The efficacy of Roux-en-Y gastric bypass (RYGB) surgery to produce weight loss has been well-documented, but few studies have measured the key components of energy balance, food intake, and energy expenditure longitudinally.

Male Sprague-Dawley rats on a high-fat diet underwent either RYGB, sham operation, or pair feeding and were compared to chow-fed lean controls. Body weight and composition, food intake and preference, energy expenditure, fecal output, and gastric emptying were monitored before and up to 4 months after intervention.

Despite the recovery of initially decreased food intake to levels slightly higher than before surgery and comparable to sham-operated rats after about 1 month, RYGB rats maintained a lower level of body weight and fat mass for 4 months that was not different from chow-fed age-matched controls. Energy expenditure corrected for lean body mass at 1 and 4 months after RYGB was not different from presurgical levels and from all other groups. Fecal energy loss was significantly increased at 6 and 16 weeks after RYGB compared to sham operation, and there was a progressive decrease in fat preference after RYGB.

In this rat model of RYGB, sustained weight loss is achieved by a combination of initial hypophagia and sustained increases in fecal energy loss, without change in energy expenditure per lean mass. A shift away from high-fat towards low-fat/high-carbohydrate food preference occurring in parallel suggests long-term adaptive mechanisms related to fat absorption.