Introduction: Mortality decreases following bariatric surgery. We explored the extent of the reduction and whether or not it reaches the general population level in a large cohort of patients with obesity. This study aimed to compare all-cause mortality between patients with obesity who undergo bariatric surgery and those who do not, with the general Iranian population during the same period. Method: Data from Iran’s National Obesity Surgery Database were used to establish a large cohort of patients registered between 2009 and 2019. The current vital status of the patients was determined by utilizing post-surgery follow-up data for those who underwent the operation. For patients without a surgery record, a predefined checklist was filled out through telephone interviews. Death data from the National General Registrar’s office were obtained for all cohort members. Results: Of 13,313 cohort members, 12,915 were eligible for analysis. The median age at the first visit was 38 years, and 78% were women. 6,190 patients (47.9%) underwent bariatric surgery, and 6,725 patients (52.1%) were not yet operated on at the time of analysis. We observed 139 deaths during 53,880 person-years follow-ups. The median follow-ups for operated-on and not operated-on groups were 4 and 4.8 years. The mortality rates among nonoperated patients were 2.89 times higher (standardized mortality ratio [SMR] = 2.89, 95% CI: 2.36–3.53) than those in the general population, while in operated patients, the mortality rate decreased to 1.82 as high (SMR = 1.82, 95% CI: 1.34–2.46). Conclusion: The risk of death has been diminished in the operated-on group. It still remains considerably higher than the risk in the general population.

Obesity has become a global public health concern in recent decades, leading to a rise in complex chronic diseases [1‒3]. The World Health Organization reported that 13% of adults worldwide were living with obesity in 2016 [4], while the prevalence of obesity and overweight/obesity among Iranian adults in 2015 was 22.7% and 59.3%, respectively [5].

Obesity is linked to various physical, mental, and psychosocial complications, as well as obesity-associated medical problems such as hypertension, dyslipidemia (DLP), nonalcoholic fatty liver disease, and type 2 diabetes, obstructive sleep apnea, anxiety, depression, and cancer [1, 3, 6‒8]. Additionally, studies have shown that living with overweight or obesity significantly increases the risk of all-cause mortality compared with normal-weight peers [6, 9‒16]. A systematic review showed that the risk ratio of death in people with severe obesity was 1.3–3 in the reference population (with normal body mass index). Sustained weight loss has been found to reduce the risk of chronic health problems and improve treatment outcomes [17]. To achieve significant and sustainable weight loss, bariatric surgery is considered the most effective method [18, 19]. Bariatric surgery not only decreases the risk of morbidity and mortality associated with metabolic syndrome [17, 19] but also helps in improving medical issues related to obesity [1, 8, 20‒22]. Consequently, the number of bariatric surgeries performed worldwide has been steadily increasing [23].

Although acute complications and mortality associated with bariatric surgery have been a concern [24], the introduction of laparoscopic techniques has significantly decreased the risk of death within 30 days after the procedure. Early mortality (30 days) after bariatric surgery is now a rare event, with a rate of <0.2% [25‒27]. Nguyen et al. reported inhospital mortality rate for bariatric surgery over 8 years from 0.4% per year in 2002 decreased to 0.06% in 2009 [28].

Previous studies have demonstrated the reduction of long-term all-cause mortality following bariatric surgery [20, 29], but many of these studies lack external validity and focus on short-term outcomes (1–3 years). Only a few studies provide long-term mortality results for a follow-up of 10 years or more [21, 30‒32]. In this study, our objective was to assess and compare the mortality rates between patients who underwent bariatric surgery and a control group that did not undergo bariatric surgery. Furthermore, we sought to contextualize these findings by comparing the mortality rates in these groups with those of the general population, shedding light on the independent impact of obesity on mortality.

This study utilized data from the Iran National Obesity Surgery Database, which served as a prospective cohort [33, 34]. The cohort consisted of patients registered by two specific surgeons between 2009 and 2019. To assess the current status of these patients (whether deceased or alive), we conducted a comprehensive follow-up. First, we identified individuals without a recorded operation or those who had missed their follow-up for over a year. We administered a predefined checklist containing the necessary information (Table 1) for the study through telephone interviews with these individuals. Additionally, we utilized post-surgery follow-up data for patients who had previously undergone the operation and had their most recent follow-up in a recent year. Moreover, we obtained supplementary data on deaths from the National General Registrar’s Office for all cohort members. Eventually, Iranian patients who were 20–75 years old at the first visit were analyzed.

Table 1.

Baseline characteristics of operated-on and not operated-on patients

CharacteristicPatients (N = 12,915)Operated-on (N = 6,190)Not operated-on (N = 6,725)p value1
Sex (male), n (%) 2,830 (22) 1,272 (21) 1,558 (23) <0.001 
Age at first visit, median (IQR) 38 (31–48) 37 (31–46) 40 (31–50) <0.001 
Weight at first visit, median (IQR) 113 (99–130) 117 (105–133) 108 (92–126) <0.001 
BMI at first visit, median (IQR) 42 (38–47) 44 (40–49) 40 (35–46) <0.001 
Follow-up time, years, median (IQR) 4.34 (2.8–6.2) 4.00 (2.7–5.7) 4.81 (2.9–6.6) <0.001 
Smoking, n (%)    <0.001 
 Nonsmoker 10,464 (83) 5,050 (83) 5,414 (83)  
 Ex-smoker 533 (4.2) 266 (4.4) 267 (4.1)  
 Rare smoker 622 (4.9) 335 (5.5) 287 (4.4)  
 Smoker 997 (7.9) 403 (6.7) 594 (9.1)  
Marriage status, n (%)    0.057 
 Divorced 493 (3.8) 225 (3.7) 268 (4.0)  
 Married 9,341 (73) 4,473 (73) 4,868 (73)  
 Single 2,731 (21) 1,346 (22) 1,385 (21)  
 Widow 286 (2.2) 118 (1.9) 168 (2.5)  
Education, n (%)    <0.001 
 Illiterate 171 (1.3) 45 (0.7) 126 (1.9)  
 Precollege 7,622 (59) 3,597 (58) 4,025 (60)  
 College 5,032 (39) 2,521 (41) 2,511 (38)  
History of T2DM, n (%) 2,595 (20) 1,341 (22) 1,254 (19) <0.001 
History of DLP, n (%) 5,086 (39) 2,806 (45) 2,280 (34) <0.001 
History of HTN, n (%) 2,869 (22) 1,293 (21) 1,576 (23) 0.002 
History of CVD, n (%) 545 (4.2) 192 (3.1) 353 (5.2) <0.001 
History of CVA, n (%) 26 (0.2) 6 (<0.1) 20 (0.3) 0.010 
History of DVT/PE, n (%) 78 (0.6) 43 (0.7) 35 (0.5) 0.19 
History of low back pain, n (%) 6,955 (54) 3,457 (56) 3,498 (52) <0.001 
History of knee pain, n (%) 6,979 (54) 3,417 (55) 3,562 (53) <0.001 
Death, n (%) 139 (1.1) 43 (0.7) 96 (1.4) <0.001 
CharacteristicPatients (N = 12,915)Operated-on (N = 6,190)Not operated-on (N = 6,725)p value1
Sex (male), n (%) 2,830 (22) 1,272 (21) 1,558 (23) <0.001 
Age at first visit, median (IQR) 38 (31–48) 37 (31–46) 40 (31–50) <0.001 
Weight at first visit, median (IQR) 113 (99–130) 117 (105–133) 108 (92–126) <0.001 
BMI at first visit, median (IQR) 42 (38–47) 44 (40–49) 40 (35–46) <0.001 
Follow-up time, years, median (IQR) 4.34 (2.8–6.2) 4.00 (2.7–5.7) 4.81 (2.9–6.6) <0.001 
Smoking, n (%)    <0.001 
 Nonsmoker 10,464 (83) 5,050 (83) 5,414 (83)  
 Ex-smoker 533 (4.2) 266 (4.4) 267 (4.1)  
 Rare smoker 622 (4.9) 335 (5.5) 287 (4.4)  
 Smoker 997 (7.9) 403 (6.7) 594 (9.1)  
Marriage status, n (%)    0.057 
 Divorced 493 (3.8) 225 (3.7) 268 (4.0)  
 Married 9,341 (73) 4,473 (73) 4,868 (73)  
 Single 2,731 (21) 1,346 (22) 1,385 (21)  
 Widow 286 (2.2) 118 (1.9) 168 (2.5)  
Education, n (%)    <0.001 
 Illiterate 171 (1.3) 45 (0.7) 126 (1.9)  
 Precollege 7,622 (59) 3,597 (58) 4,025 (60)  
 College 5,032 (39) 2,521 (41) 2,511 (38)  
History of T2DM, n (%) 2,595 (20) 1,341 (22) 1,254 (19) <0.001 
History of DLP, n (%) 5,086 (39) 2,806 (45) 2,280 (34) <0.001 
History of HTN, n (%) 2,869 (22) 1,293 (21) 1,576 (23) 0.002 
History of CVD, n (%) 545 (4.2) 192 (3.1) 353 (5.2) <0.001 
History of CVA, n (%) 26 (0.2) 6 (<0.1) 20 (0.3) 0.010 
History of DVT/PE, n (%) 78 (0.6) 43 (0.7) 35 (0.5) 0.19 
History of low back pain, n (%) 6,955 (54) 3,457 (56) 3,498 (52) <0.001 
History of knee pain, n (%) 6,979 (54) 3,417 (55) 3,562 (53) <0.001 
Death, n (%) 139 (1.1) 43 (0.7) 96 (1.4) <0.001 

IQR, interquartile range; T2DM, type 2 diabetes mellitus; DLP, dyslipidemia; HTN, hypertension; CVD, cardiovascular diseases; CVA, cerebrovascular accident; DVT/PE, deep vein thrombosis or pulmonary embolism.

1Pearson’s χ2 test; Wilcoxon rank sum test; Fisher’s exact test.

To compare the all-cause mortality of the cohort members with the general Iranian population during the same period, we calculated standardized mortality ratios (SMRs) stratified by age groups and sex. The mortality data for the general population were obtained from the Global Burden of Disease Report [35].

Statistics

We described parametric values using mean and standard deviation, non-parametric values using the median and interquartile range, and nominal variables using frequencies and percentages.

To calculate the observed mortality rate, we divided the number of deaths by the number of person-years. We determined the expected number of deaths for each age and sex group by multiplying the crude death rate per 1,000 people in the general population by the corresponding number of participants in each group. We considered the age of patients at the last contact as the value of person-years.

In the regression model, we included the date of surgery for operated-on patients and the date of the initial visit for not operated-on patients as influential factors. We used the Kaplan-Meier estimate and log-rank test to describe the time to death and assess the difference between the two groups. Furthermore, we employed a Cox proportional hazard regression approach to assess the impact of the operation on the mortality hazard.

We considered a statistical significance level of less than 0.05 (p < 0.05) for all tests. We prepared and analyzed the data using Stata version 14.2.

We collected data from a total of 13,313 patients registered in the Iran National Obesity Surgery Database between 2009 and 2019, under the care of two surgeons. Among them, there were 9,311 patients who had not undergone bariatric surgery and had not returned for follow-up after their initial visits, as well as 864 patients who had undergone surgery but had not attended their last follow-up for over a year. During a 1-year period, we successfully conducted interviews with 6,509 patients (63.97%). Additionally, we obtained the current status of these patients through the National General Registrar’s Office, identifying 9,666 individuals (72.61%). By combining data from these three sources, we were able to ascertain information about the date of the last contact, current status, and bariatric surgery history, including surgeries performed at other centers. However, for 1,182 patients (8.88%), data were unavailable from these sources, except for the medical evaluations conducted during their initial visits (shown in Fig. 1).

Fig. 1.

Patients flowchart.

Fig. 1.

Patients flowchart.

Close modal

Ultimately, 12,915 patients were eligible for analysis, including 6,190 (47.92%) who had undergone bariatric surgery and 6,725 (52.07%) who had not. The minimum required data for individuals who did not undergo bariatric surgery and were completely unreachable were their age at the first visit. We considered the age at this visit as the survival time, and their status was censored. Unfortunately, 398 patients did not have a recorded age at their first or last interview. Therefore, we excluded them from the study. During the study, a total of 139 deaths were reported. The median follow-up duration for operated-on patients was 4 years, while for not operated-on patients, it was 4.8 years. The operated-on group had a median age at the initial visit that was 2 years lower than the not operated-on group. Furthermore, the operated-on group had a higher BMI at the first visit but a significantly lower BMI at the last visit than the not operated-on group. The operated-on group also had a higher prevalence of diabetes, DLP, and musculoskeletal problems at the first visit. In comparison, the prevalence of cardiovascular diseases, high blood pressure, and stroke was lower (Table 1).

Among the operated-on patients, the most common surgery was the one-anastomosis gastric bypass, accounting for 2,720 cases (45%). A total of 123 patients (2%) underwent a second surgery, and 4 patients (less than 0.1%) underwent a third surgery. Hospital readmission occurred in 272 patients (5.7%), and only 8 out of 45 deaths (17.8%) were recorded within the first 30 days after discharge (Table 2).

Table 2.

Surgical characteristics of operated patients

CharacteristicOperated patients (N = 6,190)
Death, n (%) 
 Dead in 30 days after surgery 8 (18.6) 
 Dead after 30 days after surgery 35 (81.4) 
Age at surgery, median (IQR) 38 (31–46) 
Times of operation, n (%) 
 1 6,063 (98) 
 2 123 (2.0) 
 3 4 (<0.1) 
Surgery type, n (%) 
 Gastric sleeve 1,333 (21.5) 
 OAGB 2,720 (44.0) 
 RYGB 1,777 (28.7) 
 Others 234 (3.8) 
 Unknown 126 (2.0) 
Readmission, n (%) 272 (5.7) 
Number of readmissions, n (%) 
 1 246 (5.2) 
 2 25 (0.5) 
 3 1 (<0.1) 
CharacteristicOperated patients (N = 6,190)
Death, n (%) 
 Dead in 30 days after surgery 8 (18.6) 
 Dead after 30 days after surgery 35 (81.4) 
Age at surgery, median (IQR) 38 (31–46) 
Times of operation, n (%) 
 1 6,063 (98) 
 2 123 (2.0) 
 3 4 (<0.1) 
Surgery type, n (%) 
 Gastric sleeve 1,333 (21.5) 
 OAGB 2,720 (44.0) 
 RYGB 1,777 (28.7) 
 Others 234 (3.8) 
 Unknown 126 (2.0) 
Readmission, n (%) 272 (5.7) 
Number of readmissions, n (%) 
 1 246 (5.2) 
 2 25 (0.5) 
 3 1 (<0.1) 

IQR, interquartile range; OAGB, one-anastomosis gastric bypass; RYGB, Roux-en-Y gastric bypass.

The SMR for all not operated-on patients with obesity was 2.89 (95% CI, 2.36–3.53), indicating that the death rate in nonoperated patients was about three times higher than the average ten-year death rate in the general population. While the SMR for operated-on patients was 1.82 (95% CI, 1.34–2.46), indicating a reduction to twice times death rate in the general population (Table 3).

Table 3.

SMR of operated-on and not operated-on patients

OperationSexPerson-timeObserved failuresExpected failuresSMR95% CIp value
Operated-on Female 18,049.45 31 10.34 3.00 2.11–4.26 0.1893 
Not operated-on  24,772.13 57 14.19 4.02 3.10–5.21  
Operated-on Male 4,318.83 11 6.45 1.71 0.94–3.08 0.0197 
Not operated-on  6,739.97 37 10.07 3.67 2.66–5.07  
Operated-on Both 22,368.28 42 23.11 1.82 1.34–2.46 0.0110 
Not operated-on 31,512.11 94 32.56 2.89 2.36–3.53 
OperationSexPerson-timeObserved failuresExpected failuresSMR95% CIp value
Operated-on Female 18,049.45 31 10.34 3.00 2.11–4.26 0.1893 
Not operated-on  24,772.13 57 14.19 4.02 3.10–5.21  
Operated-on Male 4,318.83 11 6.45 1.71 0.94–3.08 0.0197 
Not operated-on  6,739.97 37 10.07 3.67 2.66–5.07  
Operated-on Both 22,368.28 42 23.11 1.82 1.34–2.46 0.0110 
Not operated-on 31,512.11 94 32.56 2.89 2.36–3.53 

The survival probability of patients with obesity who underwent bariatric surgery is significantly higher than those who did not (p = 0.0003) (shown in Fig. 2). The outcomes from both univariate and multivariate Cox proportional hazard models are displayed in Table 4. As per these results, patients who did not undergo bariatric surgery exhibited a 64% higher hazard for mortality (95% CI, 1.14–2.37). Additionally, a higher BMI at the initial visit correlated with an increased risk of mortality, where each 1 kg/m2 increment raised the hazard by 1.07 (95% CI, 1.06–1.09).

Fig. 2.

Survival probability on the age of obese-operated and nonoperated patients.

Fig. 2.

Survival probability on the age of obese-operated and nonoperated patients.

Close modal
Table 4.

Univariable (unadjusted), multivariate (adjusted) mortality hazard ratio, Cox proportional hazard model

CharacteristicUnadjustedAdjusted
HR (95% CI)p valueHR (95% CI)p value
Operated-on 
 Yes —  —  
 No 1.41 (0.98–2.02) 0.064 1.64 (1.14–2.37) 0.008 
Sex 
 Male —  —  
 Female 0.33 (0.23–0.47) <0.001 0.38 (0.26–0.54) <0.001 
BMI at first visit 1.08 (1.07–1.10) <0.001 1.07 (1.06–1.09) <0.001 
History of T2DM 
 Yes —  —  
 No 0.57 (0.40–0.79) <0.001 0.56 (0.39–0.82) 0.003 
History of DLP 
 Yes —  —  
 No 1.44 (1.03–2.02) 0.033 1.44 (1.01–2.07) 0.044 
History of HTN 
 Yes —  —  
 No 0.86 (0.61–1.22) 0.40 0.97 (0.67–1.41) 0.88 
History of CVD 
 Yes —  —  
 No 1.31 (0.72–2.38) 0.38 1.44 (0.78–2.65) 0.24 
CharacteristicUnadjustedAdjusted
HR (95% CI)p valueHR (95% CI)p value
Operated-on 
 Yes —  —  
 No 1.41 (0.98–2.02) 0.064 1.64 (1.14–2.37) 0.008 
Sex 
 Male —  —  
 Female 0.33 (0.23–0.47) <0.001 0.38 (0.26–0.54) <0.001 
BMI at first visit 1.08 (1.07–1.10) <0.001 1.07 (1.06–1.09) <0.001 
History of T2DM 
 Yes —  —  
 No 0.57 (0.40–0.79) <0.001 0.56 (0.39–0.82) 0.003 
History of DLP 
 Yes —  —  
 No 1.44 (1.03–2.02) 0.033 1.44 (1.01–2.07) 0.044 
History of HTN 
 Yes —  —  
 No 0.86 (0.61–1.22) 0.40 0.97 (0.67–1.41) 0.88 
History of CVD 
 Yes —  —  
 No 1.31 (0.72–2.38) 0.38 1.44 (0.78–2.65) 0.24 

HR, hazard ratio; CI, confidence interval; T2DM, type 2 diabetes mellitus; DLP, dyslipidemia; HTN, hypertension; CVD, cardiovascular diseases.

Our study revealed that among the study population, 47.9% (6,190 out of 12,915 patients) belonged to the operated-on group, while 32.4% of all deaths (45 out of 139 deaths) occurred within this group. Interestingly, only 8 out of 45 deaths (17.8%) in the operated-on group were recorded within the first 30 days after discharge which can be reduced due to the advancement of surgical techniques. These deaths accounted for 5.8% of all deaths in the study population.

In comparison to a control group receiving noninvasive care, our large prospective cohort study demonstrated that the standardized death ratio among operated patients, as an invasive treatment approach, was lower, although still higher than the general population (SMR = 1.82 and SMR = 2.89 in the operated-on and not operated-on patients, respectively). A 10-center cohort study conducted by White et al. on patients with obesity undergoing bariatric surgery in the USA found no significant difference in standardized mortality compared to the general population at a 7-year follow-up (SMR = 1.02, 95% CI = 0.8–1.27). However, the study did observe significantly higher standardized death ratios within the first 30 days (SMR = 5.37, 95% CI = 1.46–13.76) and 5–7 years after surgery (SMR = 1.55, 95% CI = 1.06–2.17) [10]. These findings are consistent with our study.

A systematic review by Xu Zhou et al. reported an adjusted odds ratio for nonspecific death in morbidly patients with obesity (BMI >35) undergoing bariatric surgery compared to those who did not undergo surgery. The odds ratio of death in the operated-on group versus the not operated-on group was reported as 0.55 (95% CI: 0.46–0.65), aligning with our findings of the present study with a hazard ratio of 1.64 (95% CI: 1.14–2.37) for not operated-on group versus the operated-on group [20].

Alam et al. [12], in a study on UK hospitals’ data between 2009 and 2016, reported a 30-day post-discharge death rate of 0.08%, whereas earlier reports from 2000 to 2008, when approximately one-third of bariatric surgeries were performed as open surgeries, reported a rate of 0.3%. Another study conducted in the USA between 2005 and 2009, involving 6,118 patients, reported a 30-day post-bariatric surgery death rate of 0.3%. Similarly, a collaborative study at the University of Michigan involving 15,275 operated patients reported a rate of 0.14% [12]. Our study’s rate was 0.13% (8 out of 6,190 patients), consistent with previous findings.

In the Pontiroli et al. [30] study, the mortality hazard ratio in surgical and nonsurgical control patients, who were matched for age, sex, BMI, history of diabetes, and HTN, was 0.52 (95% C.I. 0.33–0.80, p = 0.003). Also. multivariable analysis of risk factors for mortality showed that the risk of mortality decreased in female patients who underwent surgery, while patients with a history of diabetes had an increased risk [30]. Our study shows like results, and reducing the difference in mortality ratio between the two groups can be attributed to higher BMI and prevalence of diabetes and DLP in the baseline characteristics of the group with surgery.

This study had several limitations. The main limitation was the uncertainty regarding the cause of death, as only some cases had recorded causes, resulting in reporting all-cause mortality. Furthermore, to avoid increasing the risk of type I error in subgroup analyses, the study did not conduct multiple comparisons in subgroup analyses. Another limitation was the difficulty in accessing the study population to determine their current status. However, we mitigated this limitation by extending phone calls and obtaining data from the National General Registrar’s Office. An advantage of our study is that the patients in the obesity clinic of Rasool Akram Hospital and the private department of its surgeons were referrals from across the country, making the study population representative of the population who is living with obesity in Iran.

Despite complications during and after bariatric surgery, the chances of mortality following the procedure are low. However, certain risk factors such as being male, having a higher BMI, and having a history of type 2 diabetes have been identified as factors impacting mortality. It is important to give special attention to these “at-risk” bariatric surgery patients before and after the surgery.

The authors extend their sincere thanks to all participants and National Obesity Surgery Database team, who prepared us very useful data.

This study was conducted following the Helsinki Declaration and that protocol was reviewed and approved by the Ethical Committee of the Iran University of Medical Sciences (Number: IR.IUMS.REC.1399.1394). The written informed consent was received from all patients at the time of the first registry in our database for any possible anonymous usage of their data.

The authors declare that they have no commercial associations that might be a conflict of interest in relation to this article.

This study has been supported by the Iran National Institute for Medical Research Development (NIMAD), Tehran, Iran (Grant No. 958713).

Solaymani-Dodaran M., Pazouki A., Setarehdan S.A., Mokhber S., Sheidaei A., and Abdolhosseini M.R. were responsible for the study design process and conception as well as writing and approving the final version of the manuscript. Solaymani-Dodaran M., Setarehdan S.A., and Mokhber S. conceived and designed the study and wrote the manuscript. Solaymani-Dodaran M., Setarehdan S.A., and Sheidaei A. analyzed and interpreted the data and wrote the results. Pazouki A. and Abdolhosseini M.R. as clinicians evaluated individuals as well as checked the quality control of data and provided critical revisions.

Additional Information

Seyed Amin Setarehdan and Somayeh Mokhber are co-first authors and contributed equally to this work.

The data supporting this study’s findings were available on request from the corresponding author from the Minimally Invasive Surgery Research Center, Rasool Akram Hospital. The data are not publicly available due to privacy reasons but are available from the corresponding author upon reasonable request in the form of a joint research project. The project protocol should be approved by MISRC’s scientific and ethical advisory board.

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