AIM:To determine tolerance to fiber supplementation of semi-elemental tube feeds in critically ill patients and measure its effect on colonic microbiota and fermentation.METHODS:Thirteen intensive care unit patients receiving jejunal feeding with a semi-elemental diet for predominantly necrotizing pancreatitis were studied.The study was divided into 2 parts:first,short-term (3-9 d)clinical tolerance and colonic fermentation as assessed by fecal short chain fatty acid(SCFA)concentrations and breath hydrogen and methane was measured in response to progressive fiber supplementation increasing from 4 g tid up to normal requirement levels of 8 g tid;second,4 patients with diarrhea were studied for 2-5 wk with maximal supplementation to additionally assess its influence on fecal microbiota quantitated by quantitative polymerase chain reaction (qPCR)of microbial 16S rRNA genes and Human Intestinal Tract Chip(HITChip)microarray analysis.Nearly all patients were receiving antibiotics(10/13)and acid suppressants(11/13)at some stage during the studies.RESULTS:In group 1,tolerance to progressive fiber supplementation was good with breath hydrogen and methane evidence(P=0.008 and P<0.0001,respectively)of increased fermentation with no exacerbation of abdominal symptoms and resolution of diarrhea in 2 of 4 patients.In group 2 before supplementation,fecal microbiota mass and their metabolites,SCFA,were dramatically lower in patients compared to healthy volunteers.From qPCR and HITChip analyses we calculated that there was a 97%reduction in the predominant potential butyrate producers and starch degraders.Following 2-5 wk of fiber supplementation there was a significant increase in fecal SCFA(acetate 28.4±4.1μmol/g to 42.5±3.1μmol/g dry weight,P=0.01;propionate 1.6±0.5 vs 6.22±1.1,P=0.006 and butyrate 2.5±0.6 vs 5.9±1.1,P=0.04)and microbial counts of specific butyrate producers,with resolution of diarrhea in 3 of 4 patients.CONCLUSION:Conventional management of critically ill patients,which includes the use of elemental diets and broad-spectrum antibiotics,was associated with gross suppression of the colonic microbiota and their production of essential colonic fuels,i.e.,SCFA.Our investigations show that fiber supplementation of the feeds has the potential to improve microbiota mass and function,thereby reducing the risks of diarrhea due to dysbiosis. A determination of tolerance to fiber supplementation of semi-elemental tube feeds in critically ill patients and measure its effect on colonic microbiota and fermentation. METHODS: Thirteen intensive care unit patients receiving jejunal feeding with a semi-elemental diet for predominantly necrotizing pancreatitis were studied The study was divided into 2 parts: first, short-term (3-9 d) clinical tolerance and colonic fermentation as assessed by fecal short chain fatty acid (SCFA) concentrations and breath hydrogen and methane was measured in response to progressive fiber supplementation increasing from 4 g tid up to normal requirement levels of 8 g tid; second, 4 patients with diarrhea were studied for 2-5 wk with maximal supplementation to than assess its influence on fecal microbiota quantitated by quantitative polymerase chain reaction (qPCR) of microbial 16S rRNA genes and Human Intestinal Tract Chip (HITChip) microarray analysis .Nearly all patients were receiving antibiotics (10/13) and Acid suppressingants (11/13) at some stage during the studies .RESULTS: In group 1, tolerance to progressive fiber supplementation was good with breath hydrogen and methane evidence (P = 0.008 and P <0.0001, respectively) of increased fermentation with no exacerbation of abdominal symptoms and resolution of diarrhea in 2 of 4 patients. In group 2 before supplementation, fecal microbiota mass and their metabolites, SCFA, were decreased lower in patients compared to healthy volunteers. Flash qPCR and HITChip analyzes we calculated that there was a 97 % reduction in the predominant potential butyrate producers and starch degraders. Popular 2-5 wk of fiber supplementation there was a significant increase in fecal SCFA (acetate 28.4 ± 4.1 μmol / g to 42.5 ± 3.1 μmol / g dry weight, P = 0.01; propionate 1.6 ± 0.5 vs 6.22 ± 1.1, P = 0.006 and butyrate 2.5 ± 0.6 vs 5.9 ± 1.1, P = 0.04) and microbial counts of specific butyrate producers, with resolution of diarrhea in 3 of 4 patients. CONCLUSION: Conventional management of critically i ll patients, ​​which includes the use of elemental diets and broad-spectrum antibiotics, was associated with gross suppression of the colonic microbiota and their production of essential colonic fuels, ie, SCFA. Our investigations show that fiber supplementation of the feeds has the potential to improve microbiota mass and function, thereby reducing the risks of diarrhea due to dysbiosis.