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 shortchain fatty acids (SCFAs) reflect intestinal microbial ecology. Faecal SCFAs andwater content were determined in 93 healthy volunteers to establish the normal valuesin man and the correlations between various SCFAs and between SCFAs and faecal water.SCFAs were also deterrnined in some physiological and pathophysiological conditionswith presumable affection of intestinal microbial ecology. We therefore studied:I) the establishment and development of the micro flora in 49 infants with respectto the type of feeding, 2) the effect of metronidazole in 77 rats, 3) 33 adult lactosemalabsorbers with different lactose tolerance patterns, 4) 33 children with acutebacterial diarrhoea and the effect of treatment with Lactobacillus GG and 5) theeffect 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 weightand 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 abouthalf 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 iswide: tenfold for the total and even wider for individual acids. SCFAs do not behaveas a single entity. The major acids, correlating most strongly be tween themselves,form one group, while the minor ones form another group. The major acids showed asignificant positive correlation with the faecal water content, the minor ones showedan inverse correlation (iso-acids) or no correlation at all. This grouping is mostlikely due to the different substrate types and functional groups of colonic florathat are involved and regional differences in fermentation. In infants theassumed establishment of a specific flora in response to breast-feeding was reflectedby a particular faecal pattern of SCFAs dominated by acetic acid. This pattern maybe associated with better protection against diarrhoea and respiratory infectionsin the first months of life. Metronidazole reduced the faecal concentrationsand changed the relative distribution of SCFAs. Although the faecal flora remainedunchanged, the anaerobic mucosal flora of the colon was significantly suppressed.Therefore faecal SCFAs may be of value in detecting changes in the flora that arenot reflected by faecal cultures. Lactose malabsorbers with different lactosetolerance showed differences in SCFAs. This suggests that faecal SCFAs may be indicativeof types of intestinal microflora with different metabolic capabilities, that mayplay a role in adaptation to lactose malabsorption in man. Acute bacterialdiarrhoea causes major disturbances in the microbial ecology of the gut, asreflected by very low faecal SCFA values in acute salmonellosis and shigellosis.The differences in the SCFAs between the two diagnoses may indicate different characteristicsof the disturbances. Deterrninations of the faecal SCFAs enable one to follow therecovery of the ecosystem. Treatment with a probiotic (Lactobacillus GG) promotesthat recovery, as shown by the pattern of faecal SCFAs. Olestra caused significantincrease in faecal SCFA concentrations in volunteers. Although all individual valueswere within the range of a healthy population, the effects were consistent, indicatinga possible affection of the intestinal ecosystem. The results suggest that the faecal SCFA concentrations reflect some changes anddifferences in the intestinal microbial ecosystem. Key words: Intestinal microflora, SCFAs, Faecal water, Breast-feeding, Metronidazole,Lactose malabsorption, Bacterial diarrhoea, Olestr