Most people taking vitamin D3 don’t realise they may need K2 alongside it.
The two vitamins share a job: regulating where calcium ends up in your body. D3 increases calcium absorption from food and supplements. K2 activates the proteins that direct that calcium into bones and teeth — and away from soft tissue and arteries, where excess calcium causes problems.
Take D3 without K2, and you may be absorbing more calcium than your body knows what to do with. That’s the core case for combining them.
What Is Vitamin D3?
Vitamin D3 (cholecalciferol) is the form of vitamin D your skin produces when exposed to UVB sunlight. In the UK, meaningful sun exposure for D3 synthesis is only possible between April and September — and even then, factors like skin tone, cloud cover, and time indoors significantly reduce production.
This is why vitamin D deficiency is widespread in the UK. NHS data estimates that around 1 in 5 people in the UK have low vitamin D levels, with older adults, people who spend little time outdoors, and those with darker skin at highest risk.
D3 is the preferred supplemental form over D2 (ergocalciferol) because it raises blood levels of vitamin D more effectively and sustains them for longer, according to a 2012 meta-analysis in the American Journal of Clinical Nutrition. For a full breakdown of what vitamin D does in the body, see our article on the health benefits of vitamin D.
What Is Vitamin K2?
Vitamin K2 (menaquinone) is a fat-soluble vitamin distinct from vitamin K1, which is primarily involved in blood clotting. K2’s main role is activating proteins that regulate calcium metabolism.
The most bioavailable and well-studied form is MK-7 (menaquinone-7), found in fermented foods — particularly natto, a Japanese fermented soybean product. In practice, most people in the UK get very little K2 from food, making supplementation meaningful for those concerned about bone and cardiovascular health. See our full guide to vitamin K2 benefits for more detail on forms, food sources, and dosing.
K1 and K2 are often grouped together, but they function differently. K1 is well-absorbed from leafy green vegetables. K2 from food sources is far harder to obtain in meaningful amounts without specifically eating fermented dairy or natto.
Benefits of Vitamin D3
Bone Strength
D3 is essential for calcium absorption. Without adequate D3, the gut absorbs as little as 10–15% of dietary calcium. With sufficient D3, absorption rises to 30–40%. Over time, chronically low D3 weakens bones, contributing to rickets in children and osteomalacia and osteoporosis in adults.
The NHS recommends everyone in the UK consider supplementing vitamin D through autumn and winter for this reason.
Immune Function
Vitamin D receptors are present on virtually every immune cell. D3 helps regulate both the innate immune response (your first line of defence against pathogens) and the adaptive immune system (the targeted response that develops over time).
A 2017 meta-analysis in the BMJ analysed data from 25 randomised controlled trials and found vitamin D supplementation reduced the risk of acute respiratory tract infections, with the protective effect strongest in those who were deficient at baseline.
Mood and Mental Health
Low vitamin D levels are consistently associated with higher rates of depression and seasonal affective disorder (SAD). In the UK, where sunlight is limited for half the year, this is a particularly relevant concern. A 2020 meta-analysis in Nutrients found vitamin D supplementation was associated with significant improvements in depression scores compared to placebo, though researchers noted the evidence base needs larger, longer trials to draw firm conclusions. For a detailed look at the research, see our article on vitamin D and mood.
Cardiovascular Health
Vitamin D receptors are present in heart tissue and blood vessel walls. D3 may help regulate blood pressure through its effect on the renin-angiotensin system, and low D3 levels are associated with increased risk of heart failure and hypertension. The evidence here is observational rather than conclusive from trials, but the relationship is consistent across studies.
Benefits of Vitamin K2
Calcium Routing — the Core Function
K2 activates two critical proteins: osteocalcin, which binds calcium into bone matrix, and matrix Gla protein (MGP), which inhibits calcium from depositing in arterial walls. Without adequate K2, both proteins remain inactive regardless of how much calcium you consume or how much D3 you take.
This is the primary reason K2 matters, and why it’s particularly relevant when supplementing D3 at higher doses.
Bone Density
A 2013 randomised controlled trial published in Osteoporosis International found that MK-7 supplementation over three years significantly reduced age-related bone mineral density decline and improved bone strength in postmenopausal women compared to placebo.
Arterial Health
The Rotterdam Study — a large Dutch cohort study — found that higher dietary K2 intake was associated with a 57% lower risk of dying from cardiovascular disease and significantly less aortic calcification. K1 intake showed no such association, underlining the importance of K2 specifically.
Why D3 and K2 Work Better Together
The combination matters because D3 increases calcium absorption, but without K2, that extra calcium has no guidance system. It may end up in soft tissue and arterial walls rather than bone.
Animal studies have shown that excess vitamin D without adequate K2 can accelerate arterial calcification — precisely the outcome D3 supplementation is meant to prevent. While direct evidence in humans is still emerging, the mechanistic case for combining them is robust.
Taking D3 alongside K2 (as MK-7) addresses this: D3 increases calcium availability, K2 ensures it goes where it’s needed.
Our Vitamin D3 with K2, Zinc, Boron and MCT Oil combines these nutrients in a single capsule with MCT oil to support absorption, since both D3 and K2 are fat-soluble and absorb significantly better when taken with dietary fat.
Dosage and How to Take Them
Both D3 and K2 are fat-soluble, so taking them with a meal containing fat (including MCT oil) improves absorption. A 2013 study found taking vitamin D with the largest meal of the day increased blood levels by around 50% compared to taking it on an empty stomach.
Vitamin D3: The NHS recommends 10 micrograms (400 IU) daily for adults, particularly from October to March. Many adults — especially those over 40, with darker skin, or who spend little time outdoors — may benefit from 1,000–2,000 IU daily. For people with confirmed deficiency, GPs may prescribe higher doses temporarily.
Vitamin K2 (MK-7): Most research uses doses of 90–200 micrograms daily. MK-7 has a longer half-life than MK-4 (another K2 form), meaning it stays active in the body for longer with a single daily dose.
Note on anticoagulants: If you take warfarin or other blood-thinning medication, speak to your GP before taking K2, as it may interact with these drugs.
Frequently asked questions
Do I need to take K2 with vitamin D3?
You don’t have to, but there’s a reasonable case for it. D3 increases calcium absorption — K2 helps direct that calcium into bone rather than soft tissue and arteries. If you’re supplementing D3 at doses of 1,000 IU or above, adding K2 as MK-7 is worth considering.
What’s the difference between K1 and K2?
K1 (phylloquinone) is primarily involved in blood clotting and is well-obtained from leafy green vegetables. K2 (menaquinone) activates proteins involved in bone formation and arterial health — and is much harder to get in meaningful amounts from a typical UK diet.
What form of K2 is best?
MK-7 is the most widely studied and has the longest half-life, meaning a single daily dose is effective. MK-4 is also active but has a shorter half-life requiring higher doses. Most quality D3+K2 supplements use MK-7.
How much vitamin D3 should I take?
The NHS recommends 400 IU daily for general supplementation. Adults over 40, those with limited sun exposure, or those with darker skin may benefit from 1,000–2,000 IU. If you’re unsure, a blood test (25-OH vitamin D) will tell you your current level.
Can you take too much vitamin D3?
Yes. Vitamin D toxicity is rare but possible at very high doses, typically above 10,000 IU daily for extended periods. The NHS advises against taking more than 4,000 IU daily without medical supervision. Symptoms of excess include nausea, weakness, and elevated blood calcium.
References
- Tripkovic L, et al. (2012). Comparison of vitamin D2 and vitamin D3 supplementation. American Journal of Clinical Nutrition, 95(6), 1357–1364.
- Martineau AR, et al. (2017). Vitamin D supplementation to prevent acute respiratory tract infections. BMJ, 356, i6583.
- 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.
- Knapen MHJ, et al. (2013). Three-year low-dose menaquinone-7 supplementation helps decrease bone loss in healthy postmenopausal women. Osteoporosis International, 24(9), 2499–2507.
- NHS. Vitamin D. (2023).
This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before starting any supplement, especially if you are pregnant, have a medical condition, or take prescription medication.
