1. Introduction
Bariatric surgery is the most efficient therapy for generating long-term weight loss and decreasing mortality and comorbidity symptomatology in patients with grave obesity [
1,
2]. Therefore, the latest European and U.S. Guidelines for Obesity Management in adults recommended bariatric surgery according to BMI levels and associated obesity-related diseases [
3,
4]. As a result, the global number of patients receiving bariatric surgery has increased in the past years [
5]. Studies show that the chosen procedure might also play a significant role in long-term weight loss outcome and that different procedures may lead to different outcomes at different times [
6]. The most recent procedure is the Roux-en-Y gastric bypass, followed by sleeve gastrectomy and the one-anastomosis gastric bypass.
Nevertheless, a considerable amount of patients encounter long-term complications like limited weight loss outcome, weight regain over time, or negative psychological health consequences [
7,
8]. Empirical evidence suggests that, besides bariatric-surgery-induced changes in physiology, the long-term success of bariatric surgery strongly depends on the patients’ motivation to adhere to healthier dietary behavior [
9,
10,
11,
12]. As a consequence, the measurement of dietary behavior is an indicator for post-operative health outcome and long-term success of bariatric surgery and subsequent weight loss.
Dietary behavior can be described as a great variety of manners and attitudes towards nourishment and food intake [
13]. It not only consists of the volume, nutritional value, and energy of the consumed food, but also of concrete and typical diet-related habits associated with dietary intake [
13]. Engelmann and colleagues provided an overview of the different aspects regarding the assessment of dietary behavior in the general population and developed an instrument according to the guidelines of the World Health Organization [
14].
Problematically, to date, there is no instrument available assessing adherence to relevant dietary behavior guidelines for post-bariatric-surgery patients. The application of the already validated “General Dietary Behavior Inventory” on this specific patient group is not possible, as the current dietary guidelines for bariatric surgery patients diverge strongly from those for a normal population group [
13,
15,
16,
17,
18,
19,
20]. However, the assessment of dietary behavior after bariatric surgery is beneficial and important for bariatric surgery patients, as it might detect the indication of dietary and behavioral interventions, and thus prohibit the onset of problematic eating behaviors and negative long-term health outcomes [
21]. As the long-term success of bariatric surgery strongly depends on the adherence to healthy dietary behavior [
9,
10,
11,
12], it is on behalf of both clinicians and patients to evaluate and initiate (healthy) dietary behavior after bariatric surgery in accordance with international guidelines and recommendations. A standardized dietary behavior inventory for patients after bariatric surgery should be applied as a standard in the scope of post-surgery medical examination in order to ensure appropriate post-surgical aftercare and long-term success of the treatment.
Therefore, the aim of this study was the development and validation of a standardised questionnaire, which addresses dietary behavior in patients after bariatric surgery. The Dietary Behavior Inventory—Surgery (DBI-S) for patients after bariatric surgery was developed based on nutritional recommendations for adult after bariatric surgery patients by Sherf-Dagan and colleagues, recently published in Advances in Nutrition [
17], and on several medical guidelines published inter alia by the German Obesity Society (DAG) [
15,
16,
20,
22].
In their review, Sherf-Dagan and colleagues [
17] gathered ongoing evidence and expert opinions on peri- and post-operative nutritional care to enhance the long-term success of bariatric surgery and prevent negative health outcomes. The given recommendations were divided into pre-surgery diet and supplementation, post-surgery diet, eating-related behaviors and gastrointestinal symptoms, and lifelong vitamin and mineral supplementation, as well as dietary recommendations. It became obvious that the long-term success of bariatric surgery strongly depends on the adherence to specific dietary behavior guidelines [
17]. On this account, the DBI-S was designed under the conditions laid down in the given review by Sherf-Dagan and colleagues [
17]. In order to follow the strong empirical evidence summarized by Sherf-Dagan and colleagues, the DBI-S was constructed based on their aggregated nutrition recommendations for patients after bariatric surgery.
To sum up, this study targets the following objectives. First, the goal of this study was to develop an instrument assessing dietary behavior in post-operative bariatric surgery patients, which evaluates the adherence to post-surgery dietary recommendations summarized by Sherf-Dagan and colleagues [
17] and dietary behavior guidelines published inter alia by the DAG [
15,
16,
17,
20,
22]. Second, it was the goal to examine convergent and criterion validity of the instrument. Third, the instrument should discriminate between healthy and unhealthy dietary behavior according to the mentioned recommendations and guidelines.
The developed DBI-S for the assessment of post-surgery dietary behavior is intended to represent the first instrument to assess the highly relevant adherence to the recommendations for adult bariatric surgery patients by Sherf-Dagan and colleagues [
17] and several medical guidelines published inter alia by the DAG [
15,
16,
20,
22].
4. Discussion
The goal of the study was to develop and validate the first instrument to measure adherence to relevant recommendations [
17] and medical guidelines [
15,
16,
20,
22] for dietary behavior after bariatric surgery. To this end, the study aimed to confirm content, convergent, and criterion validity of the newly developed DBI-S. In addition, the study aimed to investigate whether the DBI-S can distinguish between rather unhealthy and healthy dietary behavior. To the best of our knowledge, this is the first study to pursue these goals in this context.
We were able to develop a theory-based instrument with confirmed content validity by strictly following the scientific recommendations for the development of such instruments [
23,
33]. Interdisciplinary experts reviewed the DBI-S in several review rounds. The economic DBI-S recognizes concrete and typical dietary behaviors and does not rely on the reconstruction of retrospective exceptional situations within the final 13 items. Furthermore, we were able to verify convergent as well as criterion validity by conducting several correlational analyses with measurements representing similar constructs and relevant health outcomes of dietary behavior. In addition, this study confirms the ability of the DBI-S to distinguish between rather unhealthy and healthy dietary behavior according to the named recommendations and guidelines in a sample of post-bariatric surgery patients.
The results implicate the important role of time since the bariatric surgery was conducted. Looking closely at the results of the performed analyses, the DBI-S cannot capture concrete dietary behavior in the group of participants who stated that the bariatric surgery took place less than six months ago. Considering the severe restrictions regarding nutritional aspects as well as dietary behavior shortly after a bariatric surgery, these results are valid in the context of clinical experience [
15,
16,
17,
20,
22]. Restrictions concerning nutritional aspects include the consumption of only liquids and mashed or puréed foods in the first two weeks after surgery, transitioning to very small portion sizes distributed over four to six small meals a day, a higher than average protein intake, and the supplementation of different vitamins and minerals [
3,
16,
17,
18,
20,
22]. From the results, it can be inferred that there is a period of around six months needed to see significant changes in dietary behavior, impulsivity, BMI, body weight, and attitude towards healthy food, which may be explained by food intolerances within the first month after surgery. To sum up, these results strengthen the validity of the instrument.
Therefore, the analyses were controlled for time since the bariatric surgery.
Supplementary Material Figure S1 illustrates these correlations and shows the relationships between the DBI-S score and each validation variable by the four groups of time since bariatric surgery. It is clearly shown that especially the weight loss success (represented by pre-/post-surgery body weight and pre-/post-surgery BMI difference) does not depend on the adherence to post-surgery nutrition recommendations for patients with less than six months passed since bariatric surgery (group 1). This pattern is completely different for the other groups (more than six months since bariatric surgery), concluding that the DBI-S is able to assess dietary behavior in patients who have had bariatric surgery at least six months ago, but not in patients in the first 6 months after bariatric surgery. This finding is valid from a clinical point of view considering the massive restrictions after bariatric surgery as well as likely food intolerances in this period [
17].
Furthermore, the correlation plots shown in
Supplementary Material Figure S1 indicate that, the more time elapsed since bariatric surgery, the more patients benefit from adherence to dietary guidelines and recommendations. These results can be underpinned by significant positive correlations between attitude towards healthy food and quality of life (only in regards to the DBI-S score-related validity test), and by significant negative correlations between impulsivity, pre-/post-surgery BMI difference, and pre-/post-surgery bodyweight difference, while these relations were controlled for time since bariatric surgery.
In consequence of the application of a very conservative Bonferroni-corrected significance level of
p = 0.01, the outcome ‘quality of life’ did not show a significant relation with the DBI-S cluster assignment (
Table 6). Considering the given
p-value of 0.013, it is nevertheless plausible to presume a given tendency towards a significant positive relation between time passed since bariatric surgery and quality of life. However, looking at the correlations analysis to verify criterion validity, quality of life is strongly correlated with the DBI-S score.
Considering the non-significant difference in the DBI-S score, it can be concluded that the DBI-S is able to assess the dietary behavior independently of the surgery method. Therefore, future trials investigating weight loss after surgery could include the DBI-S as an additional co-variable.
Looking at the methodical framework of the DBI-S, a formative measurement model was applied rather than a reflective model. Because dietary behavior reflects many different aspects, which are incoherent, the formative measurement model was indicated. Following, it is not possible to assess a factorial structure of the DBI-S, which is not common in the development of psychometric instruments.
This study represents important strengths. The developed DBI-S reflects the most important aspects of dietary behavior in the context of post-bariatric-surgery patients. The instrument is based on theoretical reliable recommendations and clinical guidelines [
15,
16,
20,
22]. Therefore, the development process followed a deductive procedure on the relevant theoretical and clinical information. To the best of our knowledge, this is the first instrument measuring dietary behavior in the clinical vulnerable post-bariatric-surgery patient group by assessing more than just nutrient intake. Furthermore, the DBI-S is an economic tool to use in clinical practice and research projects. The instrument compromises 13 items and can be completed in under 5 min. The easy to evaluate and to interpret DBI-S score (sum-score) gives clinicians and researchers an economic and reliable information source regarding the adherence to respective recommendations and guidelines. Based on the results of the DBI-S, clinicians will be able to derive interventions for a more adherent diet. The interpretation of the results can be conducted independently of the bariatric surgery method used. As our results indicate that the DBI-S is a change-sensitive instrument, clinicians and researchers will be able to assess potential changes in dietary behavior over time, and thus evaluate general changes as well as the efficacy of dietary behavior interventions.
When interpreting the results of our study, limitations should be considered. The cross-sectional study design used in this study does not allow to consider causality. Thus, the interpretation of the results does not allow causal interpretation, but shows association between the constructs. Future studies should include the DBI-S and implement a longitudinal study design. In this case, the prognostic validity of the DBI-S could be assessed. Furthermore, the sensitivity regarding changes (e.g., due to a dietary behavior intervention) could be examined. Because of the online-based data collection, a possible selection bias cannot be ruled out. Therefore, the recruitment method might led to a selection of people who were familiar with the Internet use. However, the data collection was additionally performed at the Obesity and Metabolic Surgery Center of Alfried Krupp Hospital, Essen, Germany. Therefore, not only participants’ recruitment via online channels was performed. The online data collection was solely performed in topic-related online platforms. The gender distribution highly in favour of female participants is a relevant limitation of the results. This unequal gender distribution makes it impossible to examine a gender effect. However, the gender distribution represents the clinical reality. In fact, the gender distribution in this sample is comparable to the gender distribution in other samples of patients obtaining bariatric surgery [
34]. All data were self-reported. Therefore, the possibility of biased reports should be considered. Looking at the DBI-S, methodological limitations arise. Because each item represents a specific aspect of dietary behavior, only complete DBI-S can be analyzed. Therefore, assessing the Cronbach’s alpha of the DBI-S, which represents the most common indicator for the internal consistency of a psychometric instrument, is not indicated. Unexpectedly, some of the items derived from the theory-based recommendations showed insufficient item characteristics and were excluded. Nevertheless, these items may be relevant in other sample compositions and should be considered in future studies.