The prevalence of people suffering from obesity is a globally growing health problem [1]. MBS represents the gold standard in treating severe obesity [4, 5]. However, MBS’ success has some shortcomings (e.g., weight regain) [7, 9].

The present study aimed to understand the relationship between adherence to dietary behavior recommendations and %TWL after MBS. Within 6 months after MBS, adherence seemed to peak, stabilizing at a lower level afterward. Adherence and time since MBS were associated with %TWL. In detail, a high adherence was related to a higher %TWL after MBS. Overall, adherence to dietary behavior recommendations seems crucial for maximizing MBS’ success permanently.

Firstly, this study investigated how adherence to dietary behavior recommendations has changed over time since MBS. Within 6 months after MBS, the adherence seemed to be at its peak. In the following months, it dropped, stabilizing at a lower level. Consequently, patients seem to adhere to their new dietary plan for a specific period but cannot permanently implement dietary behavior recommendations into their daily lives. According to previous research [8, 26,27,28], the first 12 months seem crucial for determining the MBS’ success over time. Willett [29] concluded that a healthy lifestyle, which is defined as a combination of a healthy diet, engaging in moderate activity, avoiding excess weight, and not smoking, is the most beneficial for one’s health—only for percent attained this healthy lifestyle [29]. Burgess et al. [30] also summarized that lifestyle change programs fail due to low adherence. Many patients might struggle to adhere to dietary behavior recommendations because there is no direct success. Sarwer et al. [13] described the potential reasons for negative outcomes in the first years after MBS. Although 20–30% of all patients are affected, the reasons for these negative outcomes have not been fully identified yet. If adherence is essential for MBS’ success, maximizing adherence and preventing its drop 6 months post-MBS should be aimed. This implies adapting the aftercare management more closely to the patients’ individual needs. MBS requires lifelong aftercare [4, 5], in which patients could receive more support and involvement (e.g., frequent follow-up appointments over a longer period, electronic lifestyle-tracking applications).

Secondly, this study investigated the role of adherence to dietary behavior recommendations on patients’ %TWL after MBS. High adherence was related to a higher %TWL. Several findings also implicate that maintaining one’s weight after a successful weight loss depends on high adherence to behavior recommendations post-MBS [12, 13, 31]. In a study with the majority of patients (> 80%) reporting weight regain after MBS (laparoscopic sleeve gastrectomy), the reasons for this development were reported [32]. Weight regain was justified by approximately half of these patients with non-adherence to the recommended guidelines [32]. In particular, lack of exercise, not meeting with a dietitian, and inefficiently controlling dietary behavior were reported as reasons [32]. Revisional MBS patients tended to have greater deficits and were less satisfied with the procedure [32]. According to their %TWL (< 25%), these patients might not be categorized as “good responders” to MBS [33]. The identified covariates in the present study are consistent with Herman et al. [31], who examined physical activity post-MBS, controlling for age, type of MBS, and time since MBS. Depression symptomatology is also relevant for adherence since obesity combined with appearance concerns, and depressive moods are associated with lower adherence (regarding physical activity) [34]. BMI pre-MBS was also a determinant of %TWL. Although only patients with obesity grade 3 took part in the present study, this finding was evident. Considering and comparing patients of all three obesity grades in the future might accelerate this finding.

In a systematic review, both the drivers and barriers to adherence were summarized [30]. The drivers identified were succeeding earlier in weight loss, having a lower BMI pre-MBS and better mood, being male and of older age, while lacking motivation, experiencing environmental, societal and social burdens, lack of time, having health limitations (mental or physical), having negative thoughts/moods, being socioeconomically restricted, knowledge/awareness gaps, and lacking joy in physical activity represented barriers to a successful change in lifestyle [30]. Special attention should also be paid to having negative moods (e.g., depression symptoms) and unrealistic %TWL expectations [30].

In the future, taking into account adherence, its drivers and barriers might emphasize optimization of the outcomes [30]. The findings of this study support and extend this conclusion. Considering and integrating the relationship between adherence to dietary behavior recommendations and %TWL after MBS into the aftercare of patients might sustainably optimize MBS’ outcomes.

Some limitations must be considered when interpreting the study results. Firstly, it was a cross-sectional study design. The results cannot be understood causally. Despite querying the adherence to dietary behavior recommendations and the BMI over time, this took place at a single point in time. Accordingly, recall biases cannot be ruled out. In addition, there was heterogeneity in the sample regarding the time since MBS. The statistical analyses established a causal model based on theoretical principles/previous findings. Nevertheless, it should be noted that neither correlation nor regression proves causality between constructs [35]. Instead, the present study provides preliminary work (including theoretically derived causality) that must be tested in a longitudinal design for causal conclusions in the future.

Furthermore, it was an online survey that had to be completed in self-report. Therefore, selection as well as self-report biases might occur. Although a web-based study is economical in terms of time and costs, it is uncertain whether and, if so, which patients did not take part in the study. In the future, a different survey medium could be chosen to increase the results’ generalizability. However, the number of Internet and social media users implies that the vast majority of the population has Internet access and uses social media [36]. The self-report was counteracted with anonymization. Despite the limitations associated with self-report, it offers the opportunity to survey patients despite inconsistent/differing aftercare and potentially varying aftercare attendance. Although self-report is a purely subjective measure, it also provides insights into aspects that cannot be objectively measured. In the future, it could be supplemented by objective measures (e.g., behavioral observation, aftercare appointments).

Furthermore, the gender distribution of the sample (90.5% female, 9.5% male) has to be considered. Due to the very high proportion of women, there is a bias compared to the “normal BMI > 40 distribution” [37]. Future studies should try to assess an even more representative sample. Additionally, ethnicity was not recorded. Regarding the sociodemographic variables of the final sample and the excluded participants, there was a significant difference in their occupational status. However, it is not believed that this had an impact on the present results.

Since this study did not distinguish between primary and revisional MBS, future research should address this. However, the comparison of long-term (> 5–15 years) outcomes between primary and revisional MBS (laparoscopic sleeve gastrectomy) showed no significant differences in weight loss or related medical outcomes [32]. It also seems particularly interesting whether the type of aftercare management correlates with or even influences adherence. However, which aftercare the participants received and used was not assessed. Since aftercare for MBS is not uniformly regulated, assessing it represents a challenge. Only patients until 3 years from MBS were included to avoid heterogeneous outcomes. In future studies, the influence of adherence to dietary behavior recommendations on %TWL even 3 years after MBS should be examined.

Since the DBI-S [17] is the first and comparatively new measure for assessing adherence to dietary behavior recommendations, there are no studies for further contextualization of the results in research. However, the present study is one of the first to lay the foundation for this.