Vitamin D

The marker is real. The medicine is not.

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In brief: Population studies consistently linked low vitamin D levels to higher rates of cardiovascular disease and premature death, and vitamin D receptors throughout the cardiovascular system made the biology plausible. (6) But large randomized trials told a different story: VITAL — nearly 26,000 US adults over more than five years — found no reduction in heart attacks, strokes, or cardiovascular death, and a meta-analysis of 21 trials with more than 83,000 participants reached the same conclusion at every dose and in every subgroup. (1,5) The most consistent explanation is that low vitamin D was a marker of poor health, not a modifiable cause of it. Vitamin D supplementation remains valid for bone health and for correcting documented deficiency; it does not prevent cardiovascular events. (1,8)

Introduction

For more than two decades, vitamin D was the most promising nutrient in cardiovascular epidemiology. The observational signal was unusually strong. (6) The biology made sense: vitamin D receptors had been identified throughout the cardiovascular system, and the proposed mechanisms — blood pressure regulation, endothelial function, anti-inflammatory effects, insulin sensitivity — were each plausible on their own. (6) When this many lines of evidence pointed the same way, supplementation looked like a near-certainty.

The randomized trials produced a different answer.

VITAL, the largest vitamin D trial ever conducted, enrolled 25,871 US adults and followed them for a median of 5.3 years. (1) There was no reduction in major cardiovascular events, and none in heart attacks, strokes, or cardiovascular death individually. The benefit that observational data had pointed to for twenty years did not appear.

VITAL was not alone. Across a series of large randomized trials in different populations and at different doses, the result was the same. (2,3,4) A meta-analysis of 21 trials with more than 83,000 participants found no cardiovascular benefit at any dose, in any subgroup. (5)

Low vitamin D was never the cause. It was reflecting something else.

This does not make vitamin D supplementation pointless. The original indication — bone health, with calcium, in older adults and those with deficiency — remains valid. (8) The cardiovascular indication does not.

Find Your Situation

The table identifies what the evidence supports for specific clinical profiles. Each entry is examined in full in the sections that follow.

Where the Evidence Stands

Vitamin D is the rare case in this series where the cardiovascular question has been answered directly rather than left open.

Mechanism and observational association: strong. Vitamin D receptors sit throughout the cardiovascular system, and low vitamin D consistently associates with higher cardiovascular risk and mortality in cohort studies. (6,7)

Cardiovascular outcome trials: multiple, large, and negative. VITAL, ViDA, and D-Health found no reduction in cardiovascular events, and a meta-analysis of 21 trials with more than 83,000 participants found no benefit at any dose or in any subgroup. (1,2,4,5)

Established uses: bone health and deficiency correction. Vitamin D with calcium reduces fracture risk in older adults with deficiency, and correcting documented deficiency is beneficial. (8)

Taken together, unlike most supplements in this series, vitamin D has been tested at the level of hard cardiovascular outcomes, repeatedly. The trials are consistent and negative for cardiovascular prevention, while the bone and deficiency uses are established. The cardiovascular question is effectively settled.

Vitamin D Physiology and the Cardiovascular Hypothesis

Vitamin D is synthesized in skin exposed to UVB sunlight or obtained from diet and supplements. It undergoes two conversions: first in the liver to 25-hydroxyvitamin D (the form measured in blood tests), then in the kidneys to 1,25-dihydroxyvitamin D (calcitriol), the active hormone.

The classical role is calcium and bone. Vitamin D increases intestinal calcium absorption, maintains serum calcium, and supports bone mineralization. Severe deficiency causes rickets in children and osteomalacia in adults. (8) These functions are firmly established and are the basis for legitimate supplementation.

The cardiovascular hypothesis was built on a different foundation. Vitamin D receptors and the enzyme that converts vitamin D to its active form had been identified throughout the cardiovascular system — in heart muscle cells, vascular smooth muscle, the endothelium (the inner lining of blood vessels), and immune cells. (6) The proposed mechanisms followed from this distribution: suppression of the renin-angiotensin system to lower blood pressure, improved endothelial function, anti-inflammatory effects on atherosclerosis, improved insulin sensitivity, and effects on cardiac remodeling.

Each mechanism had supporting laboratory evidence. None has translated into measurable cardiovascular benefit in randomized trials. (1,2,4,5) The pattern is the same one seen with vitamin E (Article 17), niacin (Article 8), and homocysteine-lowering B vitamins (Article 16): the mechanism is real, but it was never the rate-limiting step in cardiovascular disease.

The Observational Evidence: Why It Looked So Promising

The case for vitamin D’s cardiovascular benefit was built on more than a decade of observational research, much of it from independent groups using consistent methods.

The Framingham Offspring Study followed 1,739 participants without prior cardiovascular disease for a mean of 5.4 years. Individuals with 25(OH)D below 15 ng/mL had a multivariable-adjusted hazard ratio of 1.62 for incident cardiovascular events compared with those at or above that level — roughly a 62% higher risk. (6) An individual-participant meta-analysis of eight European and US cohorts (Schöttker 2014) found that low vitamin D was associated with higher all-cause, cardiovascular, and cancer mortality. (7)

By observational standards, the consistency was impressive. By trial standards, it would prove misleading.

Why the observational data misled. The dominant explanation is reverse causation: illness lowers vitamin D, not the other way around. People who are sick spend less time outside, eat less well, and have reduced capacity to synthesize and metabolize vitamin D. The low level reflects their cardiovascular risk; it does not create it.

Confounding compounds this. Low vitamin D tracks with obesity, sedentary lifestyle, older age, darker skin at northern latitudes, and chronic disease — all of which independently raise cardiovascular risk. Outdoor physical activity is the cleanest example: it produces both vitamin D from sun exposure and cardiovascular benefit from exercise, generating two correlated signals from a single underlying behavior.

The result was one of the most extensively studied nutritional associations in modern epidemiology — and one the trials would not confirm.

The Trial Evidence: What Actually Happened

Multiple large randomized controlled trials directly tested the vitamin D cardiovascular hypothesis across tens of thousands of participants.

VITAL (2019). 25,871 US adults without baseline cardiovascular disease randomized to vitamin D3 2,000 IU daily or placebo for a median of 5.3 years. The primary cardiovascular composite — myocardial infarction, stroke, and cardiovascular death — occurred at a hazard ratio of 0.97 (95% CI 0.85–1.12). (1) Secondary outcomes were similarly null: myocardial infarction (HR 0.96), stroke (HR 0.95), cardiovascular death (HR 1.11), and the expanded composite including coronary revascularization (HR 0.96). The number of cardiovascular events was nearly identical between groups (396 versus 409).

ViDA (2017). 5,110 New Zealand adults aged 50–84 randomized to monthly vitamin D (an initial 200,000 IU dose followed by 100,000 IU monthly) or placebo for a median of 3.3 years. There was no reduction in cardiovascular events, and vascular substudies found no improvement in blood pressure or arterial stiffness. (2)

D2d (2019). 2,423 adults with prediabetes randomized to vitamin D3 4,000 IU daily or placebo. There was no reduction in progression to diabetes. (3)

D-Health (2022). 21,315 Australians aged 60–84 randomized to vitamin D3 60,000 IU monthly (equivalent to about 2,000 IU daily) or placebo for five years. The primary outcome was all-cause mortality, with cardiovascular outcomes prespecified. There was no reduction in mortality or cardiovascular events. (4)

Barbarawi 2019 — the meta-analytic verdict. A JAMA Cardiology meta-analysis pooled 21 randomized trials including more than 83,000 participants. (5) Vitamin D supplementation was not associated with reduced risk of major adverse cardiovascular events, myocardial infarction, stroke, or cardiovascular death. Subgroup analyses across dose, baseline 25(OH)D level, and population characteristics showed no benefit anywhere.

A separate Cochrane review of vitamin D supplementation for prevention of mortality found at most a small reduction in all-cause mortality with vitamin D3, without a demonstrated cardiovascular-specific mortality benefit. (9)

Across multiple trials, doses, and populations, the answer is consistent: vitamin D supplementation does not reduce cardiovascular events.

The Evidence at a Glance

Observational data converged on a strong association. Trial data converged on no causal effect. Both observations are correct. The error was assuming the first implied the second.

The Marker Hypothesis

The disconnect between observational associations and trial results demands explanation, and the most consistent answer is that low vitamin D is not causing cardiovascular disease. It is reflecting it.

Consider a person who is overweight, sedentary, and managing several chronic conditions. They go outside less, get less sun, and make less vitamin D. Their level on a blood test reads low. Their heart-disease risk reads high. Both numbers are real, and both are connected — but the connection runs through their underlying health, not through the vitamin itself. Giving them a supplement raises the lab number; it does not change what is actually driving their cardiovascular risk.

If this explanation is correct — and the trial pattern strongly supports it — then supplementing vitamin D should not reduce cardiovascular events. That is exactly what the trials found, across doses, across populations, and including in subgroup analyses of people with documented baseline insufficiency. (1,2,4,5) If vitamin D were causally protective, a signal would emerge in trials using high doses or in deficient populations. It is not there.

The alternative explanations — wrong dose, wrong timing, wrong form, wrong population — have been proposed and tested. The trial portfolio spans doses from 800 IU daily to 100,000 IU monthly, achieving meaningful increases in measured blood levels. (1,2) Across this entire range, no cardiovascular benefit emerged, and dose-response trends are absent.

Vitamin D joins the list of nutrients where consistent observational associations were not confirmed by randomized trials: vitamin E and beta-carotene (Article 17), homocysteine-lowering B vitamins (Article 16), and niacin (Article 8).

What Vitamin D Supplementation Actually Does

The cardiovascular disappointment does not erase vitamin D’s legitimate uses. The original indication remains valid and clinically meaningful.

Bone health. Combined with calcium, vitamin D reduces fracture risk in older adults, particularly those with deficiency. (8) Hip fracture in elderly patients carries substantial associated mortality, so this benefit — established in randomized trials — is clinically important.

Severe deficiency correction. Profound vitamin D deficiency (below 10 ng/mL) causes bone pain, muscle weakness, and osteomalacia. (8) Correcting it is straightforwardly beneficial.

Muscle and falls. Adequate vitamin D supports muscle function, but the benefit on fall prevention is more contested than commonly stated. The US Preventive Services Task Force in 2018 specifically recommended against vitamin D supplementation to prevent falls in community-dwelling adults 65 and older (a Grade D recommendation), citing trial evidence of no benefit and possible harm at higher doses. The benefit, where it exists, appears largest in residential-care populations and those with documented deficiency, not in healthy community-dwelling older adults.

Possible immune effects. A VITAL secondary analysis found a modest reduction in autoimmune disease incidence with vitamin D supplementation. (11) This finding is intriguing and has not yet been replicated, so it is hypothesis-generating rather than an established indication.

What supplementation does not do. It does not prevent heart attacks, strokes, or cardiovascular death. (1,2,4,5) It does not prevent cancer in primary prevention — VITAL found no reduction in invasive cancer incidence. (1) It does not prevent diabetes — D2d was negative. (3)

The Vitamin D Plus K2 Combination

A widely promoted argument in supplement marketing holds that vitamin D should always be taken with vitamin K2: vitamin D increases intestinal calcium absorption, while K2 activates the proteins (matrix Gla protein in arteries, osteocalcin in bone) that direct that calcium toward the right tissues. The implication is that vitamin D without K2 may push calcium toward arterial walls. The mechanism is biologically rational. The trial evidence does not support the cardiovascular framing of it.

Diederichsen 2022, the most rigorous direct test of this combination, randomized men with aortic valve calcification to MK-7 720 µg plus vitamin D 25 µg daily or placebo for two years. (10) There was no reduction in calcification progression on the primary endpoint, and no outcome trial has shown that the D-plus-K2 combination reduces heart attacks, strokes, or cardiovascular death.

The honest position: at typical supplement doses, in people not on warfarin, the combination poses no clear safety concern. Taking K2 alongside vitamin D as nutritional-gap correction is reasonable for people who do not consume fermented foods or aged cheeses. Taking the combination specifically to prevent arterial calcification or cardiovascular events is not supported by current trial evidence. Article 14 covers the K2 evidence in full.

Vitamin D and Cardiovascular Guidelines

For bone health and correction of documented deficiency, vitamin D is a recommended intervention. For cardiovascular event prevention, it is not.

The Institute of Medicine recommends 600 IU daily for adults under 70 and 800 IU daily for adults 70 and older, framed primarily around bone health. (8) The Endocrine Society recommends higher amounts — 1,500 to 2,000 IU daily — for adults at risk of deficiency.

Major cardiovascular guidelines do not recommend vitamin D supplementation for cardiovascular event prevention. The 2018 ACC/AHA cholesterol guideline, the 2017 ACC/AHA hypertension guideline, the 2022 AHA/ACC/HFSA heart failure guideline, and the US Preventive Services Task Force all decline to recommend vitamin D for primary prevention of cardiovascular disease. The USPSTF specifically reviewed VITAL and other trial evidence and concluded that the data do not support cardiovascular event prevention as an indication.

What this means: Vitamin D has been mechanistically interesting, observationally associated, and widely tested. If the trials supported routine cardiovascular use, the guidelines would reflect it. They do not, because the trials have not delivered the outcomes the mechanism predicted.

Dosing and Practical Considerations

Form. Vitamin D3 (cholecalciferol) raises blood levels more efficiently than D2 (ergocalciferol) and is generally preferred. D3 appears in fatty fish, egg yolks, and fortified dairy; D2 appears in mushrooms exposed to UV light.

Absorption. Vitamin D is fat-soluble; taking it with a fat-containing meal meaningfully improves absorption.

Toxicity. Hypercalcemia (elevated blood calcium) is the primary concern at very high sustained doses. Symptoms include nausea, weakness, confusion, kidney stones, and, rarely, cardiac arrhythmias. High-dose vitamin D combined with calcium supplementation may increase kidney-stone risk in some people.

Drug interactions. Thiazide diuretics may increase calcium retention when combined with vitamin D and calcium, so periodic calcium monitoring is appropriate. Corticosteroids may reduce vitamin D effectiveness. Some anticonvulsants (phenytoin, phenobarbital) accelerate vitamin D metabolism. Cholestyramine and orlistat may reduce vitamin D absorption. Article 20 covers cardiovascular drug-supplement interactions in full.

Testing. Routine vitamin D testing in asymptomatic adults is not supported for cardiovascular purposes. Testing is appropriate for adults with osteoporosis, malabsorption conditions, prolonged anticonvulsant or corticosteroid use, or symptoms suggesting deficiency.

Two Patients, Same Supplement Aisle, Different Answers

Both profiles below are composites built from the populations studied in the trials this article reviews.

Patient A: A 41-year-old woman, four years after gastric-bypass surgery, taking the standard post-bariatric supplement regimen including vitamin D 3,000 IU daily. Her recent 25(OH)D level is 28 ng/mL. She asks her surgeon whether she still needs to take it.

This is one of the cleanest indications for vitamin D supplementation in clinical medicine. Bariatric surgery — particularly Roux-en-Y gastric bypass — bypasses the duodenum and proximal jejunum, where most fat-soluble vitamin absorption occurs. Without ongoing supplementation, vitamin D deficiency is the rule rather than the exception in this population, with consequences for bone health that accumulate over years. The cardiovascular trials are not the relevant evidence base for her decision. She is taking vitamin D to maintain absorption-corrected status in a setting where her gut anatomy makes deficiency the default; the 28 ng/mL level on her current regimen is what success looks like, and stopping the supplement is what would put her at risk. Her bariatric program manages this — the vitamin D is part of the protocol, not a cardiovascular intervention.

Patient B: A 58-year-old man with mildly elevated LDL, a BMI of 29, and a 25(OH)D level of 24 ng/mL — flagged “low” on his lab report. His primary care physician suggested 2,000 IU daily. He has read that low vitamin D associates with cardiovascular risk and is now taking 5,000 IU daily, hoping to lower his risk of a heart attack alongside his statin.

The 24 ng/mL level is below the 30 ng/mL threshold many labs flag as “insufficient,” but well above the 20 ng/mL the Institute of Medicine identifies as adequate for bone health. (8) The label on his lab report carries more rhetorical force than the underlying biology supports. More importantly, his cardiovascular reasoning is precisely the part the trials tested. (1,2,4,5) His low-normal level may correlate with his cardiovascular risk profile — that observational association is real (6) — but whether supplementing the vitamin modifies that risk is the question the trials answered, and the answer was no. The 5,000 IU dose is unlikely to harm him over this duration, but it is not buying cardiovascular protection. The tools that would modify his risk — the statin he already takes, attention to weight and activity, dietary pattern — are doing the actual work. His physician is the right person to right-size the dose to a bone-and-deficiency rationale; the heart-attack rationale is the part the evidence does not support.

Common Misconceptions

“My vitamin D level is low, so I’m at higher risk for heart disease — supplementing will lower my risk.” The first part is observationally true. The second is what the trials directly tested and disproved. (1,5) The level reflects overall health; it does not drive cardiovascular outcomes.

“Maybe higher doses, or different patients, would show benefit.” Trials have tested doses from 800 IU daily to 100,000 IU monthly across older adults, deficient adults, prediabetic adults, and adults at northern latitudes. (1,2,3,4,5) Barbarawi 2019 pooled 21 trials and more than 83,000 participants and found no reduction at any dose, in any subgroup. (5) Dose-response signals are absent.

“Vitamin D prevents cancer.” VITAL — the largest and most rigorous test — found no reduction in invasive cancer incidence with 2,000 IU daily over 5.3 years. (1) A modest reduction in cancer mortality emerged in analyses excluding the early follow-up period, but that finding was not prespecified and remains hypothesis-generating.

“Vitamin D prevents diabetes.” D2d tested this directly in 2,423 adults with prediabetes. Vitamin D 4,000 IU daily did not reduce progression to diabetes. (3)

“You must take K2 with vitamin D to prevent arterial calcification.” The mechanism is plausible, but the most rigorous direct test of the combination found no reduction in calcification progression. (10) Reasonable as nutritional-gap correction; not supported as cardiovascular protection.

“If observational studies and biology both support a benefit, surely it must be real.” This assumption has failed repeatedly: vitamin E, beta-carotene, B vitamins, niacin, and now vitamin D. Mechanism plus observational association is the reason to run a trial, not a substitute for one.

The Bottom Line

Vitamin D was the most studied nutrient of the past two decades, with one of the strongest observational signals in nutritional epidemiology and a plausible mechanism in every cardiovascular tissue that mattered. (6,7) Across populations, doses, and follow-up periods, the trials returned the same answer: vitamin D does not prevent cardiovascular events. (1,2,4,5)

Vitamin D earns its place for what the evidence supports, not for what the marketing implies. Across more than 83,000 randomized participants, the trials eventually made clear that the question had been framed the wrong way. The vitamin level was telling researchers something true about underlying health. It was not telling them about a modifiable cause.

Article 16 examines B vitamins and homocysteine — a parallel case where the biomarker predicted cardiovascular risk, the supplement moved the marker, and the trials still failed to reduce events. It is the clearest example in the series of a measurable biomarker that turned out not to be a modifiable cause.

Key Terms

25-hydroxyvitamin D (25(OH)D): The form of vitamin D measured in blood tests, reflecting status from both sun exposure and supplementation. Below 20 ng/mL is generally considered deficient for bone health; below 10 ng/mL indicates severe deficiency.

Calcitriol: The active form of vitamin D (1,25-dihydroxyvitamin D), produced in the kidneys. Vitamin D receptors throughout the body bind calcitriol — the wide tissue distribution that drove the cardiovascular hypothesis.

Cholecalciferol (D3): The vitamin D form found in fatty fish, egg yolks, and fortified dairy, and produced in skin from UVB exposure. Generally preferred over D2 for raising blood levels efficiently.

Confounding: When a third factor influences both the measured exposure (vitamin D level) and the outcome (cardiovascular disease), creating an association that does not reflect causation. Central to why the vitamin D observational data misled.

Hypercalcemia: Elevated blood calcium, the primary toxicity of vitamin D excess at very high sustained doses. Symptoms include nausea, weakness, confusion, kidney stones, and, rarely, cardiac arrhythmias.

Marker hypothesis: The explanation that low vitamin D reflects underlying poor health rather than causing it — meaning supplementation addresses the marker without addressing what the marker is signaling. The most consistent explanation for the vitamin D observational-trial disconnect.

Reverse causation: When apparent cause and effect are inverted. In vitamin D research: illness causes low vitamin D, rather than low vitamin D causing illness.

VITAL: The Vitamin D and Omega-3 Trial. 25,871 US adults randomized to vitamin D3 2,000 IU daily, omega-3 1 g daily, both, or placebo, in a 2×2 factorial design with a median 5.3-year follow-up. The largest and most definitive vitamin D cardiovascular outcome trial; it found no reduction in major cardiovascular events.

Vitamin D deficiency: Blood 25(OH)D below 20 ng/mL, associated with bone disease and muscle weakness. The cardiovascular risk associations seen in observational studies have not been reproduced as cardiovascular benefit in supplementation trials.

References

1. Manson JE, Cook NR, Lee IM, et al. Vitamin D supplements and prevention of cancer and cardiovascular disease. N Engl J Med. 2019;380(1):33–44.
2. Scragg R, Stewart AW, Waayer D, et al. Effect of monthly high-dose vitamin D supplementation on cardiovascular disease in the Vitamin D Assessment Study (ViDA). JAMA Cardiol. 2017;2(6):608–616.
3. Pittas AG, Dawson-Hughes B, Sheehan P, et al. Vitamin D supplementation and prevention of type 2 diabetes. N Engl J Med. 2019;381(6):520–530.
4. Neale RE, Baxter C, Romero BD, et al. The D-Health Trial: a randomised controlled trial of the effect of vitamin D on mortality. Lancet Diabetes Endocrinol. 2022;10(2):120–128.
5. Barbarawi M, Kheiri B, Zayed Y, et al. Vitamin D supplementation and cardiovascular disease risks in more than 83,000 individuals in 21 randomized clinical trials: a meta-analysis. JAMA Cardiol. 2019;4(8):765–776.
6. Wang TJ, Pencina MJ, Booth SL, et al. Vitamin D deficiency and risk of cardiovascular disease. Circulation. 2008;117(4):503–511.
7. Schöttker B, Jorde R, Peasey A, et al. Vitamin D and mortality: meta-analysis of individual participant data from a large consortium of cohort studies from Europe and the United States. BMJ. 2014;348:g3656.
8. Institute of Medicine. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academies Press; 2011.
9. Bjelakovic G, Gluud LL, Nikolova D, et al. Vitamin D supplementation for prevention of mortality in adults. Cochrane Database Syst Rev. 2014;(1):CD007470.
10. Diederichsen ACP, Lindholt JS, Möller S, et al. Vitamin K2 and D in patients with aortic valve calcification: a randomized double-blinded clinical trial. Circulation. 2022;145(18):1387–1397.
11. Hahn J, Cook NR, Alexander EK, et al. Vitamin D and marine omega-3 fatty acid supplementation and incident autoimmune disease: VITAL randomized controlled trial. BMJ. 2022;376:e066452.

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