Factors influencing energy intake and body weight loss after gastric bypass

Objective: The gastric bypass-induced quantitative and qualitative modifications of energy intake (En In, kcal/day) and their impact on body weight (bw) loss were evaluated. The factors influencing energy intake and body weight loss were also investigated. Design: Longitudinal study. Setting: University Hospital of Geneva. Subjects: Fifty obese women undergoing a Roux-en-Y gastric bypass. Results: The reduction of EnIn was significantly related to bw loss expressed either in kg or as percentage correction of excess bw (P<0.01 for both), whereas the post-operative modifications of diet composition did not play a role. Age and initial bw significantly influenced bw loss (P < 0.0001 and P < 0.001, respectively), as shown by multiple regression analysis. Patients were divided into four sub-groups according to their age (under or over 35 y) and initial bw (under or over 120 kg). ANOVA showed that under 35-y-old subjects reduced their EnIn significantly more than their older counterparts having similar bw (P < 0.02 and P < 0.05); consequently, bw loss, expressed in kg, was significantly (P < 0.0001 and P < 0.0005) larger in younger patients. Subjects with an initial bw over 120 kg lost significantly (P < 0.001 and P < 0.02) more weight as compared to patients with a smaller degree of obesity (under 120 kg) and similar age. Conclusions: Gastric bypass-induced body weight loss is mainly due to the reduction of EnIn, whereas the qualitative modifications of the diet do not play a role. Younger subjects have a greater capacity to reduce EnIn and, therefore, lose more weight. Pre-operative high degree of obesity leads to a larger weight reduction, probably because of a greater energy deficit.