The importance of caloric restriction in the early improvements in insulin sensitivity after Roux-en-Y gastric bypass surgery

OBJECTIVE - Many of the metabolic benefits of Roux-en-Y gastric bypass (RYGB) occur before weight loss. In this study we investigated the influence of caloric restriction on the improvements in the metabolic responses that occur within the 1st week after RYGB. RESEARCH METHODS AND DESIGN - A mixed meal was administered to nine subjects before and after RYGB (average 4 ± 0.5 days) and to nine matched, obese subjects before and after 4 days of the post-RYGB diet. RESULTS - Weight loss in both groups was minimal; the RYGB subjects lost 1.4 ± 5.3 kg (P = 0.46) vs. 2.2 ± 1.0 kg (P = 0.004) in the calorically restricted group. Insulin resistance (homeostasis model assessment of insulin resistance) improved with both RYGB (5.0 ± 3.1 to 3.3 ± 2.1; P = 0.03) and caloric restriction (4.8 ± 4.1 to 3.6 ± 4.1; P = 0.004). The insulin response to a mixed meal was blunted in both the RYGB and caloric restriction groups (113 ± 67 to 65 ± 33 and 85 ± 59 to 65 ± 56 nmol·l^-1·min ^-1, respectively; P < 0.05) without a change in the glucose response. Glucagon-like peptide 1 levels increased (9.2 ± 8.6 to 12.2 ± 5.5 pg·l^-1·min^-1; P = 0.04) and peaked higher (45.2 ± 37.3 to 84.8 ± 33.0 pg/ml; P = 0.01) in response to a mixed meal after RYGB, but incretin responses were not altered after caloric restriction. CONCLUSIONS - These data suggest that an improvement in insulin resistance in the 1st week after RYGB is primarily due to caloric restriction, and the enhanced incretin response after RYGB does not improve postprandial glucose homeostasis during this time.