Cohort studyindependent cross-checkabstract checked I-VEG-126 standard review
Possible thyroid signal for nitrate in one cohort
Nitrate intake and the risk of thyroid cancer and thyroid disease.
Overview
Ward MH et al.. Epidemiology 2010 · PMID 20335813 · DOI 10.1097/EDE.0b013e3181d6201d
In Iowa women, the highest dietary nitrate intake was linked to more thyroid cancer, but the estimate was very imprecise.
Does the study support the specific claim? Partly · Methodological quality: low
Plain-language summary
Study explained simply
- What did the researchers want to find out?
Relate nitrate to thyroid outcomes.
- How was the study done?
Cohort.
- How many people, animals or samples were studied?
21,977 women.
- What was actually found?
Weak positive association.
- How large was the observed effect?
RR 2.9 (1.0-8.1).
- What do the results mean in everyday life?
A possible signal needing confirmation, not proof of harm.
- What are the study's limitations?
Few cases.
- What does it say about Elawady's specific claim?
Weak support.
- Do the results apply to Germany and Europe?
Moderate.
- What can explicitly not be concluded from this study?
Cannot show that vegetables harm the thyroid.
Scientific in-depth review
1. Full reference
- Authors
- Ward MH et al.
- Journal
- Epidemiology
- Year
- 2010
- Identifier
- PMID 20335813 DOI 10.1097/EDE.0b013e3181d6201d
- Verification
- verified – PubMed efetch 2026-10-26; Crossref 2026-10-26
- Retracted
- no
- Corrections
- none
- Extent checked
- abstract only
- Conflicts of interest and funding
Funding (PubMed grant list): Intramural NIH HHS; NCI NIH HHS
2. Research question
Is nitrate intake from water and diet associated with thyroid cancer and thyroid disease?
3. Study design and population
- Study design
- Cohort study – Prospective cohort (Iowa Women's Health Study)
- Population
Older women in Iowa using the same water supply >10 years
- Country
USA
- Period
1986-2004
- Sample size
21,977
- Inclusion and exclusion criteria
not stated in abstract
4. Methods and endpoints
- Exposure / intervention
Nitrate in public water (1955-1988) and dietary nitrate (FFQ)
- Comparator
Lowest quartile
- Endpoints
Thyroid cancer incidence; self-reported hypo/hyperthyroidism
- Statistical methods
Relative risks; prevalence odds ratios
5. Actual results
- Results
Water nitrate >5 mg/L nitrate-N for >=5 years: thyroid cancer RR 2.6 (1.1-6.2). Dietary nitrate highest vs lowest quartile: thyroid cancer RR 2.9 (1.0-8.1; P trend 0.046); hypothyroidism prevalence OR 1.2 (1.1-1.4); no association with hyperthyroidism.
- Absolute risks
not reported
- Authors' conclusion
Nitrate may play a role in the etiology of thyroid cancer and warrants further study.
6. Strengths
- Long-term water exposure data
7. Weaknesses
- Few cases; imprecise
- Self-reported thyroid disease
- Dietary nitrate highly correlated with vegetable intake
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
Confounding and few cases.
9. Assessment of the statistical interpretation
Dietary RR 2.9 (1.0-8.1): borderline, p ~0.05 (stats/C-VEG-08_nitrate.py).
10. Relevance to the specific claim
The only signal found that points toward harm from dietary (mostly vegetable) nitrate; imprecise and not replicated.
11. Independent contrary findings
Bondonno 2021/2024; Erichsen 2025.
12. Transferability to Germany and Europe
US cohort.
13. Overall methodological judgement
Weak supporting evidence; insufficient to establish harm.
| Multidimensional assessment | |
|---|---|
| Authenticity of the source | verified |
| Methodological quality | low |
| Risk of bias | some concerns |
| Precision of results | low |
| Directness | indirect |
| Reproducibility | not replicated |
| Consistency with the overall evidence | inconsistent |
| Regional transferability | moderate |
| Adequacy of the interpretation (by the archive) | not applicable |
14. Open questions
Replication for thyroid outcomes.
15. Linked fact checks
- Nitrate from fertilisers (together with pesticides) in lettuce, rocket and other leaves harms the people who eat them. – Studies supporting the claim