Carbohydrate malabsorption. Its measurement and its contribution to diarrhea. The major purpose of this research was to gain insight into the effect of carbohydrate malabsorption on fecal water output. To do this we measured daily fecal output of total carbohydrate, reducing sugars, and organic acids (a product of bacterial fermentation). Normal subjects were studied in their native state and when diarrhea was induced by mechanisms that did and did not involve carbohydrate malabsorption. Patients with malabsorption syndrome were also studied. We concluded that: (a) Excretion of carbohydrate and its breakdown products can be expressed as a single number by converting organic acids to their monosaccharide equivalents. (b) Diarrhea per se causes only a trivial increase in fecal carbohydrate excretion. (c) The molar output of osmotic moieties in feces due to unabsorbed carbohydrate can be determined by adding fecal reducing sugars to organic acids and their obligated cations. This expression parallels almost exactly the effect of increasing doses of lactulose (a nonabsorbable sugar) on fecal water output; one excreted millimole obligates 3.5 g of stool water. This relationship can be used to predict the effect of carbohydrate malabsorption on stool water output in patients with diarrhea. (d) 12 of 19 patients with malabsorption syndrome due to various diseases had excessive fecal excretion of carbohydrate and its breakdown products; of the diseases that cause malabsorption syndrome, combined small and large bowel resection is most likely to result in excessive fecal excretion of carbohydrate and monosaccharide equivalents. In 6 of these 19 patients carbohydrate malabsorption appeared to be the major cause of diarrhea. 