Associations between basal metabolic rate, body composition and energy intake in gastric bypass patients pre- and post-surgery

Abstract

Gastric Bypass Surgery (GBP) continues to be remarkably efficient in achieving sustainable weight loss in individuals with severe obesity. However, the physiological weight loss mechanisms have not been fully elucidated yet. This thesis aimed to objectively examine the relationship between body composition, basal metabolic rate (BMR) and its association with ad-libitum eating intake (EI) in humans up to 2-years following GBP. It also examined other potential mechanisms contributing to successful weight loss: 1) preoperative and early postoperative factors predicting the variation in post-surgical weight loss, 2) impact of GBP on meal timings and weight loss as well as 3) energy balance following GBP.

The prospective case-control study (REC 16/WS/0056, IRAS 200567) included adults eligible for GBP (n=31) and time-matched weight-stable controls (n=30). All participants were assessed for changes in EI (direct food weighing), eating patterns (covert video monitoring), food reward (liking/wanting, Leeds food preference questionnaire), BMR (indirect calorimetry) and body composition (DXA) at 1-month pre-surgery and 3-,12-, and 24- months post-surgery at a residential unit, Ulster University. In a sub-group analysis (n=7), energy balance was calculated using objective measures of EI and TEE (doubly labelled water method). Physical Activity Energy Expenditure (PAEE, Recent Physical Activity Questionnaire), and thermic effect of foods (TEF, calculated) was also examined. The presence of metabolic adaptation (an additional down-regulation in BMR that cannot be explained by reduction in body composition during weight loss) was assessed with regression analysis. All statistical analyses were performed on SPSS v.26.0, UK.

One-year following GBP, 23 (74.2%) patients achieved >50% excess weight loss that consisted of mainly fat mass (FM) with relative preservation of metabolically active fat-free mass - and this was sustained at 2-years post-surgery. Successful weight loss at 1- and 2-years post-surgery was associated with mainly a higher pre-surgery body mass index. Overall, reductions in mean weight, FM, fat free mass (FFM), BMR and EI from baseline were larger in patients than controls at all follow-ups (preference for high fat and sweet foods reducing by 50% post-surgery (P0.05) but post-surgical changes in body composition correlated with BMR (P0.05). In the subgroup analysis (n 7), mean absolute TEE, BMR, PAEE, TEF and EI values remained the same post-surgery (all P> 0.05) and a negative energy balance was obtained at baseline and 1-year post-surgery.

Based on overall findings, changes in body composition and BMR are not associated with a reduction in EI. However, GBP-induced weight loss and maintenance appears to be partly achieved via pathways stemming from significant weight loss itself and longitudinal changes in body composition (FM loss and FFM preservation) that in turn attenuates the weight loss-associated reduction in metabolic adaptation. This work adds to current knowledge on weight loss mechanisms following GBP. Recommendations are made to improve GBP clinical outcomes by validating these potentially dynamic mechanisms through more well-designed research.

Date of Award	Mar 2022
Original language	English
Supervisor	Ruth Price (Supervisor) & Barbara Livingstone (Supervisor)