Dietary fatty acids modulate human gut microbiota metabolic activity: An in vitro batch culture fermentation study

Approximately 5-7% of dietary fatty acids escape absorption in the small intestine and reach the colon (1) . How these fatty acids interact with the gut microbiota remains poorly understood. The aim of this study was to determine how medium chain fatty acids (MCFA), long chain saturated (LCSFA), and unsaturated fatty acids (UFA) modulate short-chain fatty acid (SCFA) production by human gut microbiota in vitro, in the presence and absence of fermentable mixed fibre (MF). Anaerobic 24 h batch culture fermentations (pH 7, 37oC) were inoculated individually with faecal slurries from 12 healthy adults (6 males; mean (SD) age: 34.7 (7) years; BMI: 26 (4.2) kg/m 2 ). Cultures were performed for each individual fatty acid [MCFA: caprylic (C8:0), capric (C10:0), lauric (C12:0); LCSFA: myristic (C14:0), palmitic (C16:0), stearic (C18:0); UFA: oleic (C18:1), linoleic (C18:2), linolenic (C18:3) and eicosapentaenoic acid (EPA) (C20:5)], both in the presence or absence of MF (inulin, apple pectin, alpha cellulose and high resistant maize starch). Acetate, propionate and butyrate were quantified using gas chromatography with flame ionisation detector. Paired T-test or Wilcoxon signed-rank test were performed to compare fatty acids + MF with MF alone, and fatty acids alone with fibre-free basal medium. Data are presented as median and interquartile range (IQR). This study was approved by the University of Glasgow Ethics Committee (Ref: 200240004). In the presence of MF, MCFA significantly reduced total SCFA production compared with MF alone (38.4 (IQR 8) µmol/ml): caprylic acid 9.4 (IQR 5.1) µmol/ml, capric acid 11.3 (IQR 4.2) µmol/ml, and lauric acid 15 (IQR 5.7) µmol/ml (all P<0.002). Significant reductions were also found with these treatments for acetate, propionate and butyrate (P<0.02). LCSFA+MF did not change SCFA production relative to MF alone. In the presence of MF, total SCFA decreased with linoleic acid 31.3 (IQR 22.4) µmol/ml, linolenic acid 9.1 (IQR 21.1) µmol/ml and EPA 30.8 (IQR 15.9) µmol/ml compared with MF alone 38.4 (IQR 8) µmol/ml (all P<0.005), but not with oleic acid 35.3 (IQR 6.7) µmol/ml. Acetate and butyrate were also significantly reduced in these treatments (P<0.003). Notably, propionate increased with both oleic acid 7.4 (IQR 5.4) µmol/ml and EPA 11.1 (IQR 13.4) µmol/ml compared with MF alone 5.4 (IQR 2.6) µmol/ml (P<0.002 and P=0.034, respectively). Under fibre-free conditions, broadly similar patterns were observed, with MCFA causing the greatest reductions in SCFAs, UFA showing moderate decreases, and LCSFA showing no change in SCFAs compared with the no-fibre control. These findings suggest that the modifying effect of dietary fatty acids on gut microbial activity in vitro depends on their chain length and degree of unsaturation. Dietary intervention studies are needed to confirm these effects and to explore the implications to metabolic health.