D3 and K2, in Plain Words

The basics

Two vitamins, two jobs

Vitamin D helps your body take in calcium from food and use it. That is one of the health claims the European Union allows, because the evidence for it is strong.

Vitamin K2 helps decide where the calcium goes. It switches on two proteins that need vitamin K to work:

• Osteocalcin, made by bone-building cells, helps bind calcium into the bone itself.
• Matrix Gla protein, made in the walls of blood vessels, helps stop calcium from settling there.

Without enough vitamin K, both proteins stay in an "off" state. That is the logic behind pairing the two: vitamin D brings calcium in, and K2 helps direct it. The EU also allows the claim that vitamin K contributes to normal bones.

K1 and K2 are not the same. K1 comes mostly from leafy green vegetables and is used mainly for blood clotting. K2 is found in fermented foods such as natto (fermented soybeans) and sauerkraut, in egg yolks, and is also made by bacteria.

The new trial

What the Danish trial found

The DANCODE trial, from Odense University Hospital in Denmark, was published in the journal Circulation in September 2026.

• 398men and womenaverage age 71, all with a high calcium score in the heart's arteries at the start
• 2 yrsK2 with D3, or a look-alike capsuleneither the participants nor the researchers knew who took which
• ~21%slower build-up of new calciumcalcium score rose by 196 points with K2 + D3, and by 248 points in the comparison group

The calcium kept building up in both groups, as it does at that stage. It built up more slowly in the group taking both vitamins. The effect looked similar in women and men.

Read it carefully

What this does not mean

It measured a scan, not heart attacks. The result is a calcium score on a CT scan. The trial did not measure heart attacks, strokes or deaths, so it cannot say whether the slower build-up protects anyone from those.

An earlier trial from the same Danish group found no effect on heart valves. In 2022, 365 men aged 65 to 74 took the same combination for two years, and calcium in the main heart valve built up at the same rate as in the comparison group.

It was tested in older people who already had a lot of calcium in their arteries. It tells us little about younger, healthy people.

It is not a reason to start or stop anything. The logic is strong and the proof is growing, but one trial on a scan result is not the final word.

Who supplied the vitamin: the K2 used in the trial was supplied by the company that makes the ingredient. That is common in supplement research and does not make the result wrong, but you should know it.

Important

If you take a blood thinner

Vitamin K affects blood clotting. If you take a blood-thinning medicine such as warfarin, speak to your doctor before taking any vitamin K supplement, including K2. Changes in vitamin K intake can change how these medicines work.

What you can do

Calm, practical steps

• Food first. Fermented foods, egg yolks and leafy greens bring vitamin K into your plate without any bottle.
• If you already take vitamin D, ask whether K2 belongs with it. Your doctor or pharmacist can look at your full list of medicines.
• Read the label for the form. K2 is usually listed as MK-7 or MK-4. They behave differently in the body, so compare like with like.