Faecal short-chain fatty acids as an indicator of the status of intestinal microbial ecology

The aims of this study were to investigate how faecal concentrations of short chain fatty acids (SCFAs) reflect intestinal microbial ecology. Faecal SCFAs and water content were determined in 93 healthy volunteers to establish the normal values in man and the correlations between various SCFAs and between SCFAs and faecal water. SCFAs were also determined in some physiological and pathophysiological conditions with presumable affection of intestinal microbial ecology. We therefore studied: 1) the establishment and development of the micro flora in 49 infants with respect to the type of feeding, 2) the effect of metronidazole in 77 rats, 3) 33 adult lactosemal absorbers with different lactose tolerance patterns, 4) 33 children with acute bacterial diarrhoea and the effect of treatment with Lactobacillus GG and 5) the effect of an artificial dietary compound, fat substitute olestra, in 93 healthy adults.

The mean total faecal SCFA concentration in man is about 85 mmol/kg wet weight and the relative proportions of acetic / propionic / butyric / iso-butyric / iso-valeric / valeric / caproicacids are about 58 / 16.5 / 17.5 / 2 / 3 / 2.5 / 0.5 %. The first 5 acids are always present, valeric may not be detectable in individual cases and caproic is found in only about half of the samples. The total concentrations and those of the major acids (acetic, propionic, butyric) are higher in men than in women. The biological variation is wide: tenfold for the total and even wider for individual acids. SCFAs do not behave as a single entity. The major acids, correlating most strongly between themselves, form one group, while the minor ones form another group. The major acids showed a significant positive correlation with the faecal water content, the minor ones showed an inverse correlation (iso-acids) or no correlation at all. This grouping is most likely due to the different substrate types and functional groups of colonic flora that are involved and regional differences in fermentation. In infants the assumed establishment of a specific flora in response to breast-feeding was reflected by a particular faecal pattern of SCFAs dominated by acetic acid. This pattern maybe associated with better protection against diarrhoea and respiratory infections in the first months of life.

Metronidazole reduced the faecal concentrations and changed the relative distribution of SCFAs. Although the faecal flora remained unchanged, the anaerobic mucosal flora of the colon was significantly suppressed. Therefore faecal SCFAs may be of value in detecting changes in the flora that are not reflected by faecal cultures. Lactose malabsorbers with different lactose tolerance showed differences in SCFAs. This suggests that faecal SCFAs may be indicative of types of intestinal microflora with different metabolic capabilities, that may play a role in adaptation to lactose malabsorption in man. Acute bacterial diarrhoea causes major disturbances in the microbial ecology of the gut, as reflected by very low faecal SCFA values in acute salmonellosis and shigellosis. The differences in the SCFAs between the two diagnoses may indicate different characteristics of the disturbances. Determinations of the faecal SCFAs enable one to follow the recovery of the ecosystem. Treatment with a probiotic (Lactobacillus GG) promotes that recovery, as shown by the pattern of faecal SCFAs. Olestra caused significant increase in faecal SCFA concentrations in volunteers. Although all individual values were within the range of a healthy population, the effects were consistent, indicating a possible affection of the intestinal ecosystem. The results suggest that the faecal SCFA concentrations reflect some changes and differences in the intestinal microbial ecosystem.