We have analyzed data on faecal concentrations of microbial fermentation acids from 10 human 25 volunteer studies, involving 163 subjects, conducted at the Rowett Institute, Aberdeen, UK over a 26 seven year period. Analysis of baseline samples from free-living volunteers revealed that the % 27 butyrate increased significantly, whilst the % iso-butyrate and % iso-valerate decreased significantly, 28 with increasing total short chain fatty acids (SCFA) concentration. The observed decreasing 29 proportions of iso-butyrate and iso-valerate, which are derived from amino acid fermentation, are 30 consistent with the interpretation that fibre intake is mainly responsible for increased SCFA 31 concentrations. Past studies have shown that increases in gut transit rate and in fibre intake lead to 32 higher faecal SCFA concentrations. We propose that the increase in % butyrate among faecal SCFA is 33 largely driven by a decrease in colonic pH resulting from higher concentrations of SCFA in the colon. 34 Consistent with this, we found a significant increase both in total SCFA and in % butyrate with 35 decreasing pH across five studies for which faecal pH measurements were available. We propose 36 that colonic pH influences butyrate production through a combination of two mechanisms. First, 37 decreased pH has been shown to alter the stoichiometry of butyrate formation by butyrate- 38 producing species in pure and mixed cultures in vitro, resulting in increased acetate uptake and 39 butyrate formation. Second, decreased pH appears to allow butyrate-producing species (notably 40 Roseburia and Eubacterium rectale) to compete better for available substrates, thus increasing their 41 relative abundance in the microbiota.