S.U. Christl

Production, metabolism, and excretion of hydrogen in the large intestine

Hydrogen is produced during fermentation in the large intestine and may be excreted in breath and flatus or further metabolized by the flora. However, there is little information about total H2 excretion from different substrates or the extent to which it is metabolized in the colon. We have therefore measured total H2 and methane excretion in 10 healthy subjects using a whole body calorimeter. Breath gases were measured simultaneously with total excretion in response to lactulose, pectin, and banana starch. Metabolic activities of the predominant H2 consuming anaerobes (methanogenic, sulfate reducing, and acetogenic bacteria) were measured in fecal samples. Total H2 excretion on a starch and fiber-free diet was 35 +/- 6.1 mL/24 h +/- SEM. H2 from 7.5 g, 15 g, and 22.5 g lactulose was 88.1 +/- 22.4 mL, 227.0 +/- 60.7 mL, and 321.8 +/- 79.2 mL. Four of the subjects also excreted CH4, which was 51.3 +/- 5.5 mL, 97.3 +/- 18.4 mL, and 157.5 +/- 36.3 mL for the respective lactulose doses. H2 excretion was less in methanogenic subjects (7.9 mL/g lactulose) than in nonmethanogenic (17.3 mL/g), but total H2 excreted as, hydrogen + methane, was 34.9 mL/g. H2 from pectin (20 g) was 14.1% +/- 3.2% and from starch (22.2 g) 38.6% +/- 9.2% of an equivalent lactulose dose. Sixty-five percent of total H2 and CH4 was expired in breath at total excretion rates up to 200 mL/24 h. Over this the proportion decreased to 25% with an overall average of 58%. Only subjects with CH4 excretion in vivo showed methanogenesis in feces, whereas nonmethanogenic subjects showed high sulfate-reducing activity in feces (58.7 +/- 5.6 nmol 35SO4 reduced.h-1.g-1 wet wt vs. 7.9 +/- 2.0 nmol.h-1.g-1 in methanogens). Acetogenesis rates were very low in both groups. It was concluded that H2 excretion varies with different substrates. The proportion of H2 that is exhaled in breath is higher than currently accepted and varies with total excretion rate. Substantial amounts of H2 are consumed by methanogenic and sulfate-reducing bacteria.

Metabolism of dietary sulphate : absorption and excretion in humans

Dietary sulphate may affect colonic pathophysiology because sulphate availability determines in part the activity of sulphate reducing bacteria in the bowel. The main product of sulphate reducing bacterial oxidative metabolism, hydrogen sulphide, is potentially toxic. Although it is generally believed that the sulphate ion is poorly absorbed, there are no available data on how much sulphate reaches the colon nor on the relative contributions from diet and endogenous sources. To resolve these questions, balance studies were performed on six healthy ileostomists and three normal subjects chosen because they did not have detectable sulphate reducing bacteria in their faeces. The subjects were fed diets which varied in sulphate content from 1.6-16.6 mmol/day. Sulphate was measured in diets, faeces (ileal effluent in ileostomists), and urine by anion exchange chromatography with conductivity detection. Overall there was net absorption of dietary sulphate, with the absorptive capacity of the gastrointestinal tract plateauing at 5 mmol/day in the ileostomists and exceeding 16 mmol/day in the normal subjects. Endogenous secretion of sulphate in the upper gastrointestinal tract was from 0.96-2.6 mmol/day. The dietary contribution to the colonic sulphate pool ranged up to 9 mmol/day, there being linear identity between diet and upper gastrointestinal losses for intakes above 7 mmol/day. Faecal losses of sulphate were trivial (less than 0.5 mmol/day) in the normal subjects at all doses. It is concluded that diet and intestinal absorption are the principal factors affecting the amounts of sulphate reaching the colon. Endogenous secretion of sulphate by colonic mucosa may also be important in determining amounts of sulphate in the colon.

Role of dietary sulphate in the regulation of methanogenesis in the human large intestine.

Hydrogen produced during colonic fermentation may be excreted, or removed by H2 consuming bacteria such as methanogenic and sulphate reducing bacteria. In vitro, sulphate reducing bacteria compete with methanogenic bacteria for hydrogen when sulphate is present. In this study the hypothesis that sulphate in the diet could alter CH4 production in vivo has been tested. Six methane excreting volunteers were fed a low sulphate diet (1.6 mmol/d) for 34 days with the addition of 15 mmol sodium sulphate from days 11-20. Breath methane was measured and viable counts and metabolic activities of methanogenic bacteria and sulphate reducing bacteria determined in faeces. Whole gut transit time and daily stool weight were also measured. When sulphate was added to the diet, breath methane excretion decreased in three of the subjects while faecal sulphate reduction rates rose from 7.5 (0.5) to 20.3 (4.3) nmol SO4 reduced/h/g faeces. Sulphate reducing bacteria, which were not detected during the control diet, were found and viable counts of methanogenic bacteria fell from 10(7)-10(9)/g faeces to 10(6)/g. Methanogenic counts and breath CH4 recovered after sulphate addition was stopped. No change was found in the other three subjects. Faecal weights and transit times were not different between study periods. It is concluded that methanogenesis is regulated by dietary sulphate if sulphate reducing bacteria are present. Dietary sulphate may allow growth of sulphate reducing bacteria which inhibit the growth of methanogenic bacteria. This may explain the absence of CH4 in the breath of many people in western populations.

Impaired hydrogen metabolism in pneumatosis cystoides intestinalis

BACKGROUND Pneumatosis cystoides intestinalis (PCI) is characterized by high levels of breath hydrogen. Clinical features of PCI may be due to abnormal H2 metabolism. METHODS Breath levels of H2 and CH4 were measured in 3 patients and total gas in 2 patients with PCI on a polysaccharide-free (basal) diet and after administration of 15 g of lactulose. Metabolic activities and counts of methanogenic (MB) and sulfate-reducing (SRB) bacteria were measured in feces. Ten volunteers were also studied. RESULTS Total H2 levels in patients were 383-420 mL/day on the basal diet and 1430-1730 mL/day after lactulose administration compared with 35 +/- 6 mL/day and 262 +/- 65 mL/day, respectively, in controls. Basal breath H2 levels in controls were 27 +/- 6 vs. 214 +/- 27 mL/day in patients and after lactulose ingestion, 115 +/- 18 vs. 370 +/- 72 mL/day. Four controls were methanogenic and had high fecal MB counts. The other controls had high SRB counts and sulfate reduction rates. All patients were nonmethanogenic and had low sulfate reduction rates. CONCLUSIONS Patients with PCI excrete more H2 than controls. In normal subjects, H2 is consumed by MB or SRB; the activity of these bacteria is virtually absent in PCI. This may explain the gas accumulation in these patients.

Effect of L-glutamine and n-butyrate on the restitution of rat colonic mucosa after acid induced injury.

BACKGROUND: L-glutamine and n-butyrate are important nutrients for colonocytes affecting both their structure and function. The effect of these epithelial substrates on resealing of rat distal colon after acid induced injury was studied. METHODS: Isolated colonic mucosa of 32 rats was mounted in Ussing chambers and exposed to Krebs-Ringer solution for four hours. Epithelial injury was induced by short-term exposure to luminal hydrochloric acid and resealing was studied with or without added glutamine or butyrate. RESULTS: Glutamine (luminal and serosal) reduced tissue conductance, mannitol and lactulose permeability, and permeation of enteropathogenic Escherichia coli. Glutamine (serosal) diminished conductance and mannitol permeability. Both interventions stimulated bromodeoxyuridine incorporation in nuclei of colonocytes. Luminal butyrate had no measurable effect on these parameters. CONCLUSIONS: These data suggest that L-glutamine stimulates repair mechanisms of rat colonic mucosa after acid injury. This effect on the gut barrier is associated with a stimulation of crypt cell proliferation. The addition of glutamine to parenteral solutions may be beneficial for patients under intensive care whose intestinal barrier is weakened in the course of sepsis and trauma.

histological changes in the colonic mucosa following irrigation with short-chain fatty acids

Objectives: Short‐chain fatty acids (SCFAs) derived from bacterial fermentation of complex carbohydrates are preferred luminal nutrients of the colonic mucosa. Starvation of colonocytes through lack or impaired metabolism of luminal SCFAs may be a cofactor in the pathogenesis of ulcerative colitis. Design: A detailed histological evaluation of colonic biopsy specimens was performed in patients with active distal ulcerative colitis who were treated with rectal enemas containing a mixture of SCFAs, n‐butyrate alone or saline placebo. Together with light microscopic parameters of mucosal inflammation, the pattern of crypt cell proliferation (proliferating cell nuclear antigen) and the mucosal activity of factor XIII were assessed. Results: Butyrate reduced the density of polymorphonuclear leucocytes in the lamina propria (4 weeks: P = 0.063; 8 weeks: P = 0.091); other inflammatory parameters remained unchanged. Both butyrate and the SCFA mixture reduced significantly the number of proliferating cells in the upper 40% of crypts. Tissue factor XIII activity in active ulcerative colitis was significantly lower than in mucosa from normal colons; however, it was not affected by SCFA or butyrate irrigation. Conclusion: SCFAs and butyrate have a more marked effect on crypt cell proliferation than on parameters of inflammation in patients with active ulcerative colitis.

In Vitro Fermentation of High-Amylose Cornstarch by a Mixed Population of Colonic Bacteria

BACKGROUND Malabsorbed starch is probably the most important substrate for bacterial fermentation in the human large intestine. Fermentability of starch may depend on the composition of the colonic flora and its adaptation to the substrate supply. METHODS Ten healthy volunteers were fed a controlled diet containing either 7.0 to 8.3 or 50.7 to 59.7 g/d of resistant starch (Hylon VII) for 4 weeks. At the end of each diet period, fecal starch concentrations were measured. Fecal samples were incubated in 48-hour batch cultures containing 10 g/L Hylon VII or digestible Lintners starch. Bacterial breakdown of starch and short-chain fatty acid concentrations were measured at 0, 3, 6, 12, 24, and 48 hours. RESULTS Fecal starch concentrations were higher during the Hylon VII period (35.7 +/- 16 vs 8.9 +/- 3.3 mg/g). Starch was fermented rapidly and completely in vitro in all but two subjects. Fermentability of resistant starch was comparable to that of digestible starch. No differences were found between the dietary periods. Fermentation of resistant starch produced higher rates of n-butyrate. Two subjects had substantially higher fecal starch concentrations. In vitro starch breakdown in these subjects was slow and incomplete. CONCLUSIONS Fermentation of resistant starch by the colonic microflora was rapid and complete in 8 of 10 subjects. No adaptation of the fermentation capacity was observed after 4 weeks of dietary resistant starch supplementation. Fermentation of resistant starch increased the proportion of n-butyrate in vitro. In two subjects, fecal starch concentrations were substantially higher than in the other subjects and in vitro starch fermentation was slow and incomplete.