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Supplement

Vitamin D3

Vitamin · Cholecalciferol

12 min read 14 sources, checked November 13, 2026

In the BK-Score: Well supported, heavily overhyped – human evidence 8 out of 10, safety data 9 out of 10. The effect is well supported, but the marketing promises more.

Vitamin D3 (cholecalciferol) is strictly speaking a hormone, and its receptor biology is very well understood. Two large randomized trials in people who were not preselected for deficiency missed their primary endpoints.

What vitamin D3 is

Cholecalciferol forms in the skin under sunlight and is found in fatty fish and egg yolk. The liver converts it into 25-hydroxyvitamin D, the usual status marker in the blood. Via the vitamin D receptor it influences the expression of numerous genes.

Its role in calcium balance and bone mineralization is undisputed. What is up for debate is something else: whether additional intake prevents disease in people without a documented deficiency.

Molecular target

  • Agonist Vitamin D3 receptor Gene VDR · UniProt P11473

Data: ChEMBL, UniProt · assigned automatically, as of November 12, 2026. A known target does not mean an effect in humans is established.

Why a rising lab value is not a benefit

The 25-hydroxyvitamin D level is a surrogate marker: it shows that the product was absorbed, not that the course of disease changes. In the Australian D-Health trial the level rose by about 38 nmol/l - the primary endpoint, all-cause mortality, was unaffected.

This is exactly where the hype gap lies: vitamin D is promoted for nearly every disease, and the randomized trials do not show that.

The open question: true deficiency

The large trials enrolled people who were mostly sufficient. Whether people with a true deficiency - 25-hydroxyvitamin D below 30 nmol/l - benefit was therefore not answered, because this group was barely represented.

So these trials do not disprove that vitamin D works in overt deficiency or in existing osteoporosis. They show that broad intake in a largely sufficient population does not improve the endpoints tested. Those are two different statements.

New evidence comes from Heidelberg: in 2026 the German Cancer Research Center (DKFZ) emulated both trials using UK Biobank data. In people with adequate levels it found no effect, like the original trials, while in deficiency below 30 nmol/l the risk of death was about a quarter lower. This suggests that the benefit lies with those who start out deficient. It does not prove it, because this is a projection from observational data, not a randomized trial.

What is well supported

  • That intake raises the blood level is cleanly documented. In D-Health, 21,315 participants aged 60 and over taking 60,000 IU monthly for five years reached a serum level of 115 nmol/l (SD 30) versus 77 nmol/l (SD 25) on placebo - an increase of about 38 nmol/l. Biochemically, the substance reliably does what it is supposed to do.
  • The null findings come from strong trials, not from thin data. VITAL randomized 25,871 people double-blind to 2,000 IU daily over a median of 5.3 years, D-Health 21,315 people over a median of 5.7 years; the fracture analysis is based on 1,991 fractures in 1,551 people. Reading these results, you are reading a precise result, not a knowledge gap.
  • Even at a high dose over five years, no harm signal appeared for the hard endpoints. With 60,000 IU monthly, all-cause mortality was 562 (5.3 percent) versus 538 (5.1 percent), HR 1.04 (95% CI 0.93-1.18; p=0.47), and cancer incidence was 1,336 versus 1,304 diagnoses, HR 1.02 (0.95-1.10).

What the studies show

VITAL: both primary endpoints missed

Manson 2019 randomized 25,871 people (men aged 50 and over, women aged 55 and over) who were not selected for vitamin D deficiency to 2,000 IU D3 daily or placebo; median follow-up 5.3 years. Cancer occurred 793 versus 824 times, HR 0.96 (95% CI 0.88-1.06; p=0.47). Major cardiovascular events: 396 versus 409, HR 0.97 (0.85-1.12; p=0.69). As a secondary outcome, cancer mortality was HR 0.83 (0.67-1.02), not significant, and cardiovascular death HR 1.11 (0.88-1.40).

Fractures: no difference

LeBoff 2022 analyzed 1,991 fractures in 1,551 people in the same cohort over a median of 5.3 years; participation was not tied to deficiency, low bone mass or osteoporosis. Total fractures occurred in 769 of 12,927 versus 782 of 12,944, HR 0.98 (0.89-1.08; p=0.70). Nonvertebral fractures: HR 0.97 (0.87-1.07; p=0.50). Hip fractures: HR 1.01 (0.70-1.47; p=0.96). Neither age, sex, BMI nor baseline level changed the result.

D-Health: level rises, mortality does not

Neale 2022 randomized 21,315 unscreened Australians aged 60 and over, double-blind and placebo-controlled, to 60,000 IU D3 monthly for five years, median follow-up 5.7 years. The serum level reached 115 nmol/l (SD 30) versus 77 nmol/l (SD 25), so the increase of about 38 nmol/l is documented. All-cause mortality was 562 (5.3 percent) versus 538 (5.1 percent), HR 1.04 (95% CI 0.93-1.18; p=0.47); cardiovascular mortality HR 0.96 (0.72-1.28). The cohort was mostly replete.

D-Health, cancer incidence

The D-Health cancer analysis (Neale 2025, n=21,308) counted 1,336 versus 1,304 cancer diagnoses, HR 1.02 (95% CI 0.95-1.10). Excluding melanomas, the hazard ratio was 1.04 (0.95-1.14). No difference here either.

VITAL, autoimmune diseases: smaller benefit, gone after stopping

Hahn 2022 reported 123 versus 155 newly confirmed autoimmune diseases in VITAL, HR 0.78 (95% CI 0.61-0.99), about 22 percent fewer. The follow-up (Costenbader 2024) confirmed further cases from the trial period after the fact; the benefit during the randomized phase then stood at HR 0.85 (0.70-1.04), about 15 percent and no longer statistically certain. Two years after supplementation ended, the HR was 0.98 (0.83-1.17); the effect had disappeared.

VITAL, telomeres: a small effect on a lab value

Zhu 2025 measured telomere length in 1,031 VITAL participants at baseline, after two years and after four years. With vitamin D, telomeres shortened by 0.14 kb (140 base pairs) less over four years than with placebo (95% CI 0.007-0.27; p=0.039). The press release from Mass General Brigham converted this into “nearly three years” less aging; that is a conversion, not a measured endpoint. Telomere length is a surrogate marker, and the lower bound of the confidence interval is close to zero.

Fractures: 2026 meta-analysis

Massé 2026 pooled 69 randomized trials with 153,902 participants on calcium, vitamin D and the combination in the BMJ. Vitamin D alone did not lower the risk of fractures, RR 1.00 (95% CI 0.95-1.06), with high certainty (36 trials, 92,045 people). The combination with calcium came to RR 0.91 (0.84-0.99); because the absolute effects were small, the authors rated that too as little to no benefit.

Endocrine Society guideline 2024

The guideline of the US professional society (Demay 2024) rests on its own systematic review (Shah 2024). It recommends supplementation without prior testing for children and adolescents, for people aged 75 and over, during pregnancy and in high-risk prediabetes. For healthy adults under 75 it advises against supplementing beyond the usual intake recommendation. It advises against routine 25-hydroxyvitamin D testing in all populations considered, because no study shows that measuring in itself brings a benefit. For people aged 75 and over the review found a very small reduction in mortality with high certainty; for those aged 19 to 74 it found no effect on the selected outcomes.

DKFZ 2026: emulating the trials with biobank data

Wang 2026 emulated VITAL and D-Health in the UK Biobank (237,502 and 185,809 people meeting the inclusion criteria) and applied the rise in blood level achieved in the trials to the observational data. Weighted to the composition of the original trials, all-cause mortality came to HR 0.97 (0.92-1.02) and 1.02 (0.97-1.07), the same null result as the real trials. In deficiency below 30 nmol/l the HR was 0.79 (0.72-0.87) and 0.75 (0.69-0.81), and below 50 nmol/l 0.85 and 0.81. The same research group's cancer analysis found 13 percent lower cancer mortality in deficiency, HR 0.87 in both emulations. The figures are modeled: nobody received vitamin D in this analysis.

Where the data stop

  • That 2,000 IU daily prevents cancer or cardiovascular events. VITAL tested exactly that in 25,871 people and missed both primary endpoints.
  • That broad intake prevents fractures. In the fracture analysis, total, nonvertebral and hip fractures did not differ - regardless of baseline level. The 2026 BMJ meta-analysis of 69 trials confirms this for vitamin D alone with high certainty.
  • That vitamin D permanently prevents autoimmune diseases. The benefit in VITAL shrank to about 15 percent in the follow-up, was no longer statistically certain and had disappeared after supplementation stopped.
  • That longer telomeres mean slower aging. The telomere effect in VITAL is small and only just significant; “nearly three years younger” is a conversion in the press release, not a clinical finding.
  • That a raised blood value is a benefit. D-Health raised the level substantially without changing all-cause mortality or cancer incidence.
  • Unresolved, not disproven: true deficiency. People with 25-hydroxyvitamin D below 30 nmol/l were barely represented in these trials. The DKFZ emulation points to a benefit in this group but does not replace a randomized trial that specifically enrolls people with deficiency.

Status, approval and legal

In Germany, vitamin D3 is available both as a dietary supplement and as an approved medicine for preventing and treating deficiency. The German Federal Institute for Risk Assessment (BfR) assessed supplement use in its opinion 035/2020 of July 31, 2020: it considers high-dose products unnecessary, says daily intake is not required from a nutritional point of view, and notes that the evidence points to an increased risk with long-term high intake. People in risk groups should clarify intake with a doctor beforehand.

For dietary supplements, the BfR proposes in its 2023 update (opinion 007/2024) a maximum of 20 µg (800 IU) per recommended daily portion; this is a proposal, not a legal limit. In 2023 the European Food Safety Authority (EFSA) confirmed the tolerable upper intake level for adults of 100 µg (4,000 IU) per day from all sources.

For context: a high evidence score means well studied, not effective. With vitamin D3, both apply.

Safety

The safety of vitamin D3 is well studied. A sustained excessive intake from supplements can cause hypercalcemia, meaning an excessive calcium level, with fatigue, muscle weakness, vomiting, constipation and heart rhythm disorders; in the long run, according to the BfR, kidney stones, kidney calcification and impaired kidney function can follow. Overdosing through the body's own production in sunlight is not possible. The EFSA upper limit for adults is 100 µg (4,000 IU) per day from all sources.

The 25-hydroxyvitamin D level can be measured; that is a status measurement, not a recommendation for use. The Endocrine Society, however, advised against routine testing without a specific reason in 2024.

BK-Score Well supported, heavily overhyped

Human evidence8
Mechanism9
Safety data9
Hype gap5
Track record of use10

The receptor biology is very well understood, and with VITAL and comparable trials there are large RCTs. The hype gap is the weakest value: vitamin D is promoted for nearly every disease, and the studies do not show that.

The score rates the state of knowledge, not the substance. A high safety-data value means well studied – not harmless. A high track-record value means used long and widely – that is not proof of efficacy.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)

Frequently asked questions about vitamin D3

Does vitamin D3 really work?

In people without a documented deficiency, the large randomized trials show no benefit for the endpoints tested. VITAL missed cancer and major cardiovascular events, D-Health missed all-cause mortality. The role of vitamin D in calcium balance and bone mineralization remains undisputed.

Does vitamin D protect against cancer?

In VITAL, 2,000 IU daily was associated with 793 versus 824 cancer cases, hazard ratio 0.96 with a p-value of 0.47. In the Australian D-Health trial it was 1,336 versus 1,304 diagnoses, hazard ratio 1.02. Neither trial found a difference between vitamin D and placebo.

Does vitamin D prevent fractures?

In the VITAL fracture analysis, with 1,991 fractures, the groups did not differ. Total fractures had a hazard ratio of 0.98, nonvertebral fractures 0.97, hip fractures 1.01. Participation, however, was not tied to deficiency, low bone mass or osteoporosis. A 2026 meta-analysis in the BMJ of 69 randomized trials with about 154,000 participants reached the same result: vitamin D alone did not protect against fractures, with high certainty.

Does vitamin D help with a true deficiency?

That is open, not disproven, and the evidence tends to point toward a benefit. People with a 25-hydroxyvitamin D level below 30 nmol/l were barely represented in the large trials, and the Australian cohort was mostly sufficient. In 2026 the German Cancer Research Center (DKFZ) emulated VITAL and D-Health with UK Biobank data: in people with adequate levels there was no effect, as in the original trials, while in deficiency the risk of death was about a quarter lower. That is a projection from observational data, not randomized proof. A trial that specifically enrolls people with deficiency is still missing.

What are the side effects of vitamin D3?

Safety is well studied. A sustained excessive intake from supplements can lead to hypercalcemia, meaning an excessive calcium level in the blood, and in the long run to kidney stones and kidney calcification. The EFSA upper limit for adults is 100 µg (4,000 IU) per day from all sources. Overdosing through sunlight is not possible. In the large randomized trials with 2,000 IU daily or 60,000 IU monthly, this problem was not in the foreground.

Why do the large trials find no benefit?

Both trials enrolled people who were not preselected for deficiency and were mostly sufficient. If you already have enough, you can hardly benefit from more. D-Health raised the blood level by about 38 nmol/l without changing all-cause mortality. The lab value went up, the outcome did not.

The podcast episode (in German)

Episode 61

Vitamin D & K2: The Sunshine Duo, Fact-Checked

The podcast by Paul Höser (Episode 61) · with Paul & Paula. The sunshine hormone, fact-checked: the VITAL trial read honestly (including −22% autoimmune diseases; about −15% and no longer certain in the follow-up), bathtub logic, risk groups, measurement logic 25-OH-D 40–60 ng/ml – and what speaks for K2 and magnesium as partners (an added benefit is not shown by studies). Information only, no dosage or usage recommendation.

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Information only, not medical advice and not a usage or dosage recommendation. Prescription-only and unapproved substances belong in a doctor's hands. Last updated: 2026-11-13.