Conservation of mannitol, lactulose, and raffinose by the human colon

We measured fecal composition after giving oral doses of mannitol, lactulose, or raffinose to ileostomy patients and to normal subjects in order to determine the colon's capacity for metabolizing these carbohydrates and to decide whether volatile fatty acids (VFA) or unabsorbed carbohydrate initiated diarrhea. Seventy-four percent of mannitol, 100% of lactulose, and 88% of raffinose passed unabsorbed through the small intestine of six patients with ileostomies. Dose-response experiments revealed that normal subjects could tolerate 120-220 mmol of mannitol, 73-146 mmol of lactulose, or 80 mmol of raffinose before fecal output of water exceeded 400 ml in 48 h or before test carbohydrate appeared in the stool. Mannitol could be metabolized to glucose; lactulose to fructose, galactose, and glucose; and raffinose to sucrose, melibiose, fructose, galactose, and glucose. The output of VFA in fecal water did not correlate with the moles of carbohydrate metabolized, and it did not greatly exceed the output of VFA in stools of equal volume that were induced by nonfermentable MgSO4. We conclude that the human colon is capable of removing appreciable amounts of single doses of these carbohydrates from fecal water. When this capacity is exceeded, unabsorbed carbohydrate rather than VFA prompts an increased output of fecal water.