Otherindependent cross-checkabstract checked★ key study I-DRY-109 in-depth review
Genetic study: milk does not clearly cause prostate cancer but may protect against bowel cancer
Genetically proxied milk consumption and risk of colorectal, bladder, breast, and prostate cancer: a two-sample Mendelian randomization study.
Overview
Larsson SC et al.. BMC Med 2020 · PMID 33261611 · DOI 10.1186/s12916-020-01839-9
Prostate OR 1.01 (0.99-1.02) per milk allele; colorectal 0.95.
Does the study support the specific claim? No · Methodological quality: moderate
Plain-language summary
Study explained simply
- What did the researchers want to find out?
Test cause and effect using genes linked to milk drinking.
- How was the study done?
Mendelian randomisation.
- How many people, animals or samples were studied?
Hundreds of thousands of Europeans.
- What was actually found?
No overall prostate effect; lower bowel cancer.
- How large was the observed effect?
Prostate OR 1.01.
- What do the results mean in everyday life?
Milk is unlikely to be a strong cause of prostate cancer.
- What are the study's limitations?
Instrument limits; milk only.
- What does it say about Elawady's specific claim?
Weakens the causal claim.
- Do the results apply to Germany and Europe?
European data.
- What can explicitly not be concluded from this study?
Cannot exclude a small effect.
Scientific in-depth review
1. Full reference
- Authors
- Larsson SC et al.
- Journal
- BMC Med
- Year
- 2020
- Identifier
- PMID 33261611 DOI 10.1186/s12916-020-01839-9
- Verification
- verified – PubMed efetch 2026-10-26
- Retracted
- no
- Corrections
- none
- Extent checked
- abstract only
- Conflicts of interest and funding
Cancer Research UK, MRC, Wellcome (record).
2. Research question
Is milk consumption causally related to colorectal, bladder, breast and prostate cancer?
3. Study design and population
- Study design
- Other – Two-sample Mendelian randomisation using the lactase-persistence variant (rs4988235/rs182549) as instrument for milk intake
- Population
UK Biobank, FinnGen, BCAC, PRACTICAL (European ancestry)
- Country
Europe
- Period
n/a
- Sample size
UK Biobank 367,643; FinnGen 135,638; PRACTICAL 140,254 men
- Inclusion and exclusion criteria
Consortium data
4. Methods and endpoints
- Exposure / intervention
Genetically proxied milk intake
- Comparator
Per allele
- Endpoints
Cancer incidence
- Statistical methods
MR (Wald ratio/IVW across cohorts)
5. Actual results
- Results
Colorectal cancer OR 0.95 (0.91-0.99) per milk-increasing allele; bladder 0.99 (0.94-1.05); breast 1.01 (1.00-1.02); prostate 1.01 (0.99-1.02), but FinnGen prostate 1.07 (1.01-1.13).
- Absolute risks
not reported
- Authors' conclusion
Findings strengthen evidence for a protective role of milk on colorectal cancer; no or limited evidence that milk affects bladder, breast and prostate cancer.
6. Strengths
- Reduces confounding and reverse causation
- Large consortia
7. Weaknesses
- Single-variant instrument; effect per allele, not per glass
- Pleiotropy cannot be fully excluded
- Only milk, not cheese
8. Risk of bias
- Orienting tool
- none applicable
- Type of assessment
- simplified assessment (abstract only)
- Full formal assessment
- no
- Overall risk of bias
- some concerns
- Rationale
MR assumptions.
9. Assessment of the statistical interpretation
Prostate OR close to 1 with narrow CI; the slight asymmetry of 0.99-1.02 is a rounding artefact.
10. Relevance to the specific claim
Genetic evidence does not support a causal effect of milk on prostate cancer overall (one consortium positive), while supporting protection against colorectal cancer.
11. Independent contrary findings
Cohort associations (A-DRY-02, I-DRY-108).
12. Transferability to Germany and Europe
European ancestry; lactase-persistence genetics differ in Egypt.
13. Overall methodological judgement
Limits causal interpretation of the dairy-prostate association.
| Multidimensional assessment | |
|---|---|
| Authenticity of the source | verified |
| Methodological quality | moderate |
| Risk of bias | some concerns |
| Precision of results | high |
| Directness | indirect |
| Reproducibility | unknown |
| Consistency with the overall evidence | mixed |
| Regional transferability | moderate |
| Adequacy of the interpretation (by the archive) | not applicable |
14. Open questions
Power for small effects; other dairy types.
15. Linked fact checks
- Dairy consumption increases prostate cancer risk (7% per 400 g/day; 34% for more than 2.5 servings/day). – Studies contradicting or limiting the claim
- Milk intake raises circulating IGF-1, and higher IGF-1 increases prostate cancer risk; this is the mechanism behind the dairy-prostate cancer link ('the same hormone that makes a calf grow'). – Studies contradicting or limiting the claim