Summary Gastric bypass surgery is an effective long‐term treatment for individuals with severe obesity. Changes in appetite, dietary intake, and food preferences have all been postulated to contribute to postoperative body weight regulation, however, findings are inconsistent. The aim of this systematic review was to evaluate the current literature on changes in dietary intake and appetite following gastric bypass surgery, in the context of the methodology used and the analysis, interpretation, and presentation of results. Four databases were systematically searched with terms related to “gastric bypass surgery,” “appetite,” and “dietary intake,” and 49 papers (n = 2384 patients after gastric bypass) were eligible for inclusion. The evidence indicated that only a reduction in overall energy intake and an increase in postprandial satiety are maintained beyond 6‐month post‐surgery, whereas relative macronutrient intake and premeal hunger remain unchanged. However, available data were limited by inconsistencies in the methods, analysis, presentation, and interpretation of results. In particular, there was a reliance on data collected by subjective methods with minimal acknowledgment of the limitations, such as misreporting of food intake. There is a need for further work employing objective measurement of appetite and dietary intake following gastric bypass surgery to determine how these mechanisms may contribute to weight regulation in the longer term.

Background Lack of robust research methodology for assessing ingestive behaviour has impeded clarification of the mediators of food intake following gastric bypass (GBP) surgery. Objective To evaluate changes in directly measured 24hr energy intake (EI), energy density (ED) (primary outcomes), eating patterns and food preferences (secondary outcomes) in patients and time matched weight-stable comparator participants. Design Patients (n = 31,77% female, BMI 45.5±1.3) and comparators (n = 32, 47% female, BMI 27.2±0.8) were assessed for 36hr under fully residential conditions at baseline (1-mo pre-surgery) and at 3- and 12-mo post-surgery. Participants had ad libitum access to a personalised menu (n = 54 foods) based on a 6 macronutrient mix paradigm. Food preferences were assessed by the Leeds Food Preference Questionnaire. Body composition was measured by whole-body dual-energy x-ray absorptiometry. Results In the comparator group there was an increase in relative fat intake at 3-mo post-surgery, otherwise no changes were observed in food intake or body composition. At 12-mo post-surgery, patients lost 27.7±1.6% of initial body weight (p<0.001). The decline in EI at 3-mo post-surgery (-44% from baseline, P<0001)) was followed by a partial rebound at 12-mo (-18% from baseline) but at both times dietary ED and relative macronutrient intake remained constant. The decline in EI was due to eating the same foods as consumed pre-surgery and by decreasing the size (g, MJ), but not the number, of eating occasions. In patients, reduction in explicit liking at 3-mo (-11.56±4.67, P = 0.007) and implicit wanting at 3- (-15.75±7.76, P = 0.01) and 12-mo (-15.18±6.52, P = 0.022) for sweet foods was not matched by reduced intake of these foods. Patients with the greatest reduction in ED post-surgery reduced both EI and preference for sweet foods. Conclusion After GBP patients continue to eat the same foods but in smaller amounts. These findings challenge prevailing views about the dynamics of food intake following GBP surgery. Clinical Trial Registry Number: NCT03113305 (ClinicalTrials.gov)

Gastric bypass surgery leads to significant and sustained weight loss and a reduction in associated health risks in individuals with severe obesity. While reduced energy intake (EI) is the primary driver of weight loss following surgery, the underlying mechanisms accounting for this energy deficit are not well understood. The evidence base has been constrained by a lack of fit-for-purpose methodology in assessing food intake coupled with follow-up studies that are relatively short-term. This paper describes the underlying rationale and protocol for an observational, fully residential study using covert, objective methodology to evaluate changes in 24-hr food intake in patients (n = 31) at 1-month pre-surgery and 3-, 12- and 24-months post-surgery, compared to weight-stable controls (n = 32). The main study endpoints included change in EI, macronutrient intake, food preferences, and eating behaviours (speed, frequency, and duration of eating). Other physiological changes that may influence EI and weight regulation including changes in body composition, circulating appetite hormones, resting metabolic rate, total energy expenditure and gastrointestinal symptoms were also evaluated. Understanding which mechanisms contribute to a reduction in EI and weight loss post-surgery could potentially help to identify those individuals who are most likely to benefit from gastric bypass surgery as well as those that may need more targeted intervention to optimise their weight loss post-surgery. Furthermore, clarification of these mechanisms may also inform targeted approaches for non-surgical treatments of obesity.

Gastric Bypass (GB) surgery continues to be one of the few effective treatments for sustainable weight loss in obese individuals (1). Apart from the primary weight loss mechanisms of energy restriction and malabsorption, studies have postulated that GB is associated with an increased shift in energy expenditure (EE) that contributes to sustained weight loss in the longer-term (2-5). However this positive effect on EE in humans remain equivocal. The effect has mainly been observed in experimental animal studies which adjusted EE for changes in total body weight (TBW) and body composition (BC) observed postoperatively (3-5). The aim of this study was to assess the impact of GB on Basal Metabolic Rate (BMR) in 17 GB patients (5 males; BMI 45•8 ± 6•3 kg/m 2 ; 47•2 ± 13 y) and 13 time-matched and weight-stable controls (5 males; BMI 25•8 ± 4•3 kg/m 2 ; 39•4 ± 15•1 y) at baseline (one month pre-surgery) and at three months post-surgery. BMR was measured under standardised conditions using indirect calorimetry (ECAL, Metabolic Health Solutions). Lean body mass (LBM) and fat mass (FM) were measured using dual energy X-ray absorptiometry (DEXA, GE Healthcare). BMR values were expressed per kg of TBW, LBM and FM. A paired sample t-test was used to compare changes in BMR between baseline and 3 months post-surgery. Mean absolute BMR (kJ/day), TBW (kg), BMI (kg/m 2), LBM (kg) and FM (kg) significantly decreased by 16•1 ± 21%, 16•9 ± 4•3%, 16•7 ± 4•4%, 9•8 ± 4•7% and 24•7 ± 7•1% respectively in patients (P < 0•05) and by-0•5 ± 24•4% 1•1 ± 2•4%, 0•4 ± 2•8%, 0•3 ± 3•1% and 2•1 ± 8•1% respectively in controls (all non-significant). When expressed relative to TBW and LBM, GB attenuated the compensatory reductions in BMR while BMR expressed relative to FM increased three months post-surgery in patients. In conclusion, weight loss following GB is associated with a favourable increase in BMR when expressed relative to postoperative changes in TBW and BC. This may contribute to the successful weight loss outcomes observed postoperatively. Further research is warranted to elucidate the underlying mechanism for this apparent beneficial impact of GB on BMR.

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