Dietary carbohydrates (CHO) that are not absorbed in the small intestine are fermented in the colon, producing gases (CO 2 , H 2 ), short-chain fatty acids and bacterial mass. Currently, there is no available method to determine precisely the amount of unabsorbed dietary CHO and their fermentation products excreted in stools. Since carbon is present in CHO and a majority of fermentation metabolites (CO 2 , short-chain fatty acids, bacterial mass, etc), its labelling and measurement could be an interesting method to use in the investigation and follow-up of patients with a malabsorption syndrome. For this purpose, patients with a malabsorption syndrome and normal subjects were given orally CHO naturally enriched with 1 3 C and we measured the overall excretion of 1 3 C in the stools and breath ( 1 3 CO 2 ). Ten patients with the short bowel syndrome (SBS) (length 73 +/- 40 cm, mean +/- SD) with (n = 8) or without (n = 2) the colon remaining in continuity and eight healthy volunteers (HV) ingested a test meal after an overnight fast (3 318 kJ, protein-fat-CHO, 17:26:57% energy) containing naturally 1 3 C enriched CHO in the form of 50 g corn starch (-12.22 8%%), 50 g sugar cane (-11.10 8 %%) and carmine red as a faecal recovery marker. Patients consumed a low- 1 3 C diet and their stools were collected 1 day before and 3 days after the test meal. Breath samples were collected half-hourly and CO 2 production was measured using indirect calorimetry hourly for 6 h before and after the test meal ingestion. In the breath, in the premeal and red postmeal stools, 1 3 C enrichment was measured by an isotope ratio mass spectrometer (GC/CN-IRMS, Europa Scientific). Faecal and breath 1 3 C enrichment was corrected using the basal premeal value.The excess faecal output of 1 3 C was 142 +/- 85 μmol (range 14-299) and 6 +/- 7 μmol (range 0-16) in SBS and HV, respectively. The percentage of ingested dose recovered in the stools was 23 +/- 18% (range 3-61) and 1 +/- 1% (range 0-3) in SBS and HV, respectively. The percentage of ingested dose exhaled in breath for 6 h was 31 +/- 9% (range 16-42) and 25 +/- 2% (range 20-29) in SBS and HV, respectively. In SBS, the percentage of 1 3 C excreted in stools was inversely related to that excreted in breath for 6 h (Spearman correlation Rho = -0.85, P < 0.02).It was concluded that the faecal measurement of 1 3 C after the ingestion of a breakfast containing CHO naturally enriched with 1 3 C is a useful tool for assessing both the fat of CHO and their carbon containing fermentation products after their passage through the digestive tract of patients with a malabsorption syndrome. In addition, our preliminary results show that a 6 h breath 1 3 CO 2 test could be a valid indirect measurement of overall CHO digestibility carbon from dietary CHO.