Supplement
Vitamin D3
Vitamin · Cholecalciferol
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.
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.
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.
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.
- 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. For them, the results say nothing.
Status, approval and legal
For vitamin D3 as a dietary supplement, the evidence reviewed for this page contains no regulatory decision assessing supplement use. The assessment therefore rests exclusively on the randomized trials named and their populations.
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. Sustained overdosing above 10,000 IU daily can cause hypercalcemia, meaning an excessive calcium level. Before taking it, the 25-hydroxyvitamin D level can be measured; that is a status measurement, not a recommendation for use.
BK-Score Well supported, heavily overhyped
| Human evidence | 8 | |
|---|---|---|
| Mechanism | 9 | |
| Safety data | 9 | |
| Hype gap | 5 | |
| Track record of use | 10 |
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. “Safety data 9” means well studied – not harmless. “Track record of use 9” 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.
Does vitamin D help with a true deficiency?
That is open, not disproven. 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. These results therefore say nothing about overt deficiency or existing osteoporosis. The question remains explicitly open.
What are the side effects of vitamin D3?
Safety is well studied. Sustained overdosing above 10,000 international units daily can lead to hypercalcemia, meaning an excessive calcium level in the blood. 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), bathtub logic, risk groups, measurement logic 25-OH-D 40–60 ng/ml – and why K2 and magnesium belong with it. Information only, no dosage or usage recommendation.
Related
- Works together withVitamin K2 (MK-7)
- Works together withMagnesium
- Works together withOmega-3 (EPA/DHA)
- Same categoryMyo-inositol
- Same categoryMicronutrient concentrates (LaVita & Co.)
- Same categoryVitamin C
Sources
Open in the database – with search, filters and comparison (German app)
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: 2026-09-13.