Vitamin D3 and K2 — Why You Need Both and Why Most People Only Take One

By now, most Irish people are aware that vitamin D is important. Between our famously overcast skies, long winters, and growing rates of deficiency, the message that we need to supplement has well and truly landed. What far fewer people know is that taking vitamin D without vitamin K2 is only half the story — and could actually be doing you a disservice.

Here’s why vitamin D3 and K2 work as a team, and why taking one without the other doesn’t make complete sense.

Why Are So Many Irish People Deficient in Vitamin D?

Unlike almost every other essential nutrient, our primary source of vitamin D isn’t food — it’s sunlight. When skin is exposed to the sun’s ultraviolet rays, it produces vitamin D naturally. The problem for anyone living in Ireland is obvious: we simply don’t get enough reliable sunshine, particularly between October and March, to maintain adequate levels year-round.

The vitamin D receptor has been identified in nearly every tissue type in the human body — which tells us that vitamin D has far-reaching effects well beyond its most famous role in bone health. It is now understood to play a crucial part in immune function, mood regulation, cardiovascular health, and overall disease prevention. Several large-scale studies have found that optimal vitamin D levels are associated with lower all-cause mortality rates — in plain terms, getting enough vitamin D is associated with a longer, healthier life.

Public Health guidance now recommends that everyone considers supplementing with vitamin D all year round — not just in autumn and winter. For Irish people, this is particularly relevant.

So Where Does Vitamin K2 Come In?

This is where many people’s vitamin D knowledge stops — and where it really needs to go a little further.

When you supplement with vitamin D3, you increase your body’s absorption of calcium from the intestines and its retention from the kidneys. This is one of the key ways vitamin D supports bone health — more calcium becomes available in the body. But here’s the critical question: where does that calcium actually go?

The answer depends largely on vitamin K2.

The Calcium Traffic Problem

Think of vitamin D as the nutrient that gets calcium into your bloodstream, and vitamin K2 as the nutrient that directs it to the right place.

Calcium needs to be deposited in your bones and teeth — not in your arteries, kidneys, or soft tissues. Vitamin K2 activates two key proteins that manage this process:

• Osteocalcin is produced by bone-building cells and is responsible for binding calcium into bone. However, osteocalcin can only do this job after it has been activated — a process called carboxylation, which is entirely dependent on vitamin K2. Without adequate vitamin K2, osteocalcin remains inactive, calcium doesn’t get properly incorporated into bone, and bones can become porous and fragile over time.
• Matrix GLA Protein (MGP) is found in the lining of the arteries. Its job is to protect arterial walls from calcium deposits — the process known as arterial calcification, which is a significant risk factor for cardiovascular disease. MGP is also only activated in the presence of vitamin K2.

In short: without vitamin K2, calcium driven into the body by vitamin D has nowhere to go except potentially the wrong places — including the arteries.

What Does the Research Say?

The evidence supporting the combined use of vitamin D3 and K2 is compelling.

Numerous peer-reviewed studies have shown that low vitamin K levels are associated with reduced bone mineral density and increased fracture risk. Vitamin K2 supplementation has been shown to promote bone formation and reduce fracture risk — and research has found the combination of K2 and D3 to be more effective than either nutrient alone for preventing bone loss and improving bone density.

On the cardiovascular side, a large-scale European clinical trial following over 4,800 people aged over 55 for up to ten years found that dietary intake of vitamin K2 was protective against arterial calcification and cardiovascular death. Crucially, these benefits were not seen with vitamin K1 — the form found in leafy green vegetables — only with K2.

Professor Cees Vermeer, one of the world’s leading researchers on vitamin K2, has noted that MGP is the most powerful known inhibitor of soft tissue calcification — but that even healthy, non-supplementing adults typically have insufficient vitamin K2 to keep all of their MGP fully active.

Emerging research has also identified reduced vitamin K status as a potentially significant factor in poor outcomes from respiratory illness — adding another dimension to why K2 is worth taking seriously alongside vitamin D.

Which Form of Vitamin K2 is Best?

Not all vitamin K2 is equal. The form most consistently used in research and most widely recommended is MK-7 — a naturally derived form of K2 with a half-life of around three days. This longer half-life is important: it means MK-7 stays in the body long enough to build up a consistent, stable blood level, making it far more effective than shorter-acting forms.

When choosing a vitamin D3 and K2 supplement, look for one that specifies MK-7 as the form of K2 used.

What Dose Should You Take?

Research supporting the prevention of bone loss and demineralisation has used doses of 1,000 IU of vitamin D3 combined with 45mcg of vitamin K2 (as MK-7). Many people — particularly those who are significantly deficient — may benefit from higher doses of vitamin D3, but this is worth discussing with your GP or pharmacist, as vitamin D testing is available and gives a clear picture of where your levels actually sit.

The Simple Message

The science around vitamin D3 and K2 is nuanced, but the practical takeaway is straightforward:

If you’re supplementing with vitamin D — and most Irish people should be — make sure you’re also getting vitamin K2 alongside it. Together they support bone strength, help protect arterial health, and allow calcium to do its job properly in the body. Separately, vitamin D alone may be leaving part of the job undone.

At Ballyduff Pharmacy we stock a range of vitamin D3 and K2 supplements. If you’d like advice on which product and dose is right for you, our pharmacy team is here to help — visit us in store in Ballyduff, Co. Kerry, or shop online at ballyduffpharmacy.com. Shop all our Vitamin D3/K2 supplements here

Important Disclaimer

The information in this article is provided for educational purposes only and should not be considered medical advice. Food supplements should not be used as a substitute for a varied, balanced diet and a healthy lifestyle. Always seek advice from your GP or pharmacist before starting any new supplement, particularly if you have a medical condition or are taking prescribed medication.