Vitamin K2: The Nutrient That Fermentation Makes
Haelan Publishing TeamShare
Most of us met vitamin K exactly once — in a biology class, filed under “the one that helps blood clot” — and never thought about it again. But there is a second form that behaves quite differently, is made almost entirely by bacteria, and turns up most abundantly in fermented soybeans. Here is what researchers have actually learned about it, including the parts still unsettled.
Key takeaways
Vitamin K comes in two families: K1 (phylloquinone) from leafy greens, and K2 (menaquinones), which bacteria produce during fermentation. K2 helps activate proteins involved in how the body handles calcium, and one long-chain form — MK-7, abundant in fermented soybeans — stays in circulation notably longer than the others. Population studies link higher K2 intake with markers of healthy aging, though these are associations rather than proof of cause and effect. Anyone taking warfarin or another vitamin K–sensitive blood thinner should talk with their healthcare professional before changing how much vitamin K they eat.
Two vitamins sharing one name
“Vitamin K” is less a single nutrient than a family surname. The branch most people know is K1, or phylloquinone — kale, spinach, broccoli, most dark leafy things. It accounts for roughly 80% of vitamin K intake worldwide, and the body sends most of it to the liver, where it supports the clotting factors that gave the vitamin its name [1].
The other branch is K2, known collectively as the menaquinones and labelled MK-4 through MK-13 by the length of a molecular tail. Here is the interesting part: plants do not really make these. Bacteria do. So outside a few animal foods, the most concentrated sources of K2 are fermented ones — most famously natto, the sticky, pungent Japanese breakfast dish made by fermenting soybeans with Bacillus subtilis. If natto is new to you, our beginner's guide to fermented soy foods is a gentler place to start.
What K2 does once it is in you
Vitamin K's job is to switch certain proteins on, through a step called carboxylation that gives a protein the molecular hooks it needs to grab calcium. Without enough vitamin K those proteins are still made — they simply circulate undercarboxylated, not yet activated. Two get most of the attention: osteocalcin, which once carboxylated binds calcium into the bone matrix, and matrix Gla protein, which helps keep calcium in solution in soft tissue rather than settling in artery walls [1].
I like to think of K2 as a traffic director for calcium: calcium is the raw material, vitamin D helps you absorb it, and K2-dependent proteins help route it. Like all tidy metaphors about biology it flattens a lot — but it captures why researchers grew curious about a nutrient long filed under blood clotting. Conveniently, undercarboxylated osteocalcin is measurable in blood, giving a functional readout of vitamin K status that much of the research below leans on.
Why fermentation is the K2 factory
When Bacillus subtilis works through cooked soybeans it produces menaquinone-7 as part of its own metabolism. The bean is the substrate; the bacteria are the manufacturer — a different proposition from unfermented soy, which contains very little K2.
Chain length turns out to matter. In a study of healthy young women, a single dose of MK-7 produced serum levels peaking around six hours later and still detectable at 48 hours, while MK-4 at the same dose was not detectable at all; after seven days at ordinary dietary levels, MK-7 raised circulating vitamin K2 in every participant and MK-4 did not [2]. The authors concluded long-chain MK-7 is the form most likely to reach tissues beyond the liver. It was a small study of young, healthy Japanese women, so it describes the mechanism more convincingly than it describes everyone.
| Form | Typical food sources | Notable characteristic |
|---|---|---|
| K1 (phylloquinone) | Kale, spinach, broccoli, leafy greens | Most of dietary vitamin K; largely taken up by the liver |
| K2 as MK-4 | Some animal foods, egg yolk, certain meats | Short chain; not detected in serum at dietary doses in one trial |
| K2 as MK-7 | Natto by far the richest; much smaller amounts in other fermented soy foods and some aged cheeses | Long chain; stays measurable in circulation far longer |
What the research shows — and what it does not
Three strands are worth separating, because they carry very different weight. The first is well supported: eating fermented soy raises vitamin K2 status. Research in healthy individuals found natto intake increased circulating MK-7 and gamma-carboxylated osteocalcin [3]. Food goes in, the marker moves. Not controversial.
The second goes further. In a randomized, double-blind, placebo-controlled trial, 55 healthy children given 45 micrograms of MK-7 daily for eight weeks showed reduced undercarboxylated osteocalcin and an improved carboxylation ratio versus placebo, with no adverse effects on coagulation [4]. We cite a study in children because it is one of the cleanest controlled trials of this specific question, not because the finding transfers directly to adults — and note what it measured: a biochemical marker over two months, not a health outcome over years.
The third strand is where honesty matters most, and it is the weakest of the three. Population research has looked for links between dietary menaquinone intake and cardiovascular measures — the most cited example being the Rotterdam Study, which followed 4,807 adults and reported more favourable outcomes among those with the highest K2 intakes, with no equivalent signal for K1 [5]. That paper is from 2004, and a single observational cohort is a starting point for research, not a conclusion.
That is an interesting result, and exactly the kind that deserves a cautious reading. Observational studies show associations, not proof of cause and effect: people who eat more fermented soy differ from those who eat less in many ways — diet, activity, income, family history — and researchers can adjust for the factors they measured, never the ones they did not. A 2024 review adds that the trial literature on vitamin K and bone is genuinely heterogeneous, with varying doses, designs, and populations that resist any tidy conclusion [1]. This is an active and promising area of research, not a settled one.
And to say the obvious plainly: this article is for educational purposes only. No food, fermented soy included, treats, cures, or prevents any disease, and nothing here is medical advice. If you have questions about your bone health, your heart, or your nutrient status, talk with your healthcare professional — they know your history, and a blog post does not.
Putting it on the table — and one real caution
Natto is the undisputed heavyweight and, let us be honest, an acquired texture — the traditional treatment (stirred vigorously, with mustard and soy sauce over hot rice) exists for good reason. Miso, tempeh, and doenjang contribute smaller amounts and are far easier to fold into everyday cooking — and they bring isoflavones to the table alongside the K2. Vitamin K is fat-soluble, so it absorbs better alongside some fat — and variety beats optimization, since a diet with both leafy greens and fermented foods covers both branches without anyone counting micrograms.
One caution genuinely matters here. If you take warfarin (Coumadin) or another vitamin K–sensitive anticoagulant, vitamin K intake is not a casual topic: these medications work precisely by interfering with vitamin K recycling, so swings in dietary vitamin K — up or down — can affect how the drug behaves. The usual guidance is consistency rather than avoidance, but have that conversation with whoever manages your prescription before adding natto to the breakfast rotation. The same review notes newer direct-acting anticoagulants do not interfere with vitamin K–dependent proteins the same way [1]; your prescriber can tell you which category you are in.
The bottom line
Vitamin K2 shows how nutrition science actually moves. A nutrient dismissed for decades as a clotting footnote turns out to have a second job, and the foods richest in it are ones people have made for centuries without knowing why. The evidence that fermented soy raises K2 status is solid; the evidence connecting that to long-term outcomes is suggestive, largely observational, and still being worked out. Both are true at once — and either way, fermented soy is an easy, pleasant thing to eat more of however the research settles.
Haelan 951 comes from that same tradition of fermenting soy rather than simply eating it — a concentrated organic, non-GMO fermented soy beverage made through a proprietary nitrogen fermentation process, and classified by the FDA as a food, not a supplement. For serving details, please refer to the product label.
References: [1] Aaseth JO et al. The Importance of Vitamin K and the Combination of Vitamins K and D for Calcium Metabolism and Bone Health: A Review. Nutrients. 2024;16(15):2420. [2] Sato T, Schurgers LJ, Uenishi K. Comparison of menaquinone-4 and menaquinone-7 bioavailability in healthy women. Nutrition Journal. 2012;11:93. [3] Intake of fermented soybean (natto) increases circulating vitamin K2 (menaquinone-7) and gamma-carboxylated osteocalcin concentration in normal individuals. J Bone Miner Metab. 2000. [4] van Summeren MJH et al. The effect of menaquinone-7 (vitamin K2) supplementation on osteocalcin carboxylation in healthy prepubertal children. British Journal of Nutrition. 2009;102(8):1171-1178. [5] Geleijnse JM et al. Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study. Journal of Nutrition. 2004;134(11):3100-3105.
This content is for educational purposes only and is not intended to diagnose, treat, cure, or prevent any disease. Always consult your healthcare professional before making changes to your diet or health routine.