Measurement / residue studyindependent cross-checkabstract checked I-VEG-119 standard review
Boiling greens removes much of their oxalate
Effect of different cooking methods on vegetable oxalate content.
Overview
Chai W et al.. J Agric Food Chem 2005 · PMID 15826055 · DOI 10.1021/jf048128d
Boiling cut soluble oxalate by 30-87% in nine vegetables; steaming removed less.
Does the study support the specific claim? Partly · Methodological quality: moderate
Plain-language summary
Study explained simply
- What did the researchers want to find out?
Measure oxalate loss with cooking.
- How was the study done?
Food analysis.
- How many people, animals or samples were studied?
9 vegetables.
- What was actually found?
Boiling most effective.
- How large was the observed effect?
30-87% reduction.
- What do the results mean in everyday life?
Boiled greens (water discarded) contain far less soluble oxalate than raw juice.
- What are the study's limitations?
No human outcomes.
- What does it say about Elawady's specific claim?
Limits the generalisation; supports the juicing-specific risk.
- Do the results apply to Germany and Europe?
Universal.
- What can explicitly not be concluded from this study?
Cannot show stone risk directly.
Scientific in-depth review
1. Full reference
- Authors
- Chai W et al.
- Journal
- J Agric Food Chem
- Year
- 2005
- Identifier
- PMID 15826055 DOI 10.1021/jf048128d
- Verification
- verified – PubMed efetch 2026-10-26; Crossref 2026-10-26
- Retracted
- no
- Corrections
- none
- Extent checked
- abstract only
- Conflicts of interest and funding
not stated in abstract
2. Research question
How do boiling, steaming and baking affect soluble and insoluble oxalate in vegetables?
3. Study design and population
- Study design
- Measurement / residue study – Laboratory food analysis of raw and cooked vegetables (enzymatic oxalate assay)
- Population
Nine vegetable types
- Country
USA
- Period
not stated in abstract
- Sample size
9 vegetable types
- Inclusion and exclusion criteria
not stated in abstract
4. Methods and endpoints
- Exposure / intervention
Boiling, steaming, baking
- Comparator
Raw vegetables
- Endpoints
Soluble and insoluble oxalate content
- Statistical methods
Descriptive comparisons
5. Actual results
- Results
Most raw vegetables had a high proportion of water-soluble oxalate; boiling reduced soluble oxalate by 30-87%, steaming by 5-53%, baking (potatoes) no loss; ~100% of the lost oxalate recovered in the cooking water; insoluble oxalate losses 0-74%.
- Absolute risks
not reported
- Authors' conclusion
Cooking methods that reduce soluble oxalate may help decrease oxaluria in people predisposed to kidney stones.
6. Strengths
- Direct measurement
- Raw versus cooked comparison
7. Weaknesses
- Food chemistry only; no human outcomes
8. Risk of bias
- Orienting tool
- none applicable
- Type of assessment
- simplified assessment (abstract only)
- Full formal assessment
- no
- Overall risk of bias
- not assessable
- Rationale
Laboratory study.
9. Assessment of the statistical interpretation
Boiling leaves 13-70% of soluble oxalate (stats/C-VEG-05_oxalate.py).
10. Relevance to the specific claim
Raw vs cooked: raw juices keep all soluble oxalate; boiling (discarding water) markedly lowers it.
11. Independent contrary findings
none
12. Transferability to Germany and Europe
Universal.
13. Overall methodological judgement
Explains why extreme raw juicing differs from normal cooked greens and offers a practical mitigation for stone formers.
| Multidimensional assessment | |
|---|---|
| Authenticity of the source | verified |
| Methodological quality | moderate |
| Risk of bias | not assessable |
| Precision of results | not applicable |
| Directness | indirect |
| Reproducibility | partly replicated |
| Consistency with the overall evidence | consistent |
| Regional transferability | high |
| Adequacy of the interpretation (by the archive) | not applicable |
14. Open questions
Bioavailability in vivo.
15. Linked fact checks
- Oxalate-rich greens (spinach, beet greens, chard) cause kidney stones; each 100 mg of dietary oxalate raises urinary oxalate by about 2.7 mg. – Studies contradicting or limiting the claim
- Heavy juicing of oxalate-rich greens (about 1,500 mg oxalate per day) can cause acute oxalate nephropathy, especially in people with chronic kidney disease. – Studies with mixed or unclear results