Weight Loss Measurements Under the Microscope: How Type and Baseline Definition Shape Clinical Response After Metabolic-Bariatric Surgery — A 2-Year Follow-up Analysis of 17,568 Patients

Abstract Background In the era of modern metabolic-bariatric surgery, increasing attention is being paid to the assessment of clinical response after surgery. With the growing availability of multimodal obesity treatment, including preoperative pharmacological and lifestyle interventions, identification and optimization of factors associated with a suboptimal clinical response after metabolic-bariatric surgery (MBS) have become increasingly important. When considering weight loss specifically, two methodological factors substantially influence the interpretation of postoperative response: the metric used to quantify weight loss (percent total weight loss [%TWL] versus percent excess weight loss [%EWL]) and the baseline from which the weight-loss trajectory is calculated. Objective To illustrate how different weight-loss metrics and baseline definitions influence the interpretation of postoperative response and the apparent association between preoperative characteristics and suboptimal clinical response after MBS. Methods We analyzed 17,568 patients from the StuDoQ|MBE registry who underwent primary MBS and had complete 2-year postoperative follow-up data. Postoperative suboptimal clinical response was assessed using the 2024 IFSO reporting threshold of <20% TWL. Because the registry did not contain sufficiently standardized longitudinal information on improvement of obesity-related complications, the clinical component of the IFSO definition could not be incorporated. The historical threshold of <50% EWL was additionally analyzed as a surrogate criterion to facilitate comparison with previous literature. Multivariable generalized linear mixed-effects models were used to evaluate the associations of preoperative BMI and preoperative weight loss with these weight-loss-based outcomes. Analyses distinguished between cumulative weight loss, calculated from the earliest documented weight before initiation of preoperative weight reduction, and postsurgical weight loss, calculated from weight at the time of surgery to follow-up. Partial-effects plots and heatmaps were used to visualize the modeled associations. Results The proportion of patients classified according to weight-loss-based criteria for suboptimal clinical response varied substantially according to the metric used, with more patients meeting the historical <50% EWL criterion than the IFSO-consistent <20% TWL threshold. Higher preoperative BMI was associated with a greater probability of meeting the historical <50% EWL criterion, whereas the association was reversed when suboptimal response was defined using <20% TWL. The definition of the baseline weight also substantially influenced the interpretation of preoperative weight loss. Greater preoperative weight loss was associated with a lower probability of weight-loss-based suboptimal clinical response when assessed cumulatively from the beginning of the preoperative treatment trajectory, whereas greater preoperative weight loss was associated with a higher probability of meeting the postsurgical suboptimal weight-loss criteria when weight at the day of surgery was used as the baseline. Conclusion The choice of weight-loss metric and baseline definition substantially influences the classification of postoperative weight-loss response and the apparent association between preoperative characteristics and surgical outcomes. %EWL and %TWL are not interchangeable measures and can produce opposing interpretations of the association between baseline BMI and postoperative response. Consistent with the 2024 IFSO consensus, weight-loss assessment after MBS should use a baseline established before initiation of preoperative weight reduction where appropriate, particularly when evaluating the overall treatment trajectory. %TWL provides a more readily comparable metric for standardized reporting, whereas %EWL may remain useful for comparison with historical literature. Because clinical response after MBS is multidimensional, weight-loss thresholds should not be interpreted as synonymous with clinical treatment failure.