The findings in our study provide novel insights into the weight loss response experienced both after RYGBP and SG. Patients who underwent either SG or RYGBP experienced a wide variability in weight loss during the first two postoperative years. In a particularly novel aspect of the study, the variability in maximal weight loss was shown to be strikingly similar between procedures. Furthermore, the results demonstrate that early postoperative weight loss is a key predictor of ultimate weight loss response, with a greater effect on outcome than several well-established baseline clinical factors such as preoperative BMI, age, sex, and diabetes.
Interestingly, our results show temporal procedure-specific differences in the relationship between early postoperative weight loss, expressed as %WL or WLV, and maximal weight loss. In the immediate postoperative period (6 weeks post-surgery), %WL is a stronger predictor of maximal weight loss in the SG group compared to the RYGBP group. Similarly, the strength of the association between %WL in the first three postoperative months and maximal weight loss in the SG group was approximately twice that observed in the RYGBP group. In order to identify which postoperative time period may best predict ultimate weight loss outcome, we examined WLV during specific postoperative time intervals, a concept which has been previously applied in a study of RYGBP outcomes [21]. WLV experienced in the 3–6 month postoperative period was a stronger predictor of maximal weight loss, compared to the earlier postoperative time intervals, for both SG and RYGBP groups. However, in the RYGBP group, the strength of the association with maximal weight loss was approximately threefold higher for the 3–6 month postoperative period compared to the earlier postoperative time intervals, but only approximately 25–50 % higher in the SG group. Taken together, these findings support the concept that both distinct and overlapping biological mechanisms underlie the benefits of SG and RYGBP [27, 28].
Our findings have several potential clinical implications for patients undergoing either SG or RYGBP. Firstly, focus on early postoperative WLV is an effective means of identifying patients whose weight loss is ultimately suboptimal. Our ROC analysis demonstrates that approximately four out of every five patients who lose less than a 1 lb a week during the 3–6 month postoperative period will not achieve a maximal %WL of more than 20 %. Such patients could be targeted for early postoperative behavioural or intensive lifestyle interventions known to improve weight outcome after surgery [22–24], thereby providing an opportunity to enhance their maximal weight loss. Our results provide a basis for randomized trials of behavioural or exercise interventions initiated early in the postoperative course for both RYGBP and SG patients.
Secondly, the wide variability in weight loss response, previously demonstrated only in RYGBP patients [8, 9], is similar for both procedures. Therefore, bariatric health care professionals should alert patients who are considering surgery to this inherent variability as part of the informed consent process. In this regard, our normative charts provide a useful reference for expected weight loss trajectories post-SG or RYGBP and are consistent with the results of a previous single-centre study in RYGBP patients [21]. The practice of providing advice to patients regarding expected weight loss based on an average narrow range [29] is likely to be counterproductive, ultimately leading unnecessarily to a sense of disappointment or failure for many patients [26]. Indeed, a poor weight loss response is likely to merely reflect an outcome at the lower tail of a normal distribution driven by a multitude of complex biological factors [7, 30]. Patients should be advised as such preoperatively and be relieved of any sense of blame if necessary in the postoperative setting.
Thirdly, our findings are consistent with the strong biological basis that underlies the benefits of bariatric surgery [31]. Indeed, the weight loss response to RYGBP is known to be highly heritable [32], suggesting that patients’ responses to bariatric surgery may, to a large extent, be predetermined by their genotype. Genome-wide association studies have demonstrated associations of common genetic variants with 1-year weight loss response in patients after RYGBP [30, 33]. However, personalized medicine, like in many clinical specialties, has thus far had limited clinical impact, if any, in the field of bariatric surgery [34]. In this light, our findings suggest that an individual’s maximal weight loss response to SG or RYGBP may be most practically predicted by tracking their actual weight change in the early postoperative period.
A potential limitation of our study is the focus on maximal weight loss. However, this is clearly an important outcome for patients who undergo bariatric surgery [26]. Moreover, weight regain subsequent to the maximal weight loss achieved is likely to reflect a completely different biological process from that governing the initial weight loss. Such weight regain is subject to a multitude of biological, psychological, and environmental influences, and remains poorly defined [35]. In order to address a definable research question, we focused on maximal weight loss response and not subsequent weight change. Prediction of late weight regain after bariatric surgery may be equally important; however, maximization of the initial weight loss response is clearly central to optimizing long-term outcome [8, 36]. Interestingly, we found that in addition to being associated with maximal %WL, early postoperative WLV was also associated with 2-year %WL. These findings suggest that early WLV predicts longer-term postoperative weight loss outcomes but longer-term studies are required to confirm this. Another interesting question not addressed by our study is whether early postoperative weight loss also predicts resolution of comorbidities. However, the benefits of bariatric surgery in ameliorating obesity-related comorbidities are in proportion to the weight loss achieved [36]. Finally, a further potential limitation is the difference in baseline patient characteristics between bariatric centres. In particular, the population of patients who underwent SG in the UHP centre was significantly different from SG patients in the UCLH centre (smaller sample size and older age). This difference may, in turn, have contributed to the finding that centre predicted maximal weight loss in the SG group but not the RYGBP group. This limitation was unlikely to affect the robust association of early postoperative weight loss with maximal weight loss, and we believe the study benefits from its multicentre dimension.