**2. Definition of weight regain**

The general metrics to assess the success of the surgeries includes calculating % of excessive weight loss (EWL) (>50%), % of total weight loss (TWL), and % of weight regain (WR) post-surgery. Different studies have shown a large amount of variability within these values, which have been attributed to the type of surgery, the preoperative BMI, and to the race and ethnicity that the patients belong to [2–4, 10]. Literature review studies have revealed that only a limited number of them have looked at differences in weight loss patterns across different populations and specifically in the European population, where Bariatric/Metabolic procedures are performed routinely in nowadays. However, the sustained health improvements following bariatric surgery are dependent on the individual's adherence to long-term changes in lifestyle habits [11, 12]. As a result, despite its effectiveness, weight regain after bariatric surgery is a persistent problematic issue!

The first group of patients are those who do not lose the expected or anticipated average percentage of weight following surgery, while the second group are patients who lose a successful amount of weight after Bariatric/Metabolic Surgery (B/MS), but they regain some or most of the weight 5 or more years after the initial procedure [10].

According to several authors and publications about weight loss surgery, we must make a distinction between two types of WL failure post (B/MS). The first is known as insufficient WL (IWL). The second is known as weight regain (WR). IWL is defined as excess weight loss (EWL%) of <50% at 18 months after BS. Weight regain is defined as regain of weight that occurs few years after the Bariatric procedure and successful achievement of the initial weight loss. Literature review found several definitions for WR [13–15] as:


All those definitions describe one and the same problem: WR several years after B/MS. A robust review of the main causes, leading to that problem, is mandatory in order to answer to significant health and social issues about application of B/MS worldwide.
