PubMed · 9855926
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J Winney. Constipation.. https://pubmed.ncbi.nlm.nih.gov/9855926/
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Prebiotics, as currently conceived of, are all carbohydrates of relatively short chain length. To be effective they must reach the cecum. Present evidence concerning the 2 most studied prebiotics, fructooligosaccharides and inulin, is consistent with their resisting digestion by gastric acid and pancreatic enzymes in vivo. However, the wide variety of new candidate prebiotics becoming available for human use requires that a manageable set of in vitro tests be agreed on so that their nondigestibility and fermentability can be established without recourse to human studies in every case. In the large intestine, prebiotics, in addition to their selective effects on bifidobacteria and lactobacilli, influence many aspects of bowel function through fermentation. Short-chain fatty acids are a major product of prebiotic breakdown, but as yet, no characteristic pattern of fermentation acids has been identified. Through stimulation of bacterial growth and fermentation, prebiotics affect bowel habit and are mildly laxative. Perhaps more importantly, some are a potent source of hydrogen in the gut. Mild flatulence is frequently observed by subjects being fed prebiotics; in a significant number of subjects it is severe enough to be unacceptable and to discourage consumption. Prebiotics are like other carbohydrates that reach the cecum, such as nonstarch polysaccharides, sugar alcohols, and resistant starch, in being substrates for fermentation. They are, however, distinctive in their selective effect on the microflora and their propensity to produce flatulence.
We have previously shown that magnesium citrate or bisacodyl before polyethylene glycol electrolyte lavage solution (PEG-ELS) improves colonoscopy preparation. Patients prefer this to preparation with full-volume PEG-ELS alone. However, such combinations might cause undesirable changes in hemodynamics or serum electrolytes. This study examined the effects of these combinations on heart rate, blood pressure, and serum electrolytes. We randomized 68 consecutive patients undergoing colonoscopy to one of three preparations. Group 1 received 4 L of PEG-ELS, group 2 received 2 L of PEG-ELS with 20 mg bisacodyl, and group 3 received 2 L of PEG-ELS preceded by 296 mL of magnesium citrate. Heart rate, blood pressure, and serum electrolytes were measured before starting the preparation and immediately before colonoscopy. Mean serum magnesium fell by 0.01 mg/dL and 0.11 mg/dL in groups 1 and 2, respectively, and rose by 0.06 mg/dL in group 3 (p = 0.044). There were no other significant changes. Statistically significant changes in serum magnesium after preparation with PEG-ELS and a stimulant laxative are minor and probably of no clinical relevance. Two liters of PEG-ELS with a stimulant laxative is safe and effective for colonoscopy.
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