Weight loss in adult male Wistar rats by Roux-en-Y gastric bypass is primarily explained by caloric intake reduction and pre-surgery body weight.

Diets varying in macronutrient composition, energy density, and/or palatability may cause differences in outcome of bariatric surgery. In the present study, rats feeding a healthy low fat (LF) diet or an obesogenic high fat/sucrose diet (HF/S) were either subjected to Roux-en-Y gastric bypass surgery (RYGB) or sham surgery, and weight loss trajectories and various energy balance parameters were assessed. Before RYGB, rats eating a HF/S (n=14) diet increased body weight relative to rats eating a LF diet (n=20; p<0.01). After RYGB, absolute weight loss was larger in HF/S (n=6) relative to LF feeding (n=6) rats, and this was associated with reduced cumulative energy intake (EI; p<0.05) and increased locomotor activity (LA; p<0.05-0.001), finally leading to similar levels of reduced body fat content in HF/S and LF rats 3 weeks after surgery. Regression analysis revealed that variation in RYGB-induced body weight loss was best explained by models including a) post-surgery cumulative EI and pre-surgery body weight (R²=0.87) and b) post-surgery cumulative EI and diet (R²=0.79), each without significant contribution of LA. Particularly rats on the LF diet became transiently more hypothermic and circadianally arrhythmic following RYGB (i.e., indicators of surgery-associated malaise) than HF/S feeding rats. Our data suggest that relative to feeding a LF diet, continued feeding a HF/S diet does not negatively impact recovery from RYGB surgery, yet it promotes RYGB-induced weight loss explained primarily by reduced cumulative EI and higher pre-surgery body weight, finally leading to comparably low levels of body fat content in HF/S and LF feeding rats.