OBJECTIVE Improvements in diabetes after Roux-en-Y gastric bypass (RYGB) often occur days after surgery. Surgically induced hormonal changes and the restrictive postoperative diet are proposed mechanisms. We evaluated the contribution of caloric restriction versus surgically induced changes to glucose homeostasis in the immediate postoperative period.

RESEARCH DESIGN AND METHODS Patients with type 2 diabetes planning to undergo RYGB participated in a prospective two-period study (each period involved a 10-day inpatient stay and periods were separated by a minimum of 6 weeks of wash-out) in which patients served as their own controls. The presurgery period consisted of diet alone. The postsurgery period was matched in all aspects (daily matched diet) and included RYGB surgery. Glucose measurements were performed every 4 h throughout the study. A mixed meal challenge test was performed before and after each period.

RESULTS Ten patients completed the study and had the following characteristics: age, 53.2 years (95% CI, 48.0–58.4); BMI, 51.2 kg/m² (46.1–56.4); diabetes duration, 7.4 years (4.8–10.0); and HbA_1c, 8.52% (7.08–9.96). Patients lost 7.3 kg (8.1–6.5) during the presurgery period versus 4.0 kg (6.2–1.7) during the postsurgery period (P = 0.01 between periods). Daily glycemia in the presurgery period was significantly lower (1,293.58 mg/dL*day [1,096.83–1,490.33) vs. 1,478.80 mg/dL*day [1,277.47–1,680.13]) compared with the postsurgery period (P = 0.02 between periods). The improvements in the fasting and maximum poststimulation glucose and 6-h glucose area under the curve (primary outcome) were similar during both periods.

CONCLUSIONS Glucose homeostasis improved in response to a reduced caloric diet, with a greater effect observed in the absence of surgery as compared with after RYGB. These findings suggest that reduced calorie ingestion can explain the marked improvement in diabetes control observed after RYGB.