Colonic Fermentation of Ispaghula,Wheat Bran,Glucose,and Albumin to Short-Chain Fatty Acids and Ammonia Evaluated in Vitro in 50 Subjects

The production of short-chain fatty acids and ammonia was measured in 16.6% fecal homogenates from 50 subjects incubated at 37°C for 6 and 24 hours. All 50 homogenates produced ammonia and short-chain fatty acids of any chain length (C2-C5). Incubation for 24 hours with dietary fiber (ispaghula husk or wheat bran), albumin, or glucose (10 mg/ mL) increased the short-chain fatty acid production (43.6 ± 2.8, 45.4 ± 2.0, 60.3 ± 3.2, and 65.8 ± 3.1 mmol/L, respectively) compared with controls (21.4 ± 1.3 mmol/L). The degradation of different substrates was associated with the production of different amounts of ammonia and short-chain fatty acids. Ispaghula, wheat bran, albumin, and glucose were fermented to acetate (>2 mmol/L; 24-hour incubations) in 86%, 96%, 98%, and 98% of the homogenates, to propionate in 80%, 76%, 100%, and 68%, and to butyrate in 32%, 94%, 88%, and 54% of the homogenates, respectively. Isobutyrate, valerate, and isovalerate were produced from albumin in all (100%) of the homogenates, but only in 2 to 4% of the homogenates incubated with ispaghula or glucose. Ammonia was always (100%) produced after the addition of albumin and always (98%) consumed (assimilated) when glucose was fermented. Surgery (sigmoid or right- or left-sided colonic resection) did not change the pattern of ammonia and short-chain fatty acid production from these substrates.

This study suggests that the different colonic flora from a large number of subjects share general biochemical characteristics, which metabolize different substrates to specific patterns of ammonia and short-chain fatty acids. (Journal of Parenteral and Enteral Nutrition 16:433-439, 1992)