Preoperative weight loss is postulated to help assess patient compliance and assist with patient selection. Requiring preoperative weight loss might identify patients who will comply better with the dietary restrictions after surgery. Losing weight in the weeks before surgery appears to decrease liver size, which in turn might lead to shorter operative times, lesser blood loss, lower rates of conversion from laparoscopic to open procedures, and fewer per-operative complications [6].

Because a preoperative weight loss requirement in the period immediately preceding surgery might potentially exclude some patients who would refuse to lose weight, it is critical to evaluate the evidence on whether preoperative weight loss leads to improved outcomes. Several studies have found a short-term benefit for preoperative weight loss in bariatric surgery patients as per a meta-analysis done by Masha et al. [7]. Mrad et al. found that preoperative weight loss correlated with postoperative weight loss in men but not in women. A significant short-term correlation was found at 3 months, but this effect had disappeared by 12 months [8]. Alami et al. also found significantly greater weight loss in the preoperative weight loss group at 3 months [9]. Alvarado et al. showed that an increase of 1 % in preoperative weight loss correlated with an increase of 1.8 % in the postoperative percentage excess weight loss (%EWL) at 1 year [10]. Carlin et al. found no correlation between the preoperative and postoperative %EWL at 12 months, controlling for initial BMI [11]. Alger-Mayer et al. found a positive correlation between the preoperative %EWL and post-operative %EWL at 3 years [12].

The above concept is controversial, in part because of questions of the safety of relatively rapid weight loss in an obese population before undergoing major surgery. The efficacy and justification of mandating patients to lose weight before bariatric surgery has also been challenged. The findings suggest that obese patients can lose 10 % of their excess body weight in the weeks before undergoing surgery without significant peri-operative risk. Several studies demonstrated a decreased operative time for patients who had lost weight preoperatively, although mostly this did not translate to a decreased length of stay or reduction in complication rates. Overall, it appears that preoperative weight loss results in greater total postoperative weight loss, when studies of low quality were excluded.

Several authors have sought to determine whether or not a patient’s preoperative body mass index (BMI) has any bearing on their likely outcome following surgery. The majority of those studies have concluded that while a higher preoperative BMI is associated with a greater absolute weight loss, when considered in terms of %EWL, this group tends to be worse [1, 13–17].

To date no meta-analysis of the studies specific to LRYGB has been published and consequently the significance of BMI at presentation can only be considered to be based on level 3 evidence at best. The discrepancy between absolute weight loss and %EWL has led some authors to argue that %EWL is not an appropriate measure of success in the higher initial BMI group. Percentage EWL is a relative measure that diminishes the significance of the absolute amount, i.e., kilogram of weight lost. The disparity between %EWL and absolute weight loss is also magnified by the length of post-operative follow-up. The relatively short lengths of follow-up (i.e., 12 months) of many studies may not allow sufficient time for patients with higher BMIs to shed sufficient weight to reach their weight nadir.

Several studies have looked at the subject of ethnicity to see whether racial background has an influence on the degree of EWL following LRYGB. All but one of the studies that have compared Caucasians with people of Afro-Caribbean descent have found a significantly greater degree of EWL in the investigated Caucasian populations [11, 18–23]. Why these racial differences exist is not currently known. Similarly, the degree to which ethnicity can be used as a prognostic indicator is also currently unquantified but there remains a strong suggestion that certain races do experience a more favorable outcome following LRYGB than others secondary to metabolic differences.

Another factor which has been shown to influence EWL following LRYGB is marital status. Whether this should be viewed as a modifiable factor is open to debate but the fact that unmarried patients achieved a higher degree of EWL after LRYGB when compared with their married counterparts is still worthy of consideration [20, 24].

The specific aspect of eating behavior that has attracted the most attention has been that of binge eating disorder (BED) in which a person experiences episodes of eating an objectively large amount of food in association with a subjective feeling of loss of control. A literature review in 1998 by Hsu et al. concluded that BED was associated with weight regain [25].