Cohort studyindependent cross-checkfull text checked★ key study I-ABX-114 in-depth review
Danish siblings: early antibiotics not linked to overweight, weakly to asthma
Use of antibiotics in early life and development of diseases in childhood: nationwide registry study.
Overview
Brandt S et al.. BMJ Med 2025 · PMID 40519361 · DOI 10.1136/bmjmed-2024-001064
Comparing siblings, the link with overweight vanished and the link with asthma halved.
Does the study support the specific claim? Partly · Methodological quality: high
Plain-language summary
Study explained simply
- What did the researchers want to find out?
Test links between baby antibiotics and later diseases
- How was the study done?
National registers with sibling comparison
- How many people, animals or samples were studied?
518,483 children
- What was actually found?
Most links weakened within families; asthma and eczema remained
- How large was the observed effect?
Overweight sibling HR 1.02; asthma sibling HR 1.24
- What do the results mean in everyday life?
Family factors explain much of the earlier findings
- What are the study's limitations?
Overweight only from hospital records
- What does it say about Elawady's specific claim?
Contradicts antibiotics causing obesity; partly supports an asthma link
- Do the results apply to Germany and Europe?
High
- What can explicitly not be concluded from this study?
Causality for asthma
Scientific in-depth review
1. Full reference
- Authors
- Brandt S et al.
- Journal
- BMJ Med
- Year
- 2025
- Identifier
- PMID 40519361 DOI 10.1136/bmjmed-2024-001064
- Verification
- verified – PubMed efetch 2026-10-26 + Crossref
- Retracted
- no
- Corrections
- none
- Extent checked
- full text
- Conflicts of interest and funding
COPSAC, EU Horizon 2020, Danish Council for Independent Research, NIH and foundations; one author speaking fee from AstraZeneca (full text).
2. Research question
Is antibiotic use in the first year of life associated with immune-mediated diseases and overweight, and does it persist within sibships?
3. Study design and population
- Study design
- Cohort study – Nationwide registry cohort with sibling-matched analysis
- Population
Children born in Denmark 1998-2006 resident in their first year
- Country
Denmark
- Period
1998-2016; mean follow-up 13.2 years
- Sample size
518,483 children; sibling population 272,753 (126,632 sibships)
- Inclusion and exclusion criteria
No outcome before age 1
4. Methods and endpoints
- Exposure / intervention
Any systemic antibiotic before age 1 (40.3%)
- Comparator
No antibiotics; siblings
- Endpoints
Asthma, allergy, eczema, coeliac disease, juvenile arthritis, type 1 diabetes, overweight
- Statistical methods
Mixed-effects Cox; stratified sibling Cox
5. Actual results
- Results
Total population: adjusted HRs 1.20-1.53 for several outcomes, dose-response. Table 3 (full text): overweight (hospital diagnosis) adjusted HR 1.34 (1.25-1.43), sibling HR 1.02 (0.86-1.22); asthma ever (hospital) adjusted 1.52 (1.48-1.57), sibling 1.24 (1.16-1.34); asthma (drug-treatment definition) adjusted 1.53 (1.50-1.56), sibling 1.25 (1.20-1.31); asthma onset >5 years (hospital) sibling 1.04 (0.93-1.16); eczema (hospital) sibling 1.35 (1.07-1.69); allergic rhinitis sibling 0.91 (0.69-1.22); drug-defined allergy sibling 1.04 (0.99-1.09).
- Absolute risks
Rates per 1000 person-years reported (e.g. overweight hospital diagnosis 0.56); absolute risk differences not reported
- Authors' conclusion
Early-life antibiotics were linked with immune-mediated diseases, but familial factors may partly explain this; except for asthma and eczema, antibiotics might not be an independent risk factor, including for overweight.
6. Strengths
- Nationwide registry, prescription data
- Sibling design
- Long follow-up
- Full text read
7. Weaknesses
- Overweight captured only by hospital diagnosis (underestimated)
- Sibling design does not control confounding by indication within families
- Denmark has low antibiotic use
8. Risk of bias
- Orienting tool
- ROBINS-E
- Type of assessment
- simplified assessment (abstract only)
- Full formal assessment
- no
- Overall risk of bias
- some concerns
- Rationale
Sibling comparison reduces familial confounding; non-shared confounding (infections) remains.
9. Assessment of the statistical interpretation
About 93% of the log-excess risk for overweight and about half for asthma disappears within sibships (recalculated).
10. Relevance to the specific claim
Key family-based test for both obesity (C-ABX-05) and asthma/allergy (C-ABX-06).
11. Independent contrary findings
Persistence of asthma/eczema associations within siblings may still reflect respiratory infections (indication).
12. Transferability to Germany and Europe
High (Danish).
13. Overall methodological judgement
Strong evidence that the overweight association is largely familial confounding; asthma association partly persists.
| Multidimensional assessment | |
|---|---|
| Authenticity of the source | verified |
| Methodological quality | high |
| Risk of bias | some concerns |
| Precision of results | high |
| Directness | direct |
| Reproducibility | partly replicated |
| Consistency with the overall evidence | mixed |
| Regional transferability | high |
| Adequacy of the interpretation (by the archive) | not applicable |
14. Open questions
Whether residual asthma association is causal.
15. Linked fact checks
- Antibiotics make people (especially children) gain weight by altering gut bacteria, as they do in fattened poultry; early-childhood antibiotic courses raise obesity risk. – Studies contradicting or limiting the claim
- Antibiotic exposure in early life causes asthma and allergic disease. – Studies with mixed or unclear results