osteocalcin, MGP) that drive bone and arterial benefits. Look specifically for "vitamin K2" or "menaquinone-7" or "MK-7" on the label.

Pitfall 2: Taking calcium without K2

This is the calcium paradox in action. Calcium supplements without K2 may increase arterial calcification while doing less than expected for bone. Always pair calcium with K2 and vitamin D. The Gast trial used 180 micrograms of MK-7 alongside dietary calcium.

Pitfall 3: Buying MK-4 instead of MK-7

MK-4 has a short half-life (1 hour) and requires multiple daily doses. Most MK-4 supplements provide a tiny fraction of the 45 mg per day used in Japanese osteoporosis trials. MK-7 at 100 to 180 micrograms per day is the better choice for routine supplementation because of its long half-life and proven efficacy in trials.

Pitfall 4: Taking K2 while on warfarin

Warfarin's mechanism is vitamin K antagonism. Adding K2 can reduce its anticoagulant effect and increase clot risk. If you are on warfarin, do not take K2 supplements without your anticoagulation clinic's supervision. The newer direct oral anticoagulants do not interact with K2.

The bottom line

Vitamin K2, particularly as MK-7, is required to activate the proteins that move calcium into bone (osteocalcin) and out of arteries (matrix Gla protein). Without adequate K2, calcium supplements may accelerate arterial calcification while doing less than expected for bone, the "calcium paradox." The Gast 2009 trial showed that 180 micrograms per day of MK-7 for 3 years improved bone mineral content and strength in postmenopausal women. The Beulens 2009 trial showed that the same dose reduced arterial stiffening in the same population. The effective dose for routine supplementation is 100 to 180 micrograms per day of MK-7, taken with a meal containing fat. Pair with vitamin D and calcium for the strongest bone effect, and avoid K2 if you are on warfarin. Most Americans have low K2 intake because natto, pastured dairy, and aged hard cheeses are uncommon in the typical diet, which makes supplementation a reasonable consideration for people at risk of osteoporosis or cardiovascular calcification.