Vitamin D3 and K2: Why I Stopped Taking One Without the Other

So, apparently your body makes vitamin D naturally when you’re in the sun. But, I went to a doctor in Spain once who told me that even if you walked around naked in the blazing sun all day, you still wouldn’t make enough Vitamin D for what your body needs! All you’d end up with is sunburn!   What a visual.

Anyway, you’ve heard of vitamin D. You hopefully take it, or your doctor has at least mentioned it. I did the same thing for years. My levels came back low at a checkup, so I picked up a bottle at the pharmacy, took it on and off the way most people do with a supplement that doesn’t feel urgent.

What I didn’t know until I went deep is that it is actually a lot more important than we realize especially for those of us concerned with heart health.  And as I dove into the post-cardiac research, I realized that there’s a second nutrient most people have never heard of — vitamin K2 — that works alongside vitamin D and does a whole lot for our heart health.  Once I understood how the two fit together, I couldn’t un-see it.! So here’s the version I wish someone had walked me through.

First, the one you know: vitamin D

Vitamin D is the “sunshine vitamin.” Your skin makes it from sunlight, and you get smaller amounts from food like fatty fish and fortified dairy. Its best-known job is helping your body absorb calcium, which is why it gets talked about for bone health. But it does a lot more than that that we’ll get into.

Here’s the catch: being low is more common than most people expect, especially after 40. We spend more time indoors, skin makes less D as we age, and food sources for D are often limited or depleted. Most people who are low have no idea, because it doesn’t cause obvious symptoms. And it has to be extremely medically low at a blood test for a doctor to take notice. 

What the heart research actually says about vitamin D

When researchers pool all the big randomized trials together, vitamin D’s effect on heart attacks and major cardiovascular events is small, and often doesn’t reach statistical significance. It does in some trials where people were chronically low to begin with and then not in other trials where people had a sufficient base line to start with. Research is ongoing and overall it was determined it is not a magic bullet. 

The picture gets more interesting though in older, higher-risk groups, which I assume is most people reading this, like me. Hello, it’s me! The D-Health Trial, published in the BMJ in 2023, was a large double-blind randomized study of 21,302 adults aged 60 to 84, followed for five years. Overall cardiovascular events came out modestly lower in the vitamin D group, and heart attacks specifically were about 19% lower (a hazard ratio of 0.81). The researchers were careful to note the absolute difference was small, But they also flagged that further study was warranted, particularly in people taking statins or other cardiovascular drugs. That’s us, I assume. It’s certainly me.

A separate 2023 meta-analysis in Nutrients pooled 80 randomized trials covering more than 163,000 people and found vitamin D was associated with a modest reduction in all-cause mortality—with the effect showing up most clearly in the higher-quality trials.

So: not a cure. Not close. But a nutrient that shows up consistently enough in cardiovascular research to take seriously—especially when so many adults over 40 are quietly running low.

Now the one you don’t know: K2 (and no, it’s not the same as K1)

If you’ve heard of vitamin K at all, it’s probably K1—the one in leafy greens, the one that matters for blood clotting, the one your doctor warns you about supplementing beyond food if you’re on a blood thinner like warfarin. K2 though, is a different animal. Different sources, different job. It shows up in fermented foods, grass-fed animal products, and some aged cheeses—none of which are exactly abundant in the typical American diet. And unlike vitamin D, almost no one’s doctor tests for it or brings it up.

Here’s what K2 does, in plain terms. Your body has a protein called Matrix Gla Protein (MGP for short). Think of MGP as a traffic cop for calcium: its job is to wave calcium toward your bones and away from your artery walls. But MGP only works when it’s switched on, and K2 is the switch. Without enough K2, more of that traffic cop stays off duty in the donut shop rather than out there directing, and calcium that should be heading to your bones can end up parked in your arteries instead. Which is exactly what you don’t want.

This isn’t just a theory — it’s been studied

The Rotterdam Study followed nearly 4,807 people for about seven years and found that those with the highest dietary K2 intake had a 57% lower risk of dying from coronary heart disease and a 52% lower risk of severe aortic calcification. K1 intake showed no such link—different nutrients, different mechanisms entirely. Now, that’s an observational study, so it can’t prove cause and effect, but the size of the gap got a lot of researchers’ attention.

Then, just this month (June 2026) the VitaK-CAC trial was published in JAMA Cardiology. This one’s a proper randomized, double-blind trial: 180 patients with established coronary artery disease, most of them (78%) on statins, randomized to K2 (MK-7) at 360mcg a day or placebo for two years. The K2 group showed roughly 29% less progression in their calcification score and about 42% less in calcium mass score versus placebo.

The effect was real and statistically significant, but the trial was small, it wasn’t designed to measure heart attacks or deaths, and the consensus was that it’s too early to start recommending K2 to everyone. It’s a promising signal though, not a closed case. But it’s the kind of signal that’s hard to ignore, especially since the people in the trial (older, statin-taking, real cardiovascular risk) look a lot like you and I.

Why they belong in the same conversation

Here’s the part that made it click for me. Vitamin D increases how much calcium your body absorbs. That’s part of how it helps your bones. But, if you’re pulling more calcium into circulation and you don’t have enough K2 to direct where it goes, you’re increasing the supply without making sure the routing system is in place.  Some people call this the Calcium paradox … we NEED calcium , we just need it to end up in the right place!

D3 increases the supply. K2 manages the destination. Mechanistically, they’re a team.

What I’d actually do

If you’re already taking D3 and not taking K2, that’s a conversation worth having at your next appointment with your cardiologist. And one real safety note: if you’re on warfarin or another anticoagulant, make sure you check with your doctor before you add anything new, especially a K, to make sure it’s the right one. 

Because you only get one heart. Till next time, Stephen.

Thompson, B., et al. (2023). Vitamin D supplementation and major cardiovascular events: D-Health randomised controlled trial. BMJ, 381, e075230. PMID: 37380191.

Ruiz-Garcia, A., Pallares-Carratala, V., et al. (2023). Vitamin D supplementation and its impact on mortality and cardiovascular outcomes: Meta-analysis of 80 randomized clinical trials. Nutrients, 15(8), 1810. PMID: 37111028.

Geleijnse, J. M., 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. PMID: 15514282.

Li, T., et al. (2023). Vitamin K supplementation and vascular calcification: A systematic review and meta-analysis of randomized controlled trials. Frontiers in Nutrition, 10, 1115069. PMID: 37252246.

Vossen, L. M., de Leeuw, P. W., Schurgers, L. J., et al. (2026). Two years of menaquinone-7 supplementation and coronary artery calcification: A randomized clinical trial. JAMA Cardiology. Published online June 10, 2026. e261279. PMID: 42268593.