Advantages of Percent Weight Loss as a Method of Reporting Weight Loss after Roux-en-Y Gastric Bypass

OBJECTIVE: Although Roux-en-Y gastric bypass (RYGB) is a generally effective treatment for severe obesity, weight loss (WL) after this operation is highly variable. Accurate predictors of outcome would thus be useful in identifying those patients who would most benefit from this invasive therapy. WL has been characterized using several different metrics, including the number of BMI units lost (ΔBMI), percent baseline WL (%WL), and percent excess body WL (%EBWL). To identify clinically relevant predictors most sensitively it is necessary to avoid confounding by other factors, including preoperative BMI (pBMI), the strongest known predictor of RYGB-induced WL. DESIGN AND METHODS: To determine the WL measure least associated with pBMI, we analyzed outcomes of 846 patients undergoing RYGB. RESULTS: Patients in this cohort had an average pBMI of 50.0 kg/m(2). At weight nadir, they lost an average 19.4 kg/m(2), 38.7% WL, and 81.2% EBWL. pBMI was strongly and positively associated with ΔBMI at both one year (r=0.56, p=4.7×10(−51)) and nadir (r=0.58, p=2.8×10(−77)) and strongly but negatively associated with %EBWL at one year (r=−0.52, p=3.8×10(−44)) and nadir (r=−0.45, p=7.2×10(−43)). In contrast, pBMI was not significantly associated with %WL at one year (r=0.04, p=0.33), and only weakly associated at nadir (r=0.13, p=0.0002). CONCLUSIONS: Of the metrics examined, %WL is the parameter describing WL after RYGB least influenced by pBMI. It thus improves comparison of WL outcomes across studies of patients undergoing surgery and facilitates the most sensitive identification of novel predictors of surgery-induced WL. We therefore recommend that %WL be adopted more broadly in reporting weight loss after RYGB.