Lower nutrition knowledge in individuals with prior metabolic–bariatric surgery despite comparable hedonic hunger and body image: a cross-sectional study

Abstract

Aim

This cross-sectional study compared hedonic hunger, body image, and nutrition knowledge between individuals with prior metabolic–bariatric surgery (MBS) whose current BMI was 18.5–24.9 kg/m² and individuals with BMI 18.5–24.9 kg/m² without prior MBS.

Methods

Hedonic hunger (Power of Food Scale), body image (Body Image Scale), and nutrition knowledge (Nutrition Knowledge Scale) were assessed in 108 participants (54 with prior MBS, 54 without MBS). Group differences were examined using non-parametric tests and analysis of covariance adjusting for demographic and clinical variables.

Results

In unadjusted analyses, individuals with prior MBS demonstrated lower hedonic hunger, food taste sensitivity, positive body perception, and nutrition knowledge scores, and higher desire to change their body. However, after adjustment for covariates, group differences in hedonic hunger and body image were no longer statistically significant. In contrast, nutrition knowledge scores remained significantly lower in the MBS group (p < 0.001, partial η² = 0.111).

Conclusion

After adjustment for demographic and clinical covariates, differences in hedonic hunger and body image were attenuated and were no longer statistically significant, whereas nutrition knowledge remained lower in individuals with prior MBS. Given the cross-sectional design, causal inferences regarding the effects of surgery cannot be made.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40795-026-01355-8.

Introduction

Obesity is characterized by excessive adipose tissue accumulation influenced by genetic, environmental, biological, sociocultural, and behavioral factors [1, 2]. The World Health Organization recognizes the rapid increase in obesity prevalence across all age groups since the 1970s as a global epidemic [2]. Obesity is strongly associated with cardiovascular diseases, chronic obstructive pulmonary disease, type 2 diabetes, hypertension, osteoporosis, depression, and several types of cancer [3].

Metabolic–bariatric surgery (MBS) is considered the most effective long-term treatment for individuals living with obesity who do not achieve sufficient weight loss through lifestyle modification or pharmacological interventions [4]. In addition to sustained weight loss, MBS contributes to remission of obesity-related comorbidities and improved long-term outcomes [5]. Updated international guidelines indicate that MBS is recommended for individuals with BMI ≥ 35 kg/m², or BMI ≥ 30 kg/m² in the presence of metabolic disease such as type 2 diabetes [4].

Although health improvement is often the primary motivation for individuals undergoing MBS, appearance-related concerns and body image dissatisfaction have also been reported as contributing factors in treatment-seeking behavior [6]. Body image refers to the cognitive and emotional evaluation of one’s body size, shape, and appearance [7, 8]. Individuals living with obesity frequently report body dissatisfaction and psychosocial burden related to body perception [9–11].

Short- to mid-term studies generally report improvements in body image during the first 12–18 months following MBS [7, 12]. However, long-term findings appear heterogeneous, with some evidence suggesting partial decline in satisfaction over time, potentially influenced by excess skin, weight regain, and psychosocial adaptation [7, 9].

Hedonic hunger is defined as the drive to consume food for pleasure in the absence of physiological hunger [13, 14]. It reflects reward-related eating behavior and differs from homeostatic hunger. Elevated hedonic hunger has been associated with obesity and maladaptive eating patterns [15]. Longitudinal studies indicate that hedonic hunger tends to decrease during the first postoperative year following MBS [12, 16, 17], although long-term evidence remains limited. However, evidence specifically examining hedonic hunger in individuals who have achieved a BMI of 18.5–24.9 kg/m² following MBS remains scarce.

Nutritional knowledge plays a crucial role in preventing postoperative complications, supporting sustained weight loss, and improving metabolic outcomes after MBS [4]. Nutritional knowledge is considered a determinant of dietary behavior and dietary quality [18]. Higher nutritional knowledge has been associated with improved dietary quality and healthier eating patterns [18]. Although perioperative nutrition education is routinely provided, knowledge levels may not be consistently maintained over time without structured reinforcement [19, 20]. The Nutrition Knowledge Scale was developed and validated to assess nutritional knowledge in adult populations [21].

Although hedonic hunger, body image perception, and nutritional knowledge are frequently examined as separate constructs in the bariatric literature, these domains are conceptually interconnected. Hedonic hunger and reward-related eating behaviors have been shown to change following MBS and are associated with BMI and food responsiveness [16, 22]. Body image perception has also been linked to eating behavior patterns and weight-related psychological outcomes [9]. In parallel, nutritional knowledge influences dietary decision-making and dietary quality [18]. Taken together, these findings suggest that examining hedonic hunger, body image, and nutritional knowledge within the same framework may provide a more comprehensive understanding of behavioral and psychosocial outcomes following MBS.

However, most previous studies have focused on pre–post surgical comparisons or on individuals with persistent obesity after surgery. Data comparing individuals who have achieved a BMI within the range of 18.5–24.9 kg/m² following MBS with individuals within the same BMI range without surgical history in terms of hedonic hunger, body image, and nutritional knowledge remain scarce.

Therefore, this cross-sectional study aimed to compare hedonic hunger, body image perception, and nutrition knowledge between individuals with prior MBS whose current BMI was 18.5–24.9 kg/m² following MBS and individuals with BMI 18.5–24.9 kg/m² without prior surgery. Furthermore, we examined whether observed differences remained statistically significant after adjustment for potential demographic and clinical confounders.

Materials and methods

Study design and participants

This cross-sectional study was conducted using a face-to-face survey method between November 2024 and January 2025 in a private hospital located in the Gaziosmanpaşa district of Istanbul.

Sample size calculation was performed using G*Power (version 3.1.9.7) for an independent-samples t-test (two-tailed), assuming a medium effect size (Cohen’s d = 0.50), α = 0.05, and power = 0.95. The calculation was based on detecting a between-group difference in the Nutrition Knowledge Scale (NKS) total score, which was defined as the primary outcome of the study. This yielded a required sample of 54 participants per group (total n = 108).

Participants were divided into two groups: (1) individuals who underwent MBS and achieved a BMI of 18.5–24.9 kg/m², and (2) individuals without a history of MBS with a BMI of 18.5–24.9 kg/m².

BMI between 18.5 and 24.9 kg/m² was defined according to the World Health Organization classification criteria. Participants in the MBS group were required to be at least 12 months postoperative at the time of assessment (range: 12–48 months).

Individuals in the MBS group were recruited during their routine postoperative follow-up visits in the bariatric surgery clinic. All surgical participants had been operated on by the same bariatric surgeon and were followed postoperatively by the same bariatric dietitian team. Eligibility assessment and participant inclusion were conducted by the research team, which consisted of two bariatric dietitians.

Control participants were recruited from the general community through voluntary participation using social network announcements and word-of-mouth invitations. Individuals who expressed interest were screened by the research team to confirm eligibility criteria before inclusion. All questionnaires and anthropometric measurements were administered face-to-face under standardized conditions.

Adults aged 18–65 years who provided written informed consent were eligible. Individuals with cognitive impairment, a history of major psychiatric disorders, diagnosed eating disorders or pregnancy were excluded from the study. Participation was voluntary and conducted in accordance with the Declaration of Helsinki.

Data collection

Data collection was conducted face-to-face in a private consultation room to ensure privacy and minimize external distractions. All assessments were administered individually by trained bariatric dietitians. Before data collection, participants were informed about the study procedures and provided written informed consent. Participants completed the questionnaires independently after receiving standardized instructions. The research team reviewed the forms immediately after completion to prevent missing data. The entire assessment process lasted approximately 20–25 min per participant.

Data were collected using four instruments: (1) a General Information Form (2), the Turkish version of the Power of Food Scale (PFS-Tr) (3), the Body Image Scale (BIS), and (4) the Nutrition Knowledge Scale (NKS).

The General Information Form was developed by the research team to obtain sociodemographic and clinical characteristics, including age, sex, education level, income status, and postoperative duration (for the MBS group). Height was measured using a stadiometer and weight was measured using a calibrated digital scale. BMI was calculated as weight in kilograms divided by height in meters squared (kg/m²).

The Power of Food Scale-Turkish version (PFS-Tr) was originally developed by Cappelleri et al. [13] and validated in Turkish by Ülker et al. [15]. The scale consists of 15 items rated on a five-point Likert scale and includes three subdimensions: Food Available, Food Present, and Food Tasted. Higher scores indicate greater susceptibility to hedonic hunger.

The Body Image Scale (BIS), developed by Saylan and Soyyiğit [8], consists of 21 items across four subdimensions: Negative Body Perception, Evaluation Sensitivity, Positive Body Perception, and Body Change Orientation. Higher scores indicate greater body image dissatisfaction.

The Nutrition Knowledge Scale (NKS), developed and validated by Öngün Yılmaz et al. [21], consists of 31 items scored on a five-point Likert scale (0–4). The scale evaluates general nutritional knowledge, including food and nutrient information, food preparation methods, and the relationship between nutrition and health. Higher total scores indicate higher levels of nutritional knowledge.

Statistical evaluation of data

Data were analyzed using the Statistical Package for the Social Sciences (SPSS^® version 25.0) and Jamovi (version 2.6.44). Descriptive statistics were presented as median (interquartile range, Q1–Q3) for non-normally distributed continuous variables and frequency (%) for categorical variables. The normality of continuous variables was assessed using the Shapiro–Wilk test.

As most continuous variables did not meet the assumption of normality, non-parametric tests were used for unadjusted group comparisons. The Mann–Whitney U test was applied to examine differences between individuals with prior metabolic–bariatric surgery (MBS) and control participants. Effect sizes were calculated using rank-biserial correlation (r), which represents the magnitude of difference between two independent groups in non-parametric analyses. Rank-biserial correlation values were interpreted according to Cohen’s criteria (0.10 = small, 0.30 = medium, 0.50 = large).

Because baseline characteristics such as age and BMI differed significantly between groups, analysis of covariance (ANCOVA) was conducted to determine whether observed group differences remained statistically significant after adjusting for potential confounding variables (age, sex, BMI, education level, income status, and chronic disease presence). Although both groups were within the same BMI classification range, BMI was included as a covariate to account for variability within this range. Partial eta squared (ηp²) was reported as an effect size for ANCOVA models and interpreted as 0.01 = small, 0.06 = medium, and 0.14 = large. Statistical significance was set at p < 0.05.

Ethical approval

This study was approved by the Istanbul Atlas University Non-Interventional Scientific Research Ethics Committee (Approval No.: 54954; Meeting No.: 09; Approval date: 19 November 2024). All participants provided written informed consent prior to participation.

Results

Baseline characteristics

Baseline characteristics of the participants are presented in Table 1. The MBS group was significantly older than the control group (35.0 [27.0–41.8] vs. 24.0 [22.3–28.0] years, p < 0.001), demonstrating a large effect size (r = 0.54). BMI was also significantly higher in the MBS group (23.1 [21.5–24.2] vs. 21.6 [20.0–23.5] kg/m², p = 0.005), with a moderate effect size (r = 0.32).

Table 1.

General information about the participants

Variable	MBS Group (n = 54)	Control Group (n = 54)	p	Effect size
Age (years)	35 (27.0-41.8)	24 (22.3–28.0)	< 0.001	r = 0.54
BMI (kg/m2)	23.1 (21.5–24.2)	21.6 (20.0-23.5)	0.005	r = 0.32
Women n (%)	42 (77.8)	41 (75.9)	0.820	-
Married, n (%)	33 (61.1)	15 (27.8)	< 0.001	V = 0.34
High education, n (%)	28 (51.9)	43 (79.6)	0.002	V = 0.29
Working, n (%)	36 (66.7)	29 (53.7)	0.169	-
High income, n (%)	17 (31.5)	13 (24.1)	0.025	V = 0.26
Chronic illness (yes), n (%)	13 (24.1)	5 (9.3)	0.064	-
Drug use (yes), n (%)	14 (25.9)	6 (11.1)	0.048	V = 0.19
Supplement use (yes), n (%)	31 (57.4)	11 (20.4)	< 0.001	V = 0.38
MBS Group Characteristics	MBS (n = 54)
Sleeve gastrectomy, n (%)	49 (90.7)
Gastric band, n (%)	5 (9.3)
Postoperative duration (months)	17.0 (12.0–24.0)

Continuous variables are presented as median (Q1–Q3). Categorical variables are presented as n (%). Effect sizes are reported as rank-biserial correlation (r) for Mann–Whitney U tests and Cramer’s V for chi-square tests. Group comparisons without adjustment. BMI Body Mass Index. Bold values indicate statistically significant differences (p < 0.05)

Significant between-group differences were observed in marital status, education level, income level, drug use, and supplement use (all p < 0.05). Specifically, the MBS group included a higher proportion of married individuals and supplement users, whereas higher education was more frequent in the control group. No statistically significant differences were observed in sex distribution, employment status, or chronic disease prevalence.

Within the MBS group, 90.7% of participants had undergone sleeve gastrectomy and 9.3% gastric band surgery. The median postoperative duration was 17.0 (12.0–24.0) months.

Comparison of nutrition knowledge and related scale scores between groups

Initial between-group comparisons using the Mann–Whitney U test are presented in Table 2, and the distribution of scale scores is illustrated in Fig. 1.

Table 2.

Comparison of hedonic hunger, body image, and nutrition knowledge scores between MBS and control groups

	MBS Group	Control Group
	M (Q1-Q3)	M (Q1-Q3)	p	Effect size (r)
PFS-Tr Total Score	2.7 (2.4–3.2)	3.2 (2.6–3.5)	0.043	0.23
Food Availability	2.5 (2.0–3.0)	2.8 (2.0-3.3)	0.375	0.12
Food Present	2.8 (2.0-3.3)	2.9 (2.3–3.8)	0.061	0.17
Food Taste	3.0 (2.6–3.4)	3.6 (3.0–4.0)	0.005	0.31
BIS Total Score	49.0 (39.5–55.5)	46.0 (39.5–59.3)	0.931	0.01
Negative perception of the body	11.0 (10.0-15.3)	12.5 (8.0–16.0)	0.853	0.02
Body evaluation sensitivity	14.0 (8.0–17.0)	12.5 (10.0–19.0)	0.479	0.08
Positive perception of the body	10.0 (9.0–12.0)	11.5 (11.5–13.3)	0.044	0.22
Changing the size	11.0 (9.0–14.0)	10.0 (8.0–12.0)	0.028	-0.24
NKS Total Score	86.0 (79.8–90.0)	92.0 (80.8-105.5)	0.009	0.29

Values are presented as median (Q1–Q3). Between-group comparisons were performed using the Mann–Whitney U test. Effect sizes are reported as rank-biserial correlation (r). NKS Nutrition Knowledge Scale, PFS-Tr Power of Food Scale-Turkish, BIS Body Image Scale. Bold values indicate statistically significant differences (p < 0.05)

Fig. 1.

Comparisons of scale scores between MBS and control groups. Violin plots display the distribution of scores with embedded boxplots showing the median and interquartile range (IQR). Panel A Hedonic Hunger (PFS-Tr Total). Panel B Food Taste Sensitivity. Panel C Nutrition Knowledge (NKS Total). Panel D Positive Body Perception. Panel E Desire to Change Body. p < 0.05 indicates statistical significance; r = rank-biserial correlation (effect size)

The control group demonstrated significantly higher scores in hedonic hunger (PFS-Tr total), food taste sensitivity, positive body perception, and nutrition knowledge compared to the bariatric surgery group (all p < 0.05). Conversely, desire to change body size was significantly higher in the MBS group (p = 0.028, r = − 0.24). No other statistically significant differences were observed (p > 0.05).

Adjusted analyses (ANCOVA)

ANCOVA models were conducted to adjust for potential confounders, including age, sex, BMI, education level, income status, and chronic disease presence (Table 3).

Table 3.

ANCOVA results for scale scores adjusted for covariates

Outcome Variable	p (group)	partial η²	Significant covariates
PFS-Tr Total	0.466	0.006	Sex (p = 0.002)
Food Taste Sensitivity	0.168	0.020	Sex (p = 0.002)
Positive Body Perception	0.971	< 0.001	None
Desire to Change Body	0.100	0.029	Age (p = 0.002), BMI (p = 0.043)
NKS Total	< 0.001	0.111	Sex (p < 0.001), Income (p = 0.018), BMI (p = 0.017)

ANCOVA adjusted for age, sex, BMI, education level, income status, and chronic disease presence. Group effect refers to adjusted comparison between bariatric surgery and control groups. Bold values indicate statistically significant differences (p < 0.05)

After adjustment, between-group differences in hedonic hunger (p = 0.466, ηp² = 0.006), food taste sensitivity (p = 0.168, ηp² = 0.020), positive body perception (p = 0.971, ηp² < 0.001), and desire to change body size (p = 0.100, ηp² = 0.029) were no longer statistically significant.

However, the group difference in nutrition knowledge scores remained significant after adjustment (p < 0.001, ηp² = 0.111), indicating a medium effect size. In the adjusted model, sex (p < 0.001), income (p = 0.018), and BMI (p = 0.017) were independently associated with nutrition knowledge scores.

Dıscussıon

Primary findings of the study

Obesity remains a major global health concern, strongly associated with metabolic, cardiovascular, and psychosocial complications [2, 3]. MBS is currently recognized as the most effective treatment for sustained weight loss and improvement of obesity-related comorbidities [4, 5]. However, long-term behavioral, psychological, and educational aspects following surgery remain important areas of investigation.

The present study examined hedonic hunger, body image perception, and nutrition knowledge in individuals with prior MBS whose current BMI was 18.5–24.9 kg/m² and compared them with individuals within the same BMI range without surgical history. The main finding was that most initially observed between-group differences were no longer statistically significant after adjustment for demographic and clinical covariates, whereas the difference in nutrition knowledge persisted. These findings indicate that the psychological differences initially attributed to surgical status may primarily reflect underlying demographic variability.

In addition, although hedonic hunger, body image perception, and nutritional knowledge are conceptually interrelated, the relationships between these variables and whether these associations differ between groups were not examined in the present study. Future research may benefit from exploring these interactions to better understand the underlying behavioral mechanisms.

It should also be acknowledged that, in routine clinical practice, most individuals undergoing metabolic–bariatric surgery achieve an overweight BMI rather than a lean BMI at long-term follow-up. In contrast, the present study specifically included individuals whose current BMI was within the 18.5–24.9 kg/m² range. Therefore, our sample may represent a subgroup of patients who achieved greater weight reduction than typically reported in the literature. While this may limit generalizability, it also provides a unique opportunity to examine psychosocial and behavioral factors independent of persistent excess weight.

Hedonic hunger

Previous studies have consistently shown that hedonic hunger decreases during the early postoperative period following bariatric surgery [12, 17], although this reduction may attenuate over time [16]. In our unadjusted analyses, the control group demonstrated higher hedonic hunger and food taste sensitivity scores. However, these differences lost significance after adjustment.

Sex emerged as a significant covariate in the adjusted models. This finding aligns with previous literature demonstrating sex-related differences in food responsiveness and hedonic eating behavior. Women generally report higher levels of food craving and responsiveness to palatable foods compared with men [13, 14]. A systematic review by Hallam et al. [23] further highlights that gender differences in food craving and appetite regulation contribute substantially to variability in eating behavior. Therefore, the disappearance of the between-group difference after adjustment suggests that sex-related variability, rather than surgical history per se, may account for the initial differences observed.

Body image perception

Body image dissatisfaction is common in individuals with obesity and is strongly influenced by BMI and psychosocial factors [9, 11]. While MBS is generally associated with improvements in body image, the literature indicates that these improvements are not uniform across all domains [7]. Some individuals continue to experience dissatisfaction related to excess skin or residual appearance concerns despite substantial weight loss [24].

In our study, positive body perception scores were lower and desire to change body size was higher in the MBS group in unadjusted analyses. However, these differences were no longer statistically significant after controlling for age and BMI. This finding differs from several studies reporting substantial postoperative improvements in body image; however, differences in sample characteristics, time since surgery, and current BMI distribution may account for this discrepancy. These findings indicate that current BMI and demographic characteristics may play a more central role in body image perception than surgical status itself. Furthermore, we did not collect detailed information regarding body contouring procedures, which may influence postoperative body satisfaction and represent an important contextual factor.

Nutrition knowledge

Unlike hedonic hunger and body image measures, the difference in nutrition knowledge scores persisted even after adjustment for confounders. Previous studies have shown that although bariatric surgery patients often receive perioperative dietary counseling, long-term retention of structured nutrition knowledge may remain limited [19, 20]. Clinical practice guidelines emphasize the importance of ongoing nutritional education and monitoring after bariatric procedures [4].

Evidence from general populations suggests that higher nutrition knowledge is associated with healthier dietary patterns and better weight management behaviors [18]. However, in the present study, detailed data on postoperative nutrition consultation type, frequency, and duration were not systematically recorded. Therefore, while the persistence of lower nutrition knowledge in the MBS group may indicate a potential educational gap, causal interpretations should be made cautiously. Although the Nutrition Knowledge Scale provides categorization based on quartile-derived cut-off values, these thresholds are sample-specific and may not represent universally accepted clinical standards. Therefore, while the observed difference between groups was statistically significant, its clinical relevance should be interpreted with caution. However, the persistence of this difference after adjustment may still indicate a potentially meaningful gap in nutrition-related awareness.

In addition, although BMI values were within the normal range in many participants, weight trajectory and weight regain were not evaluated in this study; therefore, no direct inference can be made regarding the relationship between nutrition knowledge and weight outcomes in this sample.

Limitations

Several limitations should be acknowledged. First, the cross-sectional design precludes causal inference. Second, the sample was relatively small and recruited from a single center. Third, scale-based self-reported data may be subject to recall and social desirability bias. In addition, we did not systematically collect detailed information regarding postoperative nutrition counseling or previous body contouring procedures, both of which may influence body image and knowledge levels. Finally, although BMI values were within the normal range in many participants, weight history and weight regain data were not analyzed in this study.

Conclusion

After adjustment for demographic and clinical covariates, most initially observed differences in hedonic hunger and body image were no longer significant. In contrast, lower nutrition knowledge scores in the MBS group persisted independently of these factors. These findings highlight the potential importance of structured and sustained nutrition education as part of comprehensive long-term postoperative care.

Authors’ contributions

Study design: ABD, BNÖ,BG, DK, ETB, GÇ; Data collection: BNÖ,BG, DK, ETB, GÇ ; Data analysis: ABD, BNÖ,DK ; Draft preparation: ABD, BN; Critical review for content: ABD; Final approval of the version to be published: ABD, BNÖ,BG, DK, ETB, GÇ.

Funding

None.

Data availability

All data generated or analysed during this study are included in this published article and its supplementary information files. The analysis code (Jamovi project file and SPSS syntax) is provided in the supplementary files as well.

Declarations

Ethics approval and consent to participate

The study protocol was approved by the Istanbul Atlas University Non-Interventional Scientific Research Ethics Committee (Approval No.: 54954; Meeting No.: 09; Approval date: 19 November 2024). All procedures complied with the Declaration of Helsinki. Written informed consent was obtained from all participants prior to enrolment. Institutional permission from the hospital administration was also obtained before data collection.

Competing interests

The authors declare no competing interests.