Dynamics of human gut microbiota and short-chain fatty acids in response to dietary interventions with three fermentable fibers
Design
This randomized, parallel, four-arm dietary intervention analyzed 174 university students who completed the study: potato resistant starch, inulin, amylase-accessible corn-starch control, or resistant maize starch (Hi-Maize). Participants provided three or four baseline fecal samples, completed a 4–7 day transition beginning at half dose, and then provided three or four samples while taking the full dose. The potato-starch supplement was 40–48 g powder/day and approximately 70% resistant starch, corresponding to about 28–34 g/day resistant fraction. Only the first-semester cohort was blinded to supplement identity; sample analysts remained blinded. Analysis denominators differed because samples with insufficient replicates or assay data were excluded: the potato arm contributed 43 participants to the fecal-SCFA table and 50 to the reported sequence-response analyses. Serial V4 16S microbiome and fecal SCFA measurements were the main outcomes; symptoms and hard clinical outcomes were not endpoints.
Findings
Among the 43 potato-arm participants with paired fecal SCFA data, mean butyrate increased from 13 to 16 mmol/kg (+29%, repeated-measures ANOVA P<0.001) and acetate from 48 to 58 mmol/kg (+21%, P=0.0012). The response was heterogeneous: median butyrate increased in 63% of individuals and was unchanged or decreased in 37%. Potato starch produced a 6.5-fold mean increase (P<0.001) in a bifidobacterial V4 sequence that could not resolve B. adolescentis, B. faecale, and B. stercoris.
Bifidobacterial expansion alone did not explain butyrate yield. In the potato arm, increases in Ruminococcus bromii or a sequence 98.8% identical in V4 to the ruminal isolate Clostridium chartatabidum were each associated with increased fecal butyrate (P=0.025 and P=0.0024). Of 29 participants with detectable baseline R. bromii, 22 (76%) had higher butyrate during supplementation, compared with 5 of 14 (36%) without detectable baseline R. bromii. Change in R. bromii abundance correlated with change in the cross-feeding butyrate producer Eubacterium rectale, and E. rectale abundance correlated with fecal butyrate in the potato arm (Spearman ρ=0.42, P<0.001). These are exploratory associations, not validated predictors.
Interpretation
The study’s major contribution is separating primary substrate degradation from downstream SCFA production. A mean taxonomic or metabolite change should not be read as a universal response, and the study does not validate a clinical baseline predictor, a symptom treatment, or an aging benefit. The cohort was young (17–29 years; median 19), diets other than the supplement were not controlled, resistant-polysaccharide doses differed across arms, and fecal SCFA concentration remains an imperfect proxy for production because absorption and transit were unmeasured. Procter & Gamble partly funded the work; the authors declared no competing interests. gap/needs-replication gap/no-mechanism