Vitamin K2 is a form of vitamin K, the menaquinones, that directs calcium toward bone rather than into artery walls. Produced by gut bacteria and found in a handful of fermented foods, it has drawn research interest in bone and cardiovascular health since the 2000s. This article reviews what the evidence shows, the doses used in clinical trials, and the precautions to know, particularly for people taking blood thinners.
In brief – Vitamin K2 is a fat-soluble vitamin that activates proteins responsible for directing calcium into the skeleton and limiting its buildup in artery walls. It comes in two main forms, MK-4 (meat, egg yolk, liver) and MK-7 (natto, some aged cheeses), with different absorption profiles. A controlled trial in 244 postmenopausal women found that 180 mcg per day of MK-7 for three years slowed bone loss and improved arterial stiffness compared with placebo. The Rotterdam Study also linked higher dietary K2 intake to lower cardiovascular mortality. It antagonizes vitamin K antagonist blood thinners such as warfarin, so anyone on these medications should tell their doctor before supplementing.
Definition: What Is Vitamin K2?
Vitamin K2 refers to the menaquinones, a subgroup of the vitamin K family distinct from vitamin K1 (phylloquinone), which comes mainly from green vegetables. The liver relies mostly on vitamin K1 for blood clotting, while K2 circulates toward bone, arteries and other peripheral tissues. Menaquinones come in several chain lengths, labeled MK-4 through MK-13 depending on the number of isoprenoid units; MK-4 and MK-7 are the two forms most studied and most present in food or supplements.
The liver converts some vitamin K1 into MK-4, but most menaquinones come from food and from gut bacteria, which produce variable amounts from one person to another. This mixed origin is why K2 intake is difficult to standardize across individuals.
Mechanism: How Does Vitamin K2 Work in the Body?
It acts as a cofactor for an enzyme, gamma-glutamyl carboxylase, which activates two key proteins through carboxylation:
- Osteocalcin, produced by osteoblasts, which binds calcium to the bone matrix once activated.
- Matrix Gla protein (MGP), which blocks calcium deposits in blood vessel walls when carboxylated.
- Gas6 proteins, involved in cell regulation and less well documented in humans.
Without enough K2, these proteins stay under-carboxylated: osteocalcin binds calcium to bone less effectively, and MGP protects arteries against calcification less efficiently. This dual mechanism, in bone and in blood vessels, has driven K2 research for the past two decades.
What the Science Says About K2
The Rotterdam Study followed 4,807 adults for about ten years and compared tertiles of dietary menaquinone intake. Participants in the highest tertile had a relative risk of coronary mortality of 0.43 compared with the lowest tertile (95% CI: 0.24 to 0.77), along with less severe aortic calcification. Vitamin K1 intake, by contrast, was not linked to either outcome (Geleijnse et al., J Nutr 2004). This is an observational study: it shows an association, not proof of cause and effect.
On the bone side, a randomized, double-blind trial followed 244 postmenopausal women for three years, split between placebo and 180 mcg of MK-7 per day. After three years, the supplemented group had slower bone loss and less vertebral height reduction than the placebo group (Knapen et al., Osteoporos Int 2013).
The same cohort of women was also used to assess arterial stiffness through carotid echo-tracking. MK-7 supplementation improved local distension and stiffness indices compared with placebo over the course of the trial (Knapen et al., Thromb Haemost 2015). These results apply to healthy postmenopausal women; they do not automatically extend to other populations, and separate trials in dialysis or kidney failure patients have not found as clear a benefit on vascular calcification. According to Dr. Belghiti, “the consistency between the biological mechanism of MGP and the clinical results on osteoporosis makes the vitamin K2 lead solid, but it does not remove the need for a full iron and vitamin panel before any supplementation.”
In Practice: When and Why Consider This Vitamin?
This vitamin mainly interests postmenopausal women being monitored for bone density loss, as an addition to standard care (calcium, vitamin D3, weight-bearing exercise) rather than a replacement for it. It also appears in protocols combining high-dose vitamin D3, where it limits the theoretical calcification risk linked to improved intestinal calcium absorption. That mechanistic reasoning, however, is less supported by clinical trials than the direct effect on bone.
MK-4 and MK-7 differ in origin, half-life in the blood and the doses used in research:
| Characteristic | Menaquinone-4 (MK-4) | Menaquinone-7 (MK-7) |
|---|---|---|
| Main source | Meat, liver, egg yolk, partial liver synthesis | Natto, some aged cheeses, bacterial fermentation |
| Plasma half-life | A few hours | Several days |
| Dose tested in clinical trials | Up to 45 mg per day (high doses, Japanese studies) | 180 mcg per day for 3 years (Knapen et al.) |
| Usual dosing frequency | Several doses per day | One dose per day |
For more on nutrients tied to bone mineralization, UltraSante’s Nutrition pillar page brings together articles on key micronutrients. See also our features on vitamin D, often paired with K2 in bone protocols, on magnesium, another cofactor for bone health, and on vitamin B12 for readers reviewing their full micronutrient status.
Protocol and Usage Tips
No official reference intake exists specifically for vitamin K2 to date. Health authorities set an adequate intake for total vitamin K, covered mainly by dietary vitamin K1, as outlined on the NHS page on vitamins and minerals. MK-7 doses used in clinical research most often range from 90 to 200 mcg per day, taken over several months to several years.
- Take it with a meal containing some fat: it is fat-soluble and absorbs better with dietary fat present.
- Check the exact form and dose on the label (usually MK-7, sometimes paired with vitamin D3) rather than relying on the words “K2” alone.
- Always tell your doctor about any K2 intake if you take a vitamin K antagonist (warfarin, fluindione): it directly opposes their mechanism of action and can throw off your INR.
- Ask a doctor before supplementing if you are pregnant, breastfeeding, or have kidney or liver disease.
Frequently Asked Questions
What is the difference between vitamin K1 and vitamin K2?
Vitamin K1 (phylloquinone) comes from green vegetables and mainly supports blood clotting through the liver. K2 (menaquinones) comes from fermented foods and gut bacteria, and acts more on bone and arteries by activating osteocalcin and MGP.
How much MK-7 should you take per day?
There is no officially validated dose for K2 alone. The clinical trials that showed a bone and vascular effect used 180 mcg of MK-7 per day for three years. Any higher dose or prolonged use should be discussed with a healthcare professional.
Is vitamin K2 dangerous for people on blood thinners?
Like vitamin K1, it opposes vitamin K antagonists (warfarin, fluindione) and can reduce their effectiveness. People on this treatment should avoid supplementing without medical advice and should monitor their INR if their food or supplement intake changes.
Which foods contain vitamin K2?
Natto, a fermented Japanese soybean product, is the most concentrated source of MK-7. Aged cheeses, some fermented dairy products, egg yolk, liver and meat mainly provide MK-4, in smaller and more variable amounts depending on how the animals were raised.
Should you combine K2 and vitamin D3?
The two vitamins work in a complementary way on calcium metabolism: vitamin D3 increases its intestinal absorption, while K2 directs it toward bone rather than arteries. This combination is common in supplement protocols, but its combined benefit has been tested less in controlled trials than each vitamin taken alone.
Medical disclaimer. The information provided here is for informational purposes only and does not constitute medical advice. It does not replace a consultation. Ask a healthcare professional before changing your diet, taking dietary supplements or starting a new practice, especially if you have a medical condition, are pregnant or are under treatment. Dietary supplements do not replace a balanced diet or medical follow-up.
