Baseline patient characteristics
A total of 1,456 adults (Table 1) underwent either RYGBP (n = 918) or SG (n = 538) as a primary procedure. There were significant group differences in age, baseline BMI, and diabetes status between centres. In relation to the SG group, age was significantly higher in the UHP centre than the UCLH centre (P < 0.001) (Table 1). In relation to the RYGBP group, baseline BMI was significantly higher in the UHP centre than the UCLH centre (P < 0.001), whereas the proportion of patients with T2D (P = 0.03) was significantly higher in the UCLH centre than the UHP centre. There were no significant group differences in gender distribution between centres. Maximal weight loss data were calculated for 877 (95.5 %) patients who underwent RYGBP and 513 (95.3 %) patients who underwent SG, with weight data available for 715 (77.9 %) patients in the RYGBP group and 390 (72.5 %) patients in the SG group at the 2-year postoperative appointment.
%WL following RYGBP and SG
There was a marked variability in maximal %WL (Fig. 1) following both RYGBP (mean 32.9 %, range 4.1–60.9 %) and SG (mean 26.2 %, range 1.1–58.3 %). Maximal %WL occurred at the 12-month follow-up appointment in approximately one-third of patients in both RYGBP and SG groups (Table 2). However, a higher proportion of patients in the RYGBP group experienced maximal %WL at the 24-month appointment, with lower proportions than the SG group at the 6- and 9-month appointments (P < 0.001). In order to visualize the distribution of weight loss trajectories, normative charts were constructed based on percentiles of %WL at postoperative timepoints and showed similar weight loss variability for both RYGBP and SG patients (Fig. 2).
Association of early postoperative %WL with maximal %WL
In order to investigate whether early postoperative weight loss predicted the ultimate weight loss response, linear regression analyses were performed with maximal %WL as the outcome (Table 3). %WL at 6 weeks, 3 and 6 months was significantly associated with maximal %WL for patients in both RYGBP and SG groups (Fig. 3). The associations of early postoperative %WL with maximal %WL were stronger for SG than RYGBP at all three postoperative timepoints examined, but most notably at 6-week and 3-month assessments (Table 3).
Association of early postoperative WLV with maximal and 2-year %WL
Next, we examined the relative importance of weight loss during specific time intervals in the early postoperative period in relation to the maximal %WL achieved. In order to determine this, WLV during the 0–6 weeks, 6 weeks to 3 months, and 3–6 months postoperative time periods was used in linear regression analyses. We observed procedure-specific temporal differences in the strength of association between postoperative WLV and maximal %WL (Fig. 4). In the SG group, WLV during the 0–6 weeks and 6 weeks to 3 months postoperative periods predicted 18 and 21 % of the variability in maximal %WL respectively, increasing to 26 % for the 3–6 month period (Table 3). Whereas in the RYGBP group, WLV during the 0–6 week period and 6 weeks to 3 months period accounted for only 12 and 10 % of the variability in maximal %WL, respectively, increasing markedly to 32 % for the 3–6 month period (Table 3). Early postoperative WLV associations with 2-year %WL were comparable with those for maximal %WL (Table 4).
Multiple linear regression analyses
Multiple linear regression analyses including baseline variables of age, sex, preoperative BMI, diabetes, ethnicity and bariatric surgery centre revealed that 3–6 months WLV, baseline BMI, and age were independent predictors of maximal %WL for both SG and RYGBP groups (Table 4). In addition, gender and T2D were independently associated with maximal %WL in the RYGBP group, while ethnicity and bariatric centre were independently associated with 1-year %WL in the SG group. However, in comparison with baseline variables, 3–6 month WLV was the strongest predictor of maximal %WL following RYGBP and SG (Table 5).
A ROC curve (Fig. 5) was constructed to determine the point at which 3–6 month WLV predicted maximal %WL (using a target %WL of 20 %) with the best sensitivity/specificity combination. The inflection point corresponded to a sensitivity of 80 % and a specificity of 72 %. Using this cutoff, which occurred at a WLV of 0.4667 kg/week (1 lb/week), the maximal %WL outcome was classified correctly for 79 % of patients.