Prevalence of Obesity, Comorbidities, and Eligibility for Pre-Transplant Bariatric Surgery in 275 Patients Awaiting Kidney Transplant at a Single Center in Germany

Introduction

Obesity is a major global health concern and is one of the leading causes of cardiovascular disease, diabetes mellitus, and, consequently, chronic kidney disease (CKD). It is also associated with increased overall mortality [1]. Body mass index (BMI, kg/m2) remains the standard measure for classifying overweight and obesity, even among patients with chronic renal failure [2]. Since 1975, the global prevalence of obesity has increased substantially [3], and in Germany it has been rising since the early 1990s [4].

Obesity contributes to CKD through mechanisms including diabetes, hypertension, and cardiovascular disease [5]. Worldwide, CKD affects 8% to 16% of the total population [6]. In Germany, diabetes mellitus and hypertension are strongly associated with CKD [7]. In 2017, 87 255 patients were receiving dialysis, and this number is projected to exceed 120 000 by 2040 [8]. Kidney transplantation remains the treatment of choice for end-stage kidney disease, offering improved quality of life, reduced mortality, and cost-effectiveness compared to dialysis [9,10]. Transplants can be performed with organs from living or deceased donors [11]. In 2019, 2132 kidney transplants were performed in Germany (76% post-mortem, 24% living donors), whereas 2449 patients were newly listed, resulting in an average waiting time of 5 to 6 years [12].

In the transplant setting, obesity is clinically relevant because it can delay listing, prolong waiting time, and increase perioperative and post-transplant risks such as delayed graft function, surgical complications, and graft loss [13]. Recent evidence confirms these risks: a 2022 cohort study of 578 kidney transplant recipients demonstrated significantly higher rates of postoperative complications and adverse long-term outcomes among obese recipients [14].

Despite these implications, the prevalence and characteristics of obesity among listed transplant candidates in Germany are not well described. A 2022 national survey of German transplant centers showed that nearly 70% of centers apply explicit BMI-based listing thresholds, most commonly BMI ≥35 kg/m2, and more than 90% consider bariatric surgery an acceptable option in transplant candidates [2]. This highlights a clear clinical need to understand how many patients on the waiting list fall into these BMI categories.

First-line therapy consists of lifestyle modifications such as diet and physical activity [15]. When these fail, bariatric surgery is considered for patients with BMI >40 kg/m2 or BMI ≥35 kg/m2 in the presence of comorbidities such as type 2 diabetes mellitus [15,16]. Evidence from 2024 further shows that sleeve gastrectomy can substantially increase the likelihood of listing and subsequent kidney transplantation in candidates with severe obesity [17]. These findings underscore that evaluating eligibility for bariatric interventions is clinically meaningful, as obesity can constitute a modifiable barrier to transplantation and reduce access to organ offers.

Therefore, this study aimed to determine the prevalence of obesity among listed kidney transplant candidates at a German transplant center, to evaluate its associations with demographic and clinical factors, including common comorbidities and dialysis duration, and to estimate the proportion of candidates meeting established eligibility criteria for bariatric surgery. By addressing these aspects, the study provides needed data on obesity-related barriers within the German transplant population.

Material and Methods

STUDY DESIGN AND ETHICAL APPROVAL:

This retrospective study was conducted at the Frankfurt University Hospital and Clinics. Ethics approval was obtained from the Ethics Committee of Goethe University Frankfurt/Main, Germany (registration number: 2022-984).

PATIENTS:

All adult patients listed for kidney transplantation at the University Hospital Frankfurt/Main between November 2018 and January 2020 were included (n=275). Pediatric candidates (<18 years) were excluded.

For prevalence and BMI-based analyses, only patients with complete height and weight data allowing calculation of BMI were included (n=271). A total of 4 patients were excluded from BMI-based analyses because BMI could not be calculated due to missing or inconsistent height or weight entries (eg, incomplete measurements, self-reported values without verification, or implausible values). No self-reported BMI values were used.

All analyses were conducted as complete-case analyses; the number of missing values for major variables was recorded explicitly. No imputation procedures were applied.

DATA COLLECTION AND VARIABLES:

Demographic and clinicopathological data were extracted from hospital records, including age, sex, BMI, and comorbidities (diabetes mellitus, arterial hypertension, and coronary heart disease). BMI values were taken exclusively from objectively measured height and weight at routine transplant evaluations; if multiple measurements were available, the value closest to the date of list-status assessment was used.

The primary renal disease leading to end-stage kidney failure was recorded and categorized as diabetic nephropathy, cystic kidney disease, hypertensive nephropathy, glomerulonephritis, reflux nephropathy, or other/unknown causes. Dialysis duration was calculated as the total number of days on dialysis; temporary interruptions (eg, after failed transplant) were summed, with each interruption identified from hospital records and measured in days. Interruption periods were identified through dialysis logs and medical documentation and quantified in days using the formula:

total dialysis days=(sum of all dialysis periods in days)-(sum of all documented interruption periods in days).

Transplant list status was recorded as transplantable (T), temporarily not transplantable (NT-temp), or permanently not transplantable (NT-perm). BMI was classified according to World Health Organization (WHO) criteria: normal (<25 kg/m²), overweight (25–29.9), obesity class I (30–34.9), class II (35–39.9), class III (≥40). Bariatric surgery eligibility was defined as BMI ≥40 or BMI ≥35 with comorbid diabetes mellitus.

STUDY ENDPOINTS:

The primary endpoint was the prevalence of obesity among patients on the transplant list, reported as point estimates with 95% confidence intervals. Secondary endpoints included correlations between BMI and comorbidities, underlying kidney disease, dialysis duration, and transplant list status.

STATISTICAL ANALYSIS:

Statistical analyses were performed using International Business Machines Corporation Statistical Package for Social Science (IBM SPSS version 29.0; IBM, Chicago, IL, USA). Normality of metric variables was assessed using the Shapiro-Wilk and Kolmogorov-Smirnov tests. Categorical variables are described in frequencies and percentages. Continuous variables are presented as mean and its standard deviation (SD). The Eta coefficient (η) was used to assess associations between continuous variables (eg, BMI) and categorical variables (eg, presence of comorbidities or transplant list status). Unlike the chi-square test, which only tests for independence between categories, the Eta coefficient quantifies the proportion of variance in the continuous variable that can be attributed to differences between categories. This provides a measure of effect size, allowing assessment of both the presence and the strength of an association. Eta values range from 0 (no association) to 1 (perfect association), facilitating interpretation in a clinical context. For associations between 2 continuous variables, Spearman’s rank correlation (ρ) was used and is given together with the P value. A P value <0.05 was considered statistically significant.

For analyzing differences between BMI <30 and BMI ≥30, categorical variables were compared by the chi-squared test. Continuous variables were compared using a one-way analysis of variance (ANOVA). A P value <0.05 was considered statistically significant. Given the exploratory design of the study, no multivariable regression models were applied; this limitation is acknowledged in the Discussion section.

Results

PATIENT DEMOGRAPHICS:

The study cohort comprised 275 patients (entire cohort), with a median age of 55 years (SD, ±13, range 21–88). Ten patients were listed for transplantation but were not yet on dialysis, as they were planned for preemptive transplantation. The most common causes of end-stage kidney disease were glomerulonephritis (27.4%), diabetes mellitus (15.5%), and polycystic kidney disease (14.1%). Hypertensive nephropathy was present in 7.6%, reflux nephropathy in 2.5%. and approximately 32.4% had other/unknown causes (Figure 1). Regarding transplant eligibility, 72.3% of patients were listed as transplantable (T), 13.5% as temporarily not transplantable (NT-temp), and 14.2% as permanently not transplantable (NT-perm). Mean time on dialysis was 2575 days (SD±1761; range 260-12080).

PREVALENCE OF OBESITY AND COMORBIDITIES:

Among 271 listed patients with complete BMI data, the mean BMI was 25.9 kg/m2 (SD, ±4.99; range 16.53–43.80). 79.3% had BMI <30 (n=215), and 20.7% (n=56) were obese: 30–34.9 (15.5%; n=42), 35–39.9 (4.1%; n=11), ≥40 (1.1%; n=3). Distribution of the BMI is shown in Figure 2. The prevalence of obesity therefore was 20.7% (95% CI, 1.16–1.25).

Among all listed patients, arterial hypertension was present in 264 patients (96.0%), diabetes mellitus in 73 patients (26.6%), and coronary heart disease in 49 (17.8%). Table 1 gives an overview of descriptive patients’ characteristics.

Patients with a BMI ≥30 had statistically significant more diabetes mellitus (P=0.003) and coronary heart disease (P=0.036) compared to patients with an BMI <30. For causes for end-stage kidney disease, only diabetic nephropathy differed significantly between the groups (P≤0.005). Detailed comparison between the groups is shown in Table 2.

CORRELATIONS AMONG VARIABLES:

No significant correlations were observed between BMI and time on dialysis (ρ=0.001; 95% CI, −0.124–0.126; P=0.987) or underlying kidney disease (η=0.169, P=0.172). A significant positive correlation was noted between BMI and age (ρ=0.248; 95% CI, 0.127–0.358; P<0.001), arterial hypertension (η=0.121; P=0.047), diabetes mellitus (η=0.305; P<0.001), and coronary heart disease (η=0.150; P=0.014). BMI was not associated with transplant list status (η=0.0516, P=0.704).

BARIATRIC SURGERY ELIGIBILITY:

Among the 56 obese patients, 10 (17.9%) met established criteria for bariatric surgery. This include 4 patients with BMI ≥40, and 6 patients with BMI 35–39.9 plus diabetes mellitus. One patient had already undergone sleeve gastrectomy prior to kidney transplantation.

Discussion

STRENGTHS AND LIMITATIONS:

The strengths of this study include the comprehensive evaluation of all patients listed at a single German transplantation center and the detailed assessment of comorbidities. However, the retrospective and monocentric design can cause some limitations. As this was a single-center analysis conducted in a German cohort, the results may not be readily generalizable to other countries or healthcare systems with different patient characteristics, referral patterns, or transplantation practices. Missing BMI data limited subgroup analyses, and reliance on BMI alone does not capture adiposity distribution or muscle mass, both of which may be clinically relevant in dialysis patients. In addition, we did not perform multivariate analyses to adjust for confounders such as age, sex, or dialysis duration. Finally, the relatively small number of obese patients restricts the statistical power of subgroup analyses.

Despite these limitations, our findings highlight the clinical relevance of obesity in a German kidney transplant population, where regional epidemiological patterns and waitlist management strategies may differ from international cohorts. Approximately 1 in 5 patients were obese, a proportion that is slightly lower than national estimates, and nearly 1 in 5 of these fulfilled criteria for bariatric surgery, suggesting opportunities for intervention that could optimize transplant readiness and outcomes. With obesity rates continuing to rise in Germany [4], the proportion of transplant candidates affected is likely to increase. Future prospective studies should investigate the impact of bariatric surgery and GLP-1 receptor agonists on waitlist dynamics, perioperative risk, and long-term graft survival.

Conclusions

In this single-center cohort study of kidney transplant candidates, obesity was present in 20.7% of patients, slightly lower than national averages, and was significantly associated with both hypertension and diabetes mellitus. Nearly 1 in 5 obese patients fulfilled established criteria for bariatric surgery. These findings should be interpreted considering the study’s limitations, including the single-center design, missing BMI data for some patients, partial reliance on self-reported weight, absence of adjusted analyses, and the relatively small number of obese individuals. Obesity is a potentially modifiable barrier to transplantation, and interventions such as bariatric surgery or GLP-1 receptor agonists may be considered in selected patients to reduce BMI and comorbidity burden. Prospective studies are needed to clarify the optimal timing and modality of weight reduction in this vulnerable patient population.