Vitamin K2 is one of the most underappreciated nutrients in Irish nutritional medicine. Most Irish adults have heard of Vitamin D — the HSE has actively promoted D3 supplementation for years — and many take it daily. Far fewer know that Vitamin D3 and Vitamin K2 are intimately linked in their function, and that taking high-dose D3 without adequate K2 may create an imbalance with real health consequences.
The core issue: Vitamin D3 dramatically increases calcium absorption from the gut. This is generally beneficial for bone health. But calcium absorbed from the gut needs to go somewhere — ideally into bones and teeth. In the absence of adequate K2, that calcium can end up depositing in arteries and soft tissues instead. K2's primary role in calcium metabolism is precisely to direct calcium into bone and away from arteries.
The vitamin K family is more complex than most people realise. Vitamin K1 (phylloquinone) is abundant in leafy green vegetables and is well known for its role in blood clotting. Vitamin K2 (menaquinone) is a different compound with a different biological role — it activates proteins that regulate calcium deposition throughout the body.
Within K2, there are several sub-forms distinguished by the length of their side chains, denoted MK-4 through MK-13. Of these, MK-4 and MK-7 are the most clinically relevant and the most commonly found in supplements:
The "calcium paradox" was first articulated by researchers who noticed that populations with high calcium intake (particularly from dairy) did not consistently have lower rates of osteoporosis, and in some cases appeared to have higher rates of cardiovascular calcification. The resolution to this apparent contradiction lies largely in K2.
Two key K2-dependent proteins govern calcium fate:
This elegant system — K2 as the "calcium traffic director" — explains why K2 deficiency can simultaneously contribute to both osteoporosis and cardiovascular disease. You can have plenty of calcium and plenty of Vitamin D, but if K2 is inadequate, the calcium goes to the wrong places.
The Rotterdam Study, a landmark Dutch prospective cohort study of 4,807 people followed for over 7 years, found that high dietary intake of K2 (but not K1) was associated with a 52% reduced risk of aortic calcification and a 57% reduced risk of death from cardiovascular disease. It also found a significant association between K2 intake and bone density.
The ECKO (Effect of Combined K and D on BMD) trial showed that MK-7 supplementation (180mcg/day) for 3 years significantly maintained bone density and strength in postmenopausal women compared to placebo, and significantly reduced age-related loss of bone mineral content. A 2020 meta-analysis in Osteoporosis International confirmed that K2 supplementation significantly reduces fracture risk in postmenopausal women.
For Irish women approaching or past menopause, this research is directly relevant. Osteoporosis affects approximately 300,000 Irish adults, predominantly women, and hip fracture carries a one-year mortality rate of around 20%. K2 offers a natural, evidence-based way to support bone density as part of a comprehensive approach that also includes D3, calcium from food sources, weight-bearing exercise, and adequate protein.
Coronary artery calcification is a major risk factor for heart attack and is now routinely measured in cardiovascular risk assessment. A 2009 Dutch study (Prospect-EPIC) found that for every 10 micrograms of additional daily K2 intake, the risk of coronary heart disease decreased by 9%. No such association was found for K1.
More recently, a 2015 study in Thrombosis and Haemostasis found that 180mcg/day of MK-7 for 3 years significantly reduced arterial stiffness and improved vascular elasticity in postmenopausal women — both measures associated with reduced cardiovascular risk. The researchers attributed this to MK-7's activation of MGP, which inhibited arterial calcium deposition.
Given that Vitamin D3 increases calcium absorption and K2 directs that calcium into bone, the logical conclusion is that D3 and K2 should be taken together. Several supplement manufacturers now produce combined D3+K2 products, and this is the format Pat Coffey at The Honey Pot tends to recommend.
When D3 intake is high — anything above 2,000 IU daily — the case for co-supplementing with K2 becomes stronger. The evidence suggests that high-dose D3 (>4,000 IU/day) without K2 may increase the risk of soft-tissue calcium deposition. This does not mean D3 is dangerous — far from it — but it does suggest that combining with K2 is prudent, particularly for older adults taking higher D3 doses through the Irish winter.
The typical recommended combination is D3 (1,000–4,000 IU) with K2 as MK-7 (90–180mcg). Both are fat-soluble vitamins and should be taken with a meal containing fat for optimal absorption.
The Irish diet is generally poor in K2. Unlike K1, which is abundant in green vegetables eaten throughout the year, K2 is primarily found in fermented foods and certain animal products:
For most Irish adults who do not eat natto or organ meats regularly, dietary K2 intake falls well below the amounts shown to be beneficial in research studies. Supplementation with MK-7 (90–180mcg/day) is a practical and cost-effective strategy.
"I recommend K2 to virtually everyone who comes in asking about Vitamin D," says Pat. "They go hand in hand. I also particularly recommend it to women over 45 thinking about bone health, and to anyone who has had any discussion with their doctor about cardiovascular risk. The research on arterial calcification is really quite persuasive." The Honey Pot at 14 Abbey Street, Clonmel stocks a range of K2 supplements including standalone MK-7 and combined D3+K2 products, and ships nationwide via thehoneypotonline.ie.