Otherindependent cross-checkabstract checked★ key study I-VEG-129 standard review
Humans ferment much of the cellulose in vegetables
Neutral detergent fiber, hemicellulose and cellulose digestibility in human subjects.
Overview
Slavin JL et al.. J Nutr 1981 · PMID 6257867 · DOI 10.1093/jn/111.2.287
In 7 women on controlled diets, about 70% of cellulose from fruits and vegetables was broken down in the gut, but only 8-16% of purified cellulose.
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?
Measure fibre digestibility in humans.
- How was the study done?
Controlled feeding.
- How many people, animals or samples were studied?
7 women.
- What was actually found?
Plant-food cellulose ~70% digested; purified cellulose little.
- How large was the observed effect?
69.7% vs 15.7%.
- What do the results mean in everyday life?
Gut bacteria break down much of the cellulose in vegetables.
- What are the study's limitations?
Very small; old methods.
- What does it say about Elawady's specific claim?
Contradicts the claim.
- Do the results apply to Germany and Europe?
Applicable.
- What can explicitly not be concluded from this study?
Cannot measure health effects.
Scientific in-depth review
1. Full reference
- Authors
- Slavin JL et al.
- Journal
- J Nutr
- Year
- 1981
- Identifier
- PMID 6257867 DOI 10.1093/jn/111.2.287
- 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): NIADDK NIH HHS
2. Research question
How much fibre, hemicellulose and cellulose do humans digest?
3. Study design and population
- Study design
- Other – Controlled metabolic feeding study (low- vs high-cellulose diets, ~1 month each)
- Population
Seven women
- Country
USA
- Period
not stated in abstract
- Sample size
7
- Inclusion and exclusion criteria
not stated in abstract
4. Methods and endpoints
- Exposure / intervention
Low-cellulose mixed diet (fruits, vegetables, refined grains) vs same diet + 16 g/day purified cellulose
- Comparator
Within-subject
- Endpoints
Apparent digestibility (faecal recovery)
- Statistical methods
Pooled 20-30-day composites
5. Actual results
- Results
Cellulose digestibility 69.7% +/- 10.7% on the low-cellulose mixed diet vs 15.7% +/- 17.4% with added purified cellulose; NDF 70.4% vs 23.0%; hemicellulose 71.7% vs 51.0%; purified cellulose with liquid diet only 8% digested.
- Absolute risks
not reported
- Authors' conclusion
More than half of the fibre in a diet containing fruits, vegetables and refined grains is degraded, while the apparent digestibility of refined cellulose is minimal.
6. Strengths
- Controlled diets, long collection periods
7. Weaknesses
- n = 7
- 1981 methods (apparent digestibility)
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
Small metabolic study.
9. Assessment of the statistical interpretation
~3.8 g/day cellulose fermented on the mixed diet (stats/C-VEG-01_cellulose.py).
10. Relevance to the specific claim
Directly contradicts "humans cannot digest cellulose": cellulose from plant foods is substantially fermented in the human colon; purified cellulose is not.
11. Independent contrary findings
none
12. Transferability to Germany and Europe
Industrialised (US) adults.
13. Overall methodological judgement
Key physiological evidence against C-VEG-01 premise and C-VEG-02 conclusion.
| Multidimensional assessment | |
|---|---|
| Authenticity of the source | verified |
| Methodological quality | moderate |
| Risk of bias | some concerns |
| Precision of results | low |
| Directness | direct |
| Reproducibility | replicated |
| Consistency with the overall evidence | consistent |
| Regional transferability | high |
| Adequacy of the interpretation (by the archive) | not applicable |
14. Open questions
Inter-individual variation (see Chassard 2010).
15. Linked fact checks
- Humans, having a single stomach, cannot digest the cellulose in vegetables (unlike ruminants), so greens and legumes cause gas and bloating and are food made for animals, not for humans. – Studies contradicting or limiting the claim
- A 2024 Science paper shows that industrialised humans have lost the cellulose-degrading gut bacteria, so the modern human gut 'was never built to digest cellulose' and no longer carries the machinery to ferment plant cell walls. – Studies contradicting or limiting the claim