Cohort studyindependent cross-checkabstract checked I-WHT-113 standard review
Whole grain eaters had 29% less type 2 diabetes
Intake of whole grain foods and risk of type 2 diabetes: results from three prospective cohort studies.
Overview
Hu Y et al.. BMJ 2020 · PMID 32641435 · DOI 10.1136/bmj.m2206
In 195,000 US adults, the highest whole grain intake was linked to 29% lower diabetes risk; dark bread and oatmeal about 20% lower.
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?
Whole grains and T2D.
- How was the study done?
Three cohorts.
- How many people, animals or samples were studied?
194,784 adults.
- What was actually found?
Lower risk with whole grains.
- How large was the observed effect?
HR 0.71.
- What do the results mean in everyday life?
Choose whole grains.
- What are the study's limitations?
Observational.
- What does it say about Elawady's specific claim?
Supports the protective part; not that refined flour causes diabetes.
- Do the results apply to Germany and Europe?
Moderate.
- What can explicitly not be concluded from this study?
Cannot prove causation.
Scientific in-depth review
1. Full reference
- Authors
- Hu Y et al.
- Journal
- BMJ
- Year
- 2020
- Identifier
- PMID 32641435 DOI 10.1136/bmj.m2206 · PMC7341349
- Verification
- verified – PubMed efetch 2026-10-26; Crossref works API 2026-10-26
- Retracted
- no
- Corrections
- none
- Extent checked
- abstract only
- Conflicts of interest and funding
NIH funding; some authors with personal fees outside the work.
2. Research question
Are total and specific whole grain foods associated with T2D?
3. Study design and population
- Study design
- Cohort study – Three prospective cohorts (NHS, NHS II, HPFS)
- Population
US women and men free of T2D, CVD, cancer
- Country
USA
- Period
1984-2017
- Sample size
194,784; 18,629 T2D cases
- Inclusion and exclusion criteria
Free of T2D, CVD and cancer at baseline
4. Methods and endpoints
- Exposure / intervention
Total and individual whole grain foods
- Comparator
Lowest intake
- Endpoints
Incident T2D
- Statistical methods
Cox models; splines
5. Actual results
- Results
Highest vs lowest total whole grain: 29% lower rate (26%-33%). >=1 serving/day vs <1/month: whole grain cold cereal HR 0.81 (0.77-0.86), dark bread 0.79 (0.75-0.83), popcorn 1.08 (1.00-1.17). >=2/week: oatmeal 0.79 (0.75-0.83), brown rice 0.88 (0.82-0.94), added bran 0.85 (0.80-0.90), wheat germ 0.88 (0.78-0.98). Plateau above ~2 servings/day.
- Absolute risks
not reported
- Authors' conclusion
Higher whole grain consumption associated with lower T2D risk.
6. Strengths
- Very large, repeated measures
7. Weaknesses
- Observational
- Health-professional cohorts
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
Residual confounding.
9. Assessment of the statistical interpretation
29% (26%-33%) = HR 0.71 (0.67-0.74) (stats/C-WHT-05).
10. Relevance to the specific claim
Supports the protective side of the claim (whole grains/cereal fibre).
11. Independent contrary findings
A-WHT-04 refined grains null.
12. Transferability to Germany and Europe
Moderate.
13. Overall methodological judgement
Strong observational evidence.
| Multidimensional assessment | |
|---|---|
| Authenticity of the source | verified |
| Methodological quality | high |
| Risk of bias | some concerns |
| Precision of results | high |
| Directness | direct |
| Reproducibility | replicated |
| Consistency with the overall evidence | consistent |
| Regional transferability | moderate |
| Adequacy of the interpretation (by the archive) | not applicable |
14. Open questions
Causality.