Vitamin K2 (MK-4/MK-7): the forgotten vitamin
Most people have never heard of vitamin K2, and the ones who have usually confuse it with K1. Yet K2 governs one of the body's most consequential decisions: where calcium goes. Get it right and calcium hardens your bones; get it wrong and the same calcium hardens your arteries. The mechanism is one of the most elegant in nutrition, which is exactly why it deserves an honest evidence grade rather than hype.
Two vitamins wearing one letter
Vitamin K comes in two families that do different jobs. K1 (phylloquinone), abundant in leafy greens, is taken up mainly by the liver and used for blood clotting. K2 (menaquinone), found in animal foods, cheese, and fermented foods like natto, circulates to other tissues, bone and blood vessels, where it activates a different set of proteins.[2] Treating "vitamin K" as one nutrient misses the entire K2 story.
K2 itself comes in subtypes. MK-4 is found in animal products (liver, egg yolk, butter) and has a short half-life. MK-7, produced by bacterial fermentation (natto is the richest source), is more bioavailable and stays in circulation far longer, so it activates K-dependent proteins at modest nutritional doses.[2]
The calcium-routing mechanism
K2's job is to activate two proteins by carboxylating them:
- Osteocalcin binds calcium into the bone mineral matrix. Without enough K2, osteocalcin stays inactive and calcium isn't properly incorporated into bone.
- Matrix Gla protein (MGP) is the body's primary inhibitor of vascular calcification, it actively keeps calcium out of arterial walls.[3]
This is the "calcium paradox" resolved: K2 is the switch that sends calcium to where it helps (bone) and away from where it harms (arteries). The mechanism, sometimes called bone-vascular crosstalk, is well-characterized.[3]
The observational signal
The epidemiology is genuinely striking. In the population-based Rotterdam Study, higher dietary menaquinone (K2) intake was associated with a 41% lower risk of coronary heart disease and substantially less aortic calcification.[1] Other cohorts have echoed the direction of effect. For a nutrient most people have never heard of, that is a notable signal.
The honest counterpoint: the trials are mixed
Here is where calibration matters. A strong mechanism and a strong observational signal do not guarantee that taking K2 changes outcomes. Supplement trials have been mixed: some show improved arterial elasticity or slowed calcification markers, while a randomized double-blind trial of vitamin K2 plus D in patients with aortic valve calcification did not significantly slow progression of the calcification on its primary endpoint.[4]
That is the honest state of the evidence: mechanistically compelling, observationally promising, but not yet proven in trials. Treat K2 as a reasonable nutrient to obtain from food, not as a guaranteed cardiovascular drug.
The practical protocol
- Get K2 from food first: fermented foods (natto is by far the richest), cheese, egg yolks, and liver supply menaquinones that greens cannot.[2]
- If you supplement vitamin D, consider pairing with K2, the two fat-soluble vitamins work together in calcium handling (covered in the D+K2 cluster).
- Don't expect a supplement to undo existing calcification, the trial evidence doesn't support that claim.[4]
- Frame K2 as part of the broader case for nutrient-dense animal and fermented foods, not a magic bullet.
FAQ
K1 vs K2? K1 (greens) is for clotting; K2 (animal/fermented) directs calcium into bone and out of arteries.
MK-4 or MK-7? MK-7 is more bioavailable and longer-lasting; MK-4 comes from animal foods. Both are studied.
Does K2 prevent heart disease? Observational data and mechanism are promising, but supplement trials are mixed, a hypothesis, not proof.
References
- 1.Geleijnse JM, et al. (2004). Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study. Journal of Nutrition 134(11):3100–3105. PMID: 15514282. Link
- 2.National Institutes of Health, Office of Dietary Supplements (2026). Vitamin K, Health Professional Fact Sheet (K1 vs K2, MK-4/MK-7, food sources). NIH ODS. Link
- 3.Mandatori D, et al. (2021). The dual role of vitamin K2 in bone-vascular crosstalk: opposite effects on bone loss and vascular calcification. Nutrients 13(4):1222. PMC8067793. Link
- 4.Diederichsen ACP, et al. (2022). Vitamin K2 and D in patients with aortic valve calcification: a randomized double-blinded clinical trial. Circulation 145(18):1387–1397. DOI: 10.1161/CIRCULATIONAHA.121.057008. Link
This article is for educational purposes only and is not medical advice. It is not a substitute for professional diagnosis, treatment, or the guidance of a qualified clinician. Always consult your physician before changing your diet, starting a fast, taking supplements, or beginning a new training or heat/cold protocol, especially if you are pregnant, breastfeeding, managing a medical condition, or taking medication.