**Abstract**

Weight regain (WR) after bariatric surgery (BS) is emerging as a common clinical problem due to the increase in the number of procedures performed worldwide. Weight regain is defined as regain of weight that occurs few years after the bariatric procedure and successful achievement of the initial weight loss. Causes of WR following BS are multifactorial and can be categorized into two main groups: patient and surgical-specific causes. Several mechanisms contribute to WR following BS. These include hormonal mechanisms, nutritional non-adherence, physical inactivity, mental health causes, maladaptive eating, surgical techniques, and the selection criteria for the weight loss procedure. Higher preoperative BMI seems to be associated with WR and worse weight loss results in a long term. Patients with baseline BMI ≥ 50 kg/m<sup>2</sup> are more likely to have significant WR, while those with BMI < 50 are likely to continue losing weight at 12 months post-surgery. The aim of the chapter is to discuss and reveal all main factors, which may contribute to weight regain after bariatric surgery and emphasize how multifactorial assessment and long-term support/follow-up of patients by key medical professionals can diminish the side effects of weight regain.

**Keywords:** bariatric surgery, weight loss, weight regain, excessive weight loss, eating disorders, gastric bypass, sleeve gastrectomy, one anastomosis gastric bypass

### **1. Introduction**

There are several definitions of Obesity worldwide [1]. Interestingly, it is considered as a kind of malnutrition nowadays. Morbid Obesity is mostly a problem in highincome countries, according to statistical data with prevalence of countries in North America. However, overweight and obesity are a socially significant growing problem in low- and middle-income countries also. The estimated increase of Obesity among children is more than 30% higher in those countries than in developed countries in the last 10 years. The data confirm that 1.9 billion adults worldwide were overweight in 2016, with 650 million being Obese. People who are obese have much higher risks of many serious health problems than nonobese people [2, 3]. Obesity affects every system of the body. The results of outcome of bariatric surgery (BS) confirm the positive effects of surgery over such conditions as Diabetes type 2 (DT2), fatty liver disease, cancer. There is evidence for improvement of thyroid function, heart function, fertility, and sexual function in patients who have had weight loss surgery. More than

50 bariatric procedures have been proposed and implemented so far. As a result of time and trial, several procedures have been established as standards. The final goal of those procedures is for the Morbid Obese patient to achieve at least a loss of 50–70% of excess weight [EWL] or about 20–30% loss of his initial weight. Some authors consider a successful outcome, when achieving a Body Mass Index [BMI] < 35 kg/m<sup>2</sup> 2 years after surgery in those patients. However, the BMI as a criterion for a successful outcome after B/M surgery is under debate due to several reports that even patients with BMI of 32.0 can benefit from Metabolic surgery. So, we think that the quality benchmark for outcome of any M/B surgical procedure should be a combination of percentage of EWL in short and mid-term, extrapolated with percentage of WR in a long term—about 10 years after surgery. We support the suggestion of SOS study [4, 5] that patients should not regain more than 20–25% of their lost weight within 10 years after the primary procedure. Several studies confirm average weight regain of 12% of total body weight in patients who underwent Roux en Y Gastric bypass (RYGB), while those reported for Sleeve Gastrectomy (SG) were variable, ranging from 6% at as early as 2 years post-surgery to 76% at 6 years post-surgery [6–8]. Morbid Obesity, like other chronic diseases, persists for prolonged durations and requires a continuous close follow-up to reassess the efficacy of treatments, including Bariatric/Metabolic surgery. Most of the reports for very good and excellent results after weight loss surgery [WLS] are short or mid-long term up to 5 years' studies. Unfortunately, the studies, reporting results for more than 5 years after surgery, revealed a significant rate of WR in patients with Body Mass Index over 50 or history of comorbidity of more than 5 years [6, 9].
