Obesity among U.S. Veterans is a significant healthcare concern [1, 2]. Bariatric surgery remains a clinically and cost-effective treatment of morbid obesity, yet is performed in only a small fraction of eligible individuals [3,4,5,6]. Roux-en-y gastric bypass (RYGB) is the criterion standard treatment for severe obesity and its co-morbid conditions, and is associated with durable, long-term outcomes [7].

In addition to overweight and obesity, the Veteran population is also at risk for poverty and low socioeconomic status (SES) [8]. The SES of a patient’s community, as measured by median income, is a quantifiable measure that may approximate local resources necessary to achieve and maintain long-term weight loss, such as access to healthy food [9], exercise [10], counseling, and health care. While SES, race, and gender have been shown to correlate with rates of utilization of bariatric surgery [6, 11, 12], the impact of SES on long-term weight-loss outcomes after bariatric surgery is unclear, and has not been evaluated in the Veteran population.

In this study, we examined the impact of SES on long-term weight loss after RYGB in the Veteran population. We hypothesized an inverse association between SES and weight-loss success at 10 years after RYGB.

Materials and methods

After Institutional Review Board (IRB) approval, we performed a retrospective review of a prospective database of bariatric surgery at a single Veterans Affairs (VA) Medical Center, located in a suburban setting. We identified all patients who underwent RYGB at our institution; those individuals who did not have weight data in the electronic health record for at least 10 years after bariatric surgery were considered lost to follow-up and excluded from the study.

Patient records were evaluated for 10-year postoperative weight and BMI. SES was estimated using approximate household income, which was estimated by linking zip codes of patient residences at the time of surgery to publicly available U.S. Census data on median household income per zip code [13]. Google Maps online map software was used to obtain direct radial distances from patient residences to the primary VA hospital.

Statistical analysis

Patient characteristics, including median household income, age, race, sex, and distance to the VA, were analyzed for relationships with percent excess BMI loss (%EBMIL). Continuous variable distributions were evaluated for %EBMIL, age, median household income, and distance to the VA. Frequency distributions were evaluated for gender and race. Median household income and distance to the VA bariatric surgical center were not normally distributed; non-parametric descriptive statistics are presented. Income was grouped by quartiles, and race was characterized as white versus non-white or unreported.

The primary predictor variable of interest was median household income. We assessed relationships between income and other patient characteristics with univariate comparisons. A Fisher’s exact test was performed to assess the relationship between income and race. A Chi-square test was calculated to evaluate the association between gender and income. A Kruskal–Wallis test was performed to evaluate the association between income and distance to the VA. Analysis of variance was utilized to compare %EBMIL by income quartile groups.

A multivariate generalized linear model was calculated to assess relationships between measured patient characteristics and the dependent variable, %EBMIL. The primary independent variable was median household income quartile; the referent group was defined as the lowest income quartile. Additional covariates were included in the model to statistically adjust for age, race, sex, and distance to the VA. An alpha level of 0.05 was defined for statistical significance. Statistical analyses were performed in SAS version 9.4 (SAS Institute Inc., Cary, NC, USA).

Results

Of 108 individuals who underwent RYGB at the study site from 2001 to 2007, a total of 16 patients (14.8%) died in the follow-up period. Among patients who were alive at 10 years, 9 (9.8%) were lost to follow-up. Thus, a total of 83 individuals (90.2%) were identified with complete follow-up data. The mean 10-year postoperative %EBMIL was 57.8% (SD: 29.5%, range − 36.0 to − 132.8%) (Table 1). The mean age was 51.1 years (SD: 8.7, range 28–65), 73.5% were white, and 79.5% were male. The median census-derived household income was $51,116 (interquartile range $40,021–$69,341), and median distance to the VA was 67.7 miles (interquartile range 56.7–387.4).

Table 1 Characteristics of patients who underwent Roux-en-Y Gastric Bypass at the study site from 2001 to 2007 and survived at least 10 years
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Income was significantly associated with race (p < 0.001) and median distance to the VA (p = 0.034) (Table 2). There were no significant differences in income by gender (p = 0.73) or age (p = 0.45). Univariate analysis demonstrated meaningful variation in %EBMIL by income, but the differences did not reach statistical significance (p = 0.16).

Table 2 Univariate relationships between household income (quartiles) and patient characteristics including weight loss
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The multivariate analysis revealed significantly lower 10-year %EBMIL for patients in the lowest income group compared to patients with low-mid income and mid-high income (Table 3). Compared to the lowest income group, patients with low-mid income had 18.6% more EBMIL (p = 0.034), and patients with mid-high income had 22.1% more EBMIL (p = 0.013) after controlling for gender, race, age, and distance to the VA bariatric surgical center (Fig. 1). The highest income group achieved 11.8% more weight loss than the lowest income group, but this difference was not statistically significant (p = 0.18). On average, patients residing farther away from the VA achieved less weight loss; however, these differences were not statistically significant.

Table 3 Multivariate generalized linear model results estimating associations between weight loss and patient characteristics including socioeconomic status
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Fig. 1
figure 1

Multivariate analysis of 10-year %EBMIL for patients in the lowest income group compared to patients with low-mid income, mid-high income, and high income, adjusted for age, race, and distance to the bariatric surgical center

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Discussion

In this study, we evaluated the impact of SES on long-term weight loss after bariatric surgery in the Veteran population. The Veteran population undergoing bariatric surgery is unique in the bariatric literature in that it is mostly male, and often resides a large distance from the bariatric center. We found that individuals residing in low-SES areas had significantly lower weight loss than low-mid and mid-high income patients, independent from gender, ethnicity, age, and distance from hospital. This relationship did not persist when using the low-mid quartile as reference, suggesting that if SES indeed has an impact on long-term weight loss after bariatric surgery, as this study suggests, it significantly affects only the low-SES population and dissipates in higher income groups.

Studies with short- and mid-term follow-up have failed to demonstrate different weight-loss outcomes in different socioeconomic groups. In a study of 309 patients, Akkary et al. found no significant impact of median income on weight loss, 1 year after gastric bypass [14]. Another study of 2-year outcomes after surgery found that weight loss was not significantly related to insurance status (used as a proxy for income) in a prospective study of 131 patients who underwent vertical banded gastroplasty [15]. While Gullick et al. suggested that SES, as reflected by race, may correlate with 5-year weight loss in bariatric surgery [16].

Our study results suggest that low regional income status may be a potential barrier to optimal weight-loss outcomes in the long-term. This may be a reflection of inadequate access to postoperative bariatric care, as has been suggested by other studies [17]. Further studies are needed to determine which aspects of the patient’s socioeconomic environment contribute to surgical outcomes. Our findings suggest that individuals with a low socioeconomic status may represent a group with unmet long-term needs after bariatric surgery, who could benefit from closer long-term follow-up and specialty bariatric care. Nonetheless, this group still had very good weight-loss outcomes, further suggesting the durability of the RYGB in all patients.

Physical distance has been studied as a potential barrier to health care delivery. A national retrospective cohort study in Canada using geographic cluster analysis showed that having a bariatric facility in the same public health unit as a residential neighborhood was associated with 6.6 times higher odds of being in a bariatric high-use cluster, suggesting that geographic distance to bariatric centers influences care delivery [18]. For this reason, we wanted to ensure that our findings were a reflection of median income, and not a reflection of the patient’s physical distance from our bariatric center. While our data did show a trend towards less weight loss for those patients living farther from the bariatric team, this trend did not contribute to the observed effect of SES on weight loss.

Factors affecting access to preoperative and postoperative bariatric are not yet fully understood, and their impact on long-term outcomes are not fully elucidated. Several other factors have previously been found to affect access to care, including income, education level, gender, rural environment, race, and insurance status. An open cohort of over 22,000 individuals that underwent bariatric surgery in Sweden showed that individuals from the lowest socioeconomic groups were less likely to undergo bariatric surgery than individuals with intermediate educational and income levels despite having the highest rates of morbid obesity [17]. A population-based study in Michigan categorized patients into SES groups using zip codes, and showed that rates of bariatric surgery were low among males regardless of race [19]. In a study using the 2006 Nationwide Inpatient Sample of 774,000 patients, rural patients were 23% less likely to receive bariatric surgery than urban patients [20]. Those rural patients that were also non-white, male, poorer, sicker, and non-privately insured rarely received bariatric surgery.

This study is limited by its retrospective design and concentration on a single VA center. As such, we do not have data to account for potential migration over the 10-year follow-up period. In addition, while patient zip codes have been used by others as markers of socioeconomic environment, they remain an approximation that may not reflect the individual patient, and do not account for the possibility of patient mobility. Furthermore, our Veteran patient population sampled in this study was majority male and white, which may impact weight-loss outcomes in general.

Our data suggest that bariatric surgery is effective for weight loss at 10-year follow-up in Veterans. However, a low-income environment is a risk factor for inferior long-term weight loss. Further study is needed in larger long-term bariatric cohorts to determine which factors related to low SES most significantly affect postoperative weight loss.

Conclusion

Low socioeconomic status is associated with lower weight-loss outcomes 10 years after gastric bypass, compared to weight loss in higher socioeconomic groups. This is independent of patient’s distance to bariatric center. Nonetheless, all groups demonstrate a durable weight loss in long-term follow-up after RYGB.