To determine if organic anions contribute to the diarrhea of inflammatory bowel disease, we measured osmolality, electrolytes, short-chain fatty acids, lactic acid, and some Krebs cycle anions in 24-hr fecal collections from 18 patients with chronic ulcerative colitis, 20 with Crohn's disease of the colon, and 16 normals. Mean lactic acid concentration was significantly elevated in ulcerative and Crohn's colitis, but values correlated with fecal weight only in the former syndrome. In ulcerative colitis, concentrations of each short-chain fatty acid, especially butyrate, were decreased compared with those from normals or Crohn's disease. Lactate and short-chain fatty acids accounted for nearly half the variability in fecal weight in ulcerative colitis. Crohn's patients had elevated mean fecal water osmolality and osmotic gap not observed in ulcerative colitis. Increased lactic acid and/or deficient short-chain fatty acids may modulate the diarrhea of ulcerative colitis. This mechanism seems less important in Crohn's colitis where an additional osmotic component may be significant.

SUMMARY We determined the ionic composition of faecal fluid from 13 patients with Crohn's disease limited to the colon, 10 with diffuse ulcerative colitis, and eight with ulcerative proctitis. The Crohn's and colitis groups had similar proportions of colon surface involved radiographically and similar 24 hour faecal weights. However, Crohn's patients' faecal fluid had arithmetically lower mean sodium and statistically lower mean chloride (34.8 mmol/l ± 16.2 SD vs. 53.1 mmol/l ± 23.1 SD) and higher potassium (49.2 mmol/l ± 20.2 SD vs. 33.0 mmol/l ± 13.8 SD) concentrations (p < 0.05 for each) and much higher osmolality (487.1 mOsmol/kg ± 87.1 SD vs. 341.1 mOsmol/ kg ± 88.9 SD, P < 0.001). Separation of these patients using the faecal osmotic gap agreed with the clinical classification in 86% of cases. The diarrhoea of proctitis patients had a nearly normal ionic composition which was clearly distinguishable from that of diffuse colitis. These results suggest differences in the composition and perhaps the pathogenesis of the diarrhoea of Crohn's and ulcerative colitis. The composition of fluid may prove a useful, non-invasive method for classifying patients with inflammatory bowel disease and, in ulcerative colitis, determining the extent of the inflammatory process.

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