Vitamin K2 (and D3): The Key to Where Your Calcium Goes
- , by SANUSq Research team
- 12 min reading time
Here's a question that upends a lot of received wisdom: what if getting plenty of calcium isn't enough — and what really matters is making sure that calcium ends up in your bones rather than your arteries? That routing job belongs to vitamin K2, and it is why the vitamin D3 + K2 benefits are usually talked about as one package rather than two. Here is what K2 actually does, what the vitamin K2 heart health and bone evidence shows, and who is most likely to be running short.
The calcium paradox
We're all told to get enough calcium for strong bones. Yet calcium in the wrong place is a problem: when it settles in the walls of arteries, it stiffens them and drives cardiovascular disease. This is sometimes called the "calcium paradox" — too little calcium in the bones, too much in the blood vessels. The nutrient that helps resolve it, quietly directing calcium traffic around the body, is one most people have barely heard of: vitamin K2.
What is vitamin K2?
Vitamin K comes in two main families. Vitamin K1 (phylloquinone) is found in leafy greens and is best known for its role in blood clotting. Vitamin K2 (menaquinone) is a different beast: found in fermented foods and some animal products, and made in small amounts by gut bacteria, it activates a set of "vitamin K-dependent proteins" that manage calcium. Two matter most here: osteocalcin, which binds calcium into the bone matrix, and matrix Gla protein (MGP), which keeps calcium out of soft tissues like arteries. Without enough K2, these proteins stay inactive — and calcium is free to drift where it shouldn't.
A review of the clinical evidence concluded that Level I and II evidence supports vitamins K1 and K2 in osteoporosis, and Level II evidence supports vitamin K2 in the prevention of coronary calcification and cardiovascular disease — while noting the evidence is weaker or insufficient for other conditions (Schwalfenberg, 2017).
K2 and your heart
Because matrix Gla protein is one of the body's most powerful natural inhibitors of vascular calcification, and because K2 is what switches it on, researchers have looked hard at whether higher K2 intake tracks with healthier arteries. The signal is encouraging.
In the population-based Rotterdam Study, following nearly 4,800 adults for years, those with the highest dietary intake of menaquinone (K2) had a significantly lower risk of coronary heart disease and of severe aortic calcification, and lower all-cause mortality, than those with the lowest intake. Vitamin K1 intake showed no such association (Geleijnse et al., 2004).
It's worth reading this result for what it is: an observational study, which shows an association rather than cause and effect. The link between vitamin K2 and the heart fits the known mechanism, which makes it credible, but it remains supporting evidence rather than proof.
Vitamin K2 for bones
The flip side of keeping calcium out of arteries is getting it into bone — osteocalcin's job, and one that depends on K2. This matters most after menopause, when falling oestrogen accelerates bone loss.
In a three-year randomised controlled trial, 244 healthy postmenopausal women taking low-dose MK-7 (a long-acting form of K2) showed significantly less age-related decline in bone mineral density and bone strength than those on placebo (Knapen et al., 2013).
Vitamin D3 K2 benefits: why they work as a team
This is where vitamin D3 comes in, and why the two are so often paired. Vitamin D3's job is to increase how much calcium you absorb from food. But absorbing more calcium without directing it properly is only half the story — you also need K2 to activate the proteins that steer that calcium into bone and away from arteries. In other words, D3 gets calcium into the body; K2 makes sure it goes to the right place. Taken together (and, because both are fat-soluble, with a meal containing some fat), they make a genuinely complementary pair for bone and cardiovascular health — and most of the vitamin D3 and K2 benefits described here depend on that partnership rather than on either nutrient alone.
There's a practical wrinkle that's easy to miss: vitamin D dramatically increases calcium absorption, so supplementing D3 on its own effectively raises the amount of calcium circulating in your body. Taking vitamin K2 with D3 is what ensures that extra calcium is put to good use in the skeleton rather than left to settle in blood-vessel walls — which is why thoughtful formulations combine the two rather than selling D3 alone.
Why the pairing matters most in winter
From roughly October to March, the sun over most of Europe sits too low for your skin to make much vitamin D — the so-called vitamin D winter. For those months, what you take becomes your main source of vitamin D, and that is exactly when it makes sense to take vitamin D3 with its partner. (A quick check: if your shadow is longer than you are tall, your skin is making little vitamin D. We explain this "shadow rule", and why the gap opens in autumn, in our guide to vitamin D benefits and the winter gap.)
Our Liposomal Vitamin D3 + K2 provides 1,000 IU (25 µg) of vitamin D3 and 45 µg of vitamin K2 as MK-7 in each 1 ml daily serving. One 60 ml bottle lasts two months, so our three-bottle bundle covers the season from October to March.
Who might benefit most
The vitamin K2 benefits are not spread evenly across the population, and a few groups deserve particular attention. Postmenopausal women and older adults, who are most vulnerable to both bone loss and arterial stiffening, stand to gain most from keeping calcium correctly routed. Anyone taking higher-dose vitamin D3 or calcium supplements has a stronger case too: raising calcium intake and absorption without adequate K2 is exactly the scenario the "calcium paradox" warns about. And because K2 is so scarce in Western diets — unless you happen to eat natto regularly — most people simply aren't getting much from food in the first place.
Are you getting enough?
Vitamin K2 is scarce in a typical Western diet, and the vitamin K2 sources that do exist are narrow. The richest by far is natto, a Japanese fermented-soybean dish that's an acquired taste for many; smaller amounts appear in certain cheeses, egg yolks, butter from grass-fed cows and organ meats, and your gut bacteria make a little. Because intake is often low and there are no clear-cut vitamin K2 deficiency symptoms to watch for, a shortfall tends to go unnoticed rather than announce itself, so K2 is a common candidate for supplementation — particularly the MK-7 form, which stays active in the body far longer than the short-acting MK-4, and especially alongside vitamin D3.
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Frequently asked questions
What's the difference between vitamin K1 and K2?
K1 (phylloquinone), from leafy greens, is mainly used for blood clotting. K2 (menaquinone), from fermented and animal foods, activates the proteins that manage calcium — binding it into bone and keeping it out of arteries. They're related but do different jobs, and most people get enough K1 but little K2.
Why take vitamin D3 and K2 together?
They're complementary. D3 increases calcium absorption, while K2 activates the proteins that direct that calcium into bone and away from soft tissues. Taking D3 without adequate K2 raises calcium levels without ensuring it's routed correctly, which is why the two are so often combined.
Does vitamin K2 really help the heart?
The evidence is promising rather than definitive. K2 activates matrix Gla protein, which inhibits arterial calcification, and population studies like the Rotterdam Study link higher K2 intake with less coronary heart disease and aortic calcification. It's best seen as one supportive factor for cardiovascular health, not a treatment.
What are the best vitamin K2 foods?
Natto (fermented soybeans) is by far the richest source, with smaller amounts in certain cheeses, egg yolks, grass-fed butter and organ meats. Because these aren't staples of most Western diets, K2 intake is often low.
What are the side effects of vitamin D3 K2?
At the amounts in food supplements, vitamin D3 and K2 are generally well tolerated. The key exception: if you take a blood-thinning medication such as warfarin, vitamin K directly affects how it works, so do not start a K1 or K2 supplement without talking to your doctor first. On the vitamin D side, very high doses over long periods can raise blood calcium, which is why it's best to stay within the European upper limit of 100 µg (4,000 IU) a day from all sources. If you are pregnant or breastfeeding, check with your healthcare professional first.
Should I take calcium with vitamin D3 K2?
Most people get enough calcium from food — dairy, leafy greens, nuts, seeds and fortified foods. Vitamin D3 helps your body absorb and use that calcium, and vitamin K2 helps build it into bone, so for many people the pairing makes better use of the calcium they already eat. Only add a calcium supplement if your diet falls short or your doctor recommends it.
Should I take vitamin D3 K2 with magnesium?
It's worth making sure you get enough. The enzymes that turn vitamin D into its active form depend on magnesium, so low magnesium can hold vitamin D back. Nuts, seeds, whole grains and leafy greens are good sources; we explain the link in detail in why you should take magnesium with vitamin D.
Vitamin D3 and K2 MK-7: does the form of K2 matter?
Yes. MK-4 is a short-chain K2 from animal foods that clears from the body quickly, so it needs frequent dosing. MK-7, the form made by fermentation (as in natto), has a much longer half-life, staying active far longer after a single daily dose — which is why most modern K2 supplements, and the bone and heart research such as the MK-7 trials, favour it.
How do I choose the best vitamin D3 K2?
Four pointers are enough. The form of vitamin D: D3 (cholecalciferol) rather than D2. The form of K2: MK-7, the long-acting form. The dose: moderate and daily, staying within EFSA's safe upper limit of 100 µg (4,000 IU) of vitamin D a day from all sources. And the format: drops, capsules or a liposomal liquid — the best one is the one you'll actually take every day, ideally with a short ingredient list and no unnecessary additives.
Should I get my vitamin D tested before supplementing?
It's the most sensible approach, especially if you're considering higher doses. A 25-hydroxyvitamin D blood test, arranged through your doctor, shows where you actually stand and saves guesswork. The question of where your calcium ends up applies whatever your starting point.
References
- Geleijnse JM, Vermeer C, Grobbee DE, et al. Dietary intake of menaquinone is associated with a reduced risk of coronary heart disease: the Rotterdam Study. The Journal of Nutrition. 2004;134(11):3100–3105. doi:10.1093/jn/134.11.3100
- Knapen MHJ, Drummen NE, Smit E, Vermeer C, Theuwissen E. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporosis International. 2013;24(9):2499–2507. PMID 23525894
- Schwalfenberg GK. Vitamins K1 and K2: the emerging group of vitamins required for human health. Journal of Nutrition and Metabolism. 2017;2017:6254836. PMID 28698808
Read more on the blog
- Vitamin D: benefits, deficiency, and how to get enough →
- Why you should take magnesium with vitamin D →
- Why your vitamin D might not be working →
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The team at SANUSq.
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