Vitamin K2 is one of the most underappreciated nutrients in the supplement world — not because the evidence for it is weak, but because most people have never heard of it separately from vitamin K1. The two forms have different roles, different sources, and different effects on the body. K2 in particular has attracted serious research attention for its role in bone strength, cardiovascular health, and calcium regulation.
Here’s what vitamin K2 actually does, where to find it, and why most people in the UK don’t get enough.
What does vitamin K2 do?
Vitamin K2’s primary function is activating proteins that regulate where calcium goes in the body. Two of these proteins are particularly important: osteocalcin, which anchors calcium into bone matrix, and Matrix Gla Protein (MGP), which prevents calcium from depositing in artery walls and soft tissue.
Both osteocalcin and MGP are inactive until K2 carboxylates them — a chemical activation process that K1 cannot perform as effectively. This is the core reason K2 is distinct from K1 and why dietary K2 intake matters independently of K1.
Vitamin K2 and bone health
Strong bones require calcium, but calcium alone isn’t enough. Vitamin D3 increases calcium absorption from food, and K2 determines where that calcium ends up. Without adequate K2, osteocalcin remains undercarboxylated and calcium is not efficiently bound into bone — even if serum vitamin D levels appear normal.
Research supports this. A systematic review published in Nutrients found that vitamin K2 supplementation significantly improved bone mineral density and reduced fracture rates in postmenopausal women. The combination of D3 and K2 shows stronger bone-protective effects than either nutrient alone, which is why they are increasingly combined in supplements.
Vitamin K2 and heart health
The Rotterdam Study — a landmark 10-year cohort of over 4,800 adults — found that higher dietary intake of vitamin K2 was associated with significantly reduced cardiovascular mortality and less aortic calcification. The same association was not seen with vitamin K1, suggesting the effect is specific to K2.
The mechanism involves MGP. When K2 status is sufficient, MGP actively inhibits calcium from depositing in artery walls. When K2 is deficient, MGP remains inactive and arterial calcification can progress. This is a slow process, which is why the cardiovascular effects of K2 are most apparent in long-term observational studies rather than short trials.
Other potential benefits
Beyond bone and heart health, preliminary research suggests vitamin K2 may have broader effects. Some studies have found that K2 may influence insulin sensitivity and glucose metabolism, though the evidence is not yet sufficient to draw firm conclusions. There is also early-stage research suggesting K2 may modulate cancer cell activity — specifically through its role in regulating cell differentiation — but this research remains at an early stage and should not be overstated.
Vitamin K2 deficiency: causes and symptoms
Unlike some fat-soluble vitamins, vitamin K2 is not stored in the body for long. This makes deficiency more common, particularly in people whose diets are low in fermented foods and animal products.
In the Western diet, the majority of vitamin K consumed is K1 (from leafy green vegetables), with K2 intakes generally well below what research suggests is optimal. Gut bacteria can convert some K1 into K2, but this conversion is limited and unreliable.
The consequences of low K2 are not immediately obvious — there are no acute deficiency symptoms comparable to, say, scurvy or rickets. Instead, the effects accumulate over years: reduced bone density, underactivated MGP leaving arteries less protected, and impaired calcium regulation. A blood test measuring undercarboxylated osteocalcin or undercarboxylated MGP can give an indication of K2 functional status, though this is not a standard NHS test.
Vitamin K1 vs vitamin K2
Vitamin K1 (phylloquinone) is found mainly in leafy green vegetables and is readily absorbed by the liver, where it is used primarily for blood clotting. Vitamin K2 (menaquinone) is found in animal products and fermented foods, and has a broader distribution in the body — reaching bone, arteries, and other tissues where K1 is less active.
In the average UK diet, roughly 90% of vitamin K consumed is K1. This dietary imbalance partly explains why K2 functional deficiency is common even in people who eat relatively well.
MK-4 vs MK-7
Vitamin K2 exists in several forms called menaquinones. MK-4 and MK-7 are the two most relevant for supplementation.
MK-4 has a short half-life of 1–2 hours and requires multiple doses per day to maintain stable blood levels. MK-7 — produced by bacteria during fermentation and most concentrated in nattō — has a half-life of approximately 72 hours, making once-daily dosing effective. Research published in Thrombosis and Haemostasis found MK-7 to be more effective than MK-4 at activating osteocalcin and MGP at equivalent doses. For supplementation purposes, MK-7 is the preferred form.
All-trans vs cis MK-7
MK-7 exists in two geometric isomers: all-trans and cis. You’ll often see the active form referred to interchangeably as “all-trans MK-7”, “all-trans K2” or “all-trans vitamin K2”. All three describe the same thing. Only the all-trans form is biologically active. The cis form does not meaningfully activate osteocalcin or Matrix Gla Protein and contributes nothing to the calcium-regulating effects K2 is taken for. The clinical trials that established MK-7’s benefits on bone density and arterial stiffness were conducted using the all-trans isomer, so the all-trans content of a supplement directly determines how much of the labelled dose is actually doing the work. Quality MK-7 is specified as all-trans. The K2 in our Vitamin D3 K2 supplement is all-trans MK-7.
Food sources of vitamin K2
Vitamin K2 is primarily found in animal products and fermented foods. The richest dietary sources include:
- Nattō (fermented soybeans) — by far the highest source; a single serving can provide several hundred micrograms of MK-7
- Hard and aged cheeses — the older the cheese, the higher the K2 content
- Egg yolks — particularly from pasture-raised hens
- Dark chicken meat (leg and thigh)
- Liver and other organ meats
- Butter and full-fat dairy
Nattō is the standout source, but it’s an acquired taste rarely eaten in quantity in the UK. For most people, dietary K2 intake falls well short of what research on bone and cardiovascular outcomes suggests is optimal.
Vitamin K2 and vegan diets
Since K2 is concentrated in animal products and fermented foods, people following a vegan diet are at elevated risk of low K2 intake. Nattō is the main plant-based exception and is suitable for vegans, though availability and palatability are barriers. Other fermented plant foods such as tempeh and kombucha contain modest amounts.
Increasing dietary K1 (from dark leafy greens such as kale, spinach, and broccoli) provides some benefit, as the body can convert a small amount of K1 to K2 via gut bacteria — but conversion rates are low and variable. For vegans who are concerned about K2 status, MK-7 supplements derived from fermented chickpea or nattō extract are available and provide a reliable plant-based source.
How much vitamin K2 do you need?
The EFSA adequate intake for vitamin K in adults is 70 mcg per day — a figure that covers total vitamin K rather than K2 specifically. Most research examining bone and cardiovascular outcomes used K2 doses of 90–200 mcg MK-7 daily, which is above the EFSA reference value.
There is no established UK upper limit for vitamin K2. At doses used in research (up to 360 mcg MK-7 daily), no significant adverse effects have been reported in people not taking anticoagulant medication. If you take warfarin or any other anticoagulant, vitamin K in any form can affect your medication’s activity and should only be taken under GP supervision.
Vitamin D3 and K2: why they work together
Vitamin D3 and K2 handle two halves of the same job. D3 increases how much calcium your body absorbs from food. K2 activates the proteins that decide where that calcium goes — into bone (via osteocalcin) or away from arteries (via Matrix Gla Protein). Without K2, the extra calcium that D3 helps you absorb has no reliable routing system.
This matters most when you’re supplementing D3 at meaningful doses. At 1,000 IU or above, calcium absorption increases substantially. A study published in The Journal of Clinical Endocrinology & Metabolism (2011) found that D3 supplementation raised intestinal calcium absorption by 65% in vitamin D-deficient subjects. If K2 status is low at the same time, that additional calcium can contribute to soft tissue and arterial calcification rather than strengthening bone.
Can you take vitamin D3 and K2 together? Yes — and there’s a strong case that you should. The combination addresses the full calcium utilisation pathway rather than just the absorption step. Research in postmenopausal women (published in Osteoporosis International, 2013) found that D3 and K2 MK-7 together improved bone mineral density more effectively than either nutrient alone.
The recommended ratio used in most research is approximately 40–50 IU of D3 per 1 µg of K2 MK-7. At a practical level, if you’re taking 4,000 IU of D3, pairing it with 90–100 µg of K2 MK-7 is consistent with the research. Our Vitamin D3 K2 supplement uses this ratio, with the addition of boron and zinc and MCT oil to support the full metabolic chain.
For a detailed breakdown of the D3 and K2 mechanism, including the osteocalcin and MGP pathways and who benefits most, see our article on vitamin D and K2.
Supplementing with vitamin K2
For most people in the UK, dietary K2 intake is insufficient to meet the amounts used in research on bone and cardiovascular health. A supplement providing 100–200 mcg of K2 MK-7 daily — ideally taken alongside vitamin D3 and with a fat-containing meal, since K2 is fat-soluble — is a practical approach to bridging this gap.
Our Vitamin D3 K2 MK-7 supplement combines D3 with all-trans MK-7, the biologically active isomer used in the clinical research on bone and cardiovascular outcomes, formulated in an MCT oil base to support fat-soluble absorption without needing to take it with a meal.
Frequently asked questions
What are the main benefits of vitamin K2?
Vitamin K2 activates proteins that regulate calcium in the body — directing it into bone (via osteocalcin) and preventing it from depositing in artery walls (via Matrix Gla Protein). The main evidence-supported benefits are improved bone density, reduced fracture risk, and protection against arterial calcification. These effects are particularly relevant for older adults and postmenopausal women.
What is the difference between vitamin K1 and K2?
Vitamin K1 (phylloquinone) is found in leafy green vegetables and is used primarily by the liver for blood clotting. Vitamin K2 (menaquinone) is found in animal products and fermented foods and has a broader distribution in the body, particularly in bone and arterial tissue. K2 is more effective than K1 at activating the proteins involved in bone mineralisation and cardiovascular protection.
What is the difference between MK-4 and MK-7?
Both are forms of vitamin K2. MK-7 has a much longer half-life (around 72 hours) compared to MK-4 (1–2 hours), making once-daily dosing effective with MK-7. Research has shown MK-7 to be more effective at activating osteocalcin and Matrix Gla Protein at equivalent doses. MK-7 from fermented sources is the preferred form in most high-quality supplements.
What is all-trans MK-7 and why does it matter?
MK-7 exists in two isomeric forms, all-trans and cis (also referred to as all-trans K2 or all-trans vitamin K2). Only the all-trans form is biologically active. The cis form does not meaningfully activate osteocalcin or Matrix Gla Protein, so it contributes nothing to the bone and cardiovascular effects K2 is taken for. The clinical trials that established MK-7’s benefits used the all-trans isomer, which means the proportion of all-trans MK-7 in a supplement directly determines how much of the labelled dose is actually usable. The K2 in our Vitamin D3 K2 supplement is all-trans MK-7.
Can vegans get enough vitamin K2?
It is difficult to get adequate K2 from a vegan diet without intentional effort. Nattō is the only plant food with meaningful MK-7 content, but it is rarely eaten in the UK. Some K2 can be produced by gut bacteria from dietary K1, but conversion rates are low. Vegans concerned about K2 status may benefit from a MK-7 supplement derived from fermented plant sources.
How much vitamin K2 should I take per day?
The EFSA adequate intake for total vitamin K in adults is 70 mcg per day, but most research on bone and cardiovascular outcomes used K2 doses of 90–200 mcg MK-7 daily. At these doses, no significant adverse effects have been reported in people not taking anticoagulant medication. If you take warfarin or other blood thinners, consult your GP before taking any vitamin K supplement.
Can you take vitamin D3 and K2 at the same time?
Yes, and combining them is recommended rather than optional. Vitamin D3 increases calcium absorption from food; K2 MK-7 activates the proteins that direct that calcium into bone and away from arteries. Taking both in the same supplement ensures consistent co-administration. No adverse interactions between D3 and K2 have been reported at standard supplemental doses.
Should I take vitamin K2 with vitamin D3?
Yes, combining K2 with D3 is well-reasoned and increasingly recommended. Vitamin D3 increases calcium absorption, but without adequate K2, that calcium cannot be efficiently directed into bone or prevented from accumulating in blood vessels. Taking both together addresses both the absorption and the routing of calcium. See our full article on vitamin D3 and K2 for a detailed explanation.
References
- 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. doi:10.1093/jn/134.11.3100
- Knapen MHJ, et al. Menaquinone-7 supplementation improves arterial stiffness in healthy postmenopausal women. Thrombosis and Haemostasis. 2015;113(5):1135–1144. doi:10.1160/TH14-08-0675
- Iwamoto J, et al. High-dose vitamin K supplementation reduces fracture incidence in postmenopausal women: a review of the literature. Nutrition Research. 2009;29(4):221–228. doi:10.1016/j.nutres.2009.03.009
- Schurgers LJ, et al. Vitamin K-containing dietary supplements: comparison of synthetic vitamin K1 and natto-derived menaquinone-7. Blood. 2007;109(8):3279–3283. doi:10.1182/blood-2006-08-040709
- European Food Safety Authority (EFSA). Dietary Reference Values for vitamin K. EFSA Journal. 2017;15(5):4780. doi:10.2903/j.efsa.2017.4780
- Knapen MH, et al. Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporosis International. 2013;24(9):2499–2507. doi:10.1007/s00223-012-9691-6
This article is for informational purposes only and does not constitute medical advice. If you take anticoagulant medication or have a cardiovascular condition, speak with your GP before starting any vitamin K supplement.
