- Vitamin D plays a role in calcium absorption, immune function, and inflammation, and low levels have been associated with higher cardiovascular risk.
- However, large clinical trials have not shown that taking vitamin D supplements reduces heart attacks or strokes in people with normal vitamin D levels, the benefit appears to lie in correcting deficiency, not in supplementing beyond normal.
- Vitamin K2 helps direct calcium away from artery walls and toward bones, a mechanism that may reduce arterial calcification, though the clinical evidence is still developing.
- Combining D3 and K2 is an area of active research, with some promising early findings, but results from major trials are still awaited.
- Important: Vitamin K can interact with warfarin and other blood-thinning medications. Anyone on anticoagulants should speak with their doctor before taking vitamin K supplements.
- The best sources of vitamin D remain sunlight and food, particularly oily fish, egg yolks, and fortified dairy products.
Vitamin D has attracted more research attention over the past two decades than almost any other nutrient. The findings have been genuinely interesting, and sometimes surprising. For heart health in particular, the story is more nuanced than the supplement industry would have you believe, and understanding the distinction between correcting a deficiency and supplementing beyond normal levels is important.
This article looks at what vitamin D3 and its partner nutrient vitamin K2 actually do in the body, what the evidence shows about their relationship to cardiovascular health, and what that means in practical terms for everyday decisions about sun exposure, diet, and supplements.
What Does Vitamin D Actually Do?
Vitamin D is a fat-soluble vitamin, meaning the body stores it in fat tissue rather than excreting excess amounts daily. The body produces it when skin is exposed to sunlight, and it can also be obtained through food, primarily fatty fish, egg yolks, and fortified dairy products. The liver and kidneys convert it into the active form the body uses.
Most people know vitamin D for its role in bone health, it’s essential for the absorption of calcium from the gut. But it also has anti-inflammatory and immune-modulating properties, and receptors for vitamin D have been found in heart muscle cells and blood vessel walls, which is what prompted researchers to investigate its potential role in cardiovascular health.
Low Vitamin D and Cardiovascular Risk, What the Evidence Shows
The observational evidence, studies that track what happens to people with different vitamin D levels over time, has been reasonably consistent. A 2019 analysis published in BMC Cardiovascular Disorders combined data from 25 studies involving over 10,000 cardiovascular disease cases and found that low vitamin D levels were associated with a meaningfully higher risk of cardiovascular disease. People with low levels had a 44% higher risk of cardiovascular events overall, with a 54% higher cardiovascular mortality rate.
That sounds alarming, but observational studies have important limitations. People with low vitamin D levels often have other risk factors too, less outdoor activity, poorer diet, more chronic illness. The association may not mean that low vitamin D directly causes heart disease. Which is why the intervention trials, where researchers actually gave people supplements and measured what happened, are so important.
What Happened When Researchers Gave People Supplements?
This is where the story becomes more nuanced. A major 2020 analysis published in JAMA Cardiology combined data from 21 randomised clinical trials involving over 83,000 patients. Half received vitamin D supplements, half received a placebo. The result: vitamin D supplementation did not reduce heart attacks, strokes, or cardiovascular death and this held true regardless of the dose, the baseline vitamin D level, or whether participants took calcium at the same time.
The current evidence suggests that correcting a genuine vitamin D deficiency is worthwhile, but that taking supplements to push levels above the normal range does not appear to offer additional cardiovascular protection.
This is an important distinction. If your vitamin D level is genuinely low, which is common, particularly in people who spend little time outdoors, have darker skin, or live in less sunny climates, there are good reasons to bring it back into the normal range. But if your levels are already normal, adding more through supplementation does not appear to further reduce cardiovascular risk based on current evidence.
Food Sources of Vitamin D
For most people, a combination of sensible sun exposure and a varied diet that includes vitamin D-rich foods is the most practical approach. The table below gives a sense of how much vitamin D different foods provide.
| Food Source | Approximate Vitamin D | Notes |
|---|---|---|
| Cod liver oil | 450–1000 IU per tsp | Highly concentrated, also rich in omega-3 |
| Wild salmon | 600–1000 IU per 100g | Wild contains more D3 than farmed |
| Sardines (canned) | 250–300 IU per 100g | Also high in calcium and omega-3 |
| Mackerel | ~350 IU per 100g | Good all-round heart-healthy fish |
| Egg yolks | 40–60 IU per yolk | Varies with hen’s diet and sun exposure |
| Fortified milk | ~100 IU per 250ml | Check label, fortification varies |
| UV-exposed mushrooms | 400–700 IU per 100g | Sun-exposed mushrooms contain significantly more D2 |
Brief, regular sun exposure, around 15 minutes of direct skin exposure a few times a week, can also meaningfully contribute to vitamin D levels for most people. The amount varies considerably depending on skin tone, latitude, and season.
Where Does Vitamin K2 Fit In?
Vitamin K2 is less well known than vitamin D but increasingly studied for its potential cardiovascular role. It is distinct from vitamin K1, which is primarily involved in blood clotting. Vitamin K2 activates specific proteins, including one called Matrix Gla Protein, that help regulate where calcium ends up in the body.
The cardiovascular relevance of this is significant. Calcium in the right places, bones and teeth, is beneficial. Calcium in the wrong places, arterial walls, is harmful. Arterial calcification, where calcium deposits build up inside artery walls, is associated with increased arterial stiffness and a higher risk of heart attack and stroke. You may have heard of the coronary calcium score, a CT scan that measures exactly this kind of calcification in the arteries of the heart.
The proposed mechanism for K2 is that it helps keep calcium directed toward bones and away from arterial walls, potentially slowing the progression of arterial calcification. This is biologically plausible and supported by some observational evidence, but the clinical trial data is still developing.
The D3 and K2 Combination, Promising but Still Being Studied
There is a theoretical and increasingly evidence-supported rationale for combining D3 and K2. Vitamin D increases calcium absorption from the gut, which is generally a good thing for bone health. But without adequate K2 to direct that calcium appropriately, some researchers have raised the question of whether increased calcium absorption could contribute to arterial calcification in some circumstances.
Combining D3 with K2 is proposed to provide the benefits of improved calcium absorption while supporting its correct distribution in the body. Some studies have shown reductions in arterial stiffness with combined supplementation.
However, the major clinical trials needed to confirm a meaningful cardiovascular outcome benefit are still underway. The Danish AVADEC Trial which tested high-dose K2 and D3 in patients with aortic valve calcification over two years, did not find a significant reduction in calcification progression. This is a reminder that biologically plausible mechanisms don’t always translate into measurable clinical benefit when put to the test of a rigorous trial.
Vitamin K, including K2, can interact with anticoagulant medications, particularly warfarin. Warfarin works by blocking vitamin K-dependent clotting factors, so changes in vitamin K intake can affect how well the medication works and alter INR levels.
Anyone taking warfarin or other anticoagulant medications should not start vitamin K supplementation without first discussing it with their doctor. This applies even to dietary changes that significantly increase vitamin K intake.
What Does This Mean in Practice?
- Get your vitamin D level checked if you haven’t recently, particularly if you spend little time outdoors, have darker skin, or live in a climate with limited sunlight. A simple blood test tells you where you stand.
- If you’re deficient correcting it through sensible sun exposure, diet, or supplements (under medical guidance) is worthwhile for general health, including cardiovascular health.
- If your levels are normal the current evidence does not support taking high-dose supplements for additional cardiovascular benefit.
- Oily fish is your best food-based source salmon, sardines, and mackerel provide vitamin D alongside omega-3 fatty acids and high-quality protein, all of which support heart health.
- Vitamin K2 supplements are an area of genuine scientific interest, but the evidence base is not yet strong enough to make a general recommendation. If you are considering them, discuss with your doctor, particularly if you are on any medications.
- Never start vitamin K supplements if you are on warfarin without medical supervision.
Conclusion
Vitamins D3 and K2 are genuinely interesting nutrients from a cardiovascular perspective, and the science exploring their combined role in calcium metabolism and arterial health is evolving. The current evidence supports correcting vitamin D deficiency as a reasonable health goal, but does not support high-dose supplementation for cardiovascular protection in people with normal levels.
Vitamin K2’s role in directing calcium away from artery walls is biologically compelling, and ongoing research may eventually provide clearer guidance. For now, the most evidence-supported approach remains getting both nutrients primarily through food and sensible sun exposure, with supplements used selectively and under medical guidance, particularly for anyone on blood-thinning medication.
As with all supplement decisions, individual circumstances vary considerably and a conversation with your doctor is always the right starting point.
