Context: Type 2 diabetes mellitus (DM2) is associated with small intestinal hyperplasia and hypertrophy in rodents. Moreover, the small intestine is increasingly acknowledged to play a role in the pathophysiology of DM2. Objective: The objective of the study was to investigate the relation between plasma markers of small intestinal function and chronic hyperglycemia in man. Design, Setting, and Participants: We conducted a cross-sectional observational study of 40 severely obese subjects with chronic hyperglycemia and 30 severely obese subjects without chronic hyperglycemia who were indicated for bariatric surgery. Main Outcome Measures: We assessed plasma levels of citrulline, representing small intestinal enterocyte mass, intestinal fatty acid binding protein (I-FABP), a marker of enterocyte loss, and glucagon-like peptide-2, an intestinotrophic factor, and related them to glycated hemoglobin (HbA1c) levels. Results: Plasma citrulline and I-FABP levels were both significantly elevated in subjects with chronic hyperglycemia (HbA1c > 6.0%) compared with subjects with a normal HbA1c (</= 6.0%) (citrulline, 35 +/- 2.1 muM vs. 26 +/- 1.4 muM, P = 0.001; I-FABP, 140 +/- 22 pg/ml vs. 69 +/- 14 pg/ml, P = 0.001). Moreover, plasma citrulline and I-FABP levels correlated with HbA1c levels (citrulline, rs = 0.30, P = 0.02; I-FABP, rs = 0.33, P = 0.005). The I-FABP to citrulline ratio was higher in subjects with an elevated HbA1c (4.0 vs. 3.1, P = 0.03). Plasma glucagon-like peptide-2 levels were not related to citrulline or I-FABP levels (rs = 0.06, P = 0.67; rs 0.08, P = 0.54, respectively). Conclusion: Chronically elevated glucose levels in obese individuals are associated with increased small intestinal enterocyte mass and increased enterocyte loss. These findings argue for the further exploration of the role of the intestine in the pathophysiology of DM2.