Pain is Adversely Related to Weight Loss Maintenance Following Bariatric Surgery

Abstract

Background:

Pain and obesity are frequently comorbid health conditions; thus, it is unsurprising that pain is commonly experienced by individuals seeking bariatric surgery. While pain is generally reduced in the short-term after surgery, there is also variability in pain outcomes and less is known about how unresolved or recurring pain may relate to long-term weight loss and weight loss maintenance.

Objectives:

This study evaluated trajectories of pain scores through seven years following bariatric surgery and whether higher pain levels related to poorer weight loss and greater weight regain.

Setting:

Data were collected from three university hospitals, one private not-for-profit research institute, and one community hospital.

Methods:

Self-report measures of pain and weight change data were utilized for 1702 adults seeking Roux-en-Y gastric bypass surgery from the Longitudinal Assessment for Bariatric Surgery (LABS) cohort. A series of linear mixed models examined trajectories of pain scores and the concurrent predictive relationship between pain and weight outcomes from pre-surgery through seven years post-surgery.

Results:

Overall bodily-, hip-, and knee-pain improved through two years, deteriorated from two to five years, and then slightly improved from five to seven years following surgery (p’s < .001). Greater pain was concurrently associated with less weight loss and greater weight regain over time (p’s ≤ .006).

Conclusions:

Pain is evident in the long-term following bariatric surgery and associated with suboptimal weight outcomes. More research is needed to identify mechanisms underlying this relationship, which may ultimately help develop appropriate pain assessment and treatment strategies to ensure optimal post-surgery outcomes.

It is well established that quality of life, a multi-dimensional construct consisting of physical, psychological, and social components,⁽¹⁾ is often impaired among individuals with obesity.^(2,3) Quality of life impairments among individuals with obesity may be due to their greater experience of pain, which can negatively impact every aspect of physical and mental health via functional impairment, disability, and psychological distress.⁽⁴⁾ Up to 40% of adults with obesity have chronic pain and specific pain conditions (e.g., low back pain, migraine) that occur at higher rates compared to those at a healthy weight.^(5–8) The prevalence, persistence, and intensity of pain also increase incrementally with degree of obesity, placing adults with severe obesity at highest risk for experiencing pain that is chronic, severe, and requires treatment.^(6,9,10) Putative mechanisms substantiate the association between pain and obesity and suggest that these conditions exacerbate each other.⁽⁵⁾ Research has demonstrated that obesity-related inflammation and mechanical joint loading often increase pain, whereas pain-related decreases in physical activity and use of energy-dense and palatable foods to soothe pain may promote further weight gain.^(11–15)

Understandably, improvement in quality of life and the alleviation of pain is often cited as a motivator for weight loss.⁽¹⁶⁾ Bariatric surgery, arguably the most effective method for substantial weight loss, has been shown to be a powerful pain reduction strategy among persons with severe obesity.^(17,18) Studies consistently show that general bodily pain and pain specific to certain conditions (e.g., osteoarthritis) are significantly reduced on average during the first few years following Roux-en-Y gastric bypass (RYGB) surgery.^(19–27) However, data from the Longitudinal Assessment of Bariatric Surgery (LABS) cohort show that the improvements in pain-related quality of life and physical functioning during the initial post-surgical year begin to deteriorate over the subsequent years.⁽²⁸⁾ Other data on change in pain past three years post-surgery have been mixed and limited by methodological issues such small sample size, restricted measurement of pain, or lack of longitudinal data.^(29–32) Thus, given the short-term pattern observed in the LABS study,⁽²⁸⁾ more information about long-term changes in pain is needed from large, longitudinal samples of bariatric surgery patients.

While improvements in pain corresponded with greater weight loss among the LABS cohort,⁽²⁸⁾ it is possible that deteriorations in pain over the long-term following bariatric surgery may negatively impact weight outcomes. This notion that pain may hinder weight loss and/or encourage weight regain is consistent with evidence from a 12-month lifestyle weight loss intervention among individuals with overweight and obesity: participants reporting significant levels of pain, regardless of the location or type of the pain, experienced poorer weight loss outcomes.⁽³³⁾ However, this study did not evaluate the relationship between pain and maintenance of weight loss. Information on long-term change in pain and weight outcomes is particularly important following bariatric surgery given the trend for increased weight regain and re-emergence of other medical comorbidities over time (e.g., diabetes, hypertension).⁽³⁴⁾ To our knowledge, no study has explored whether pain is adversely associated with long-term weight change and maintenance of weight loss following bariatric surgery.

This study used long-term (i.e., through seven years) data from the LABS cohort to characterize changes in pain and evaluate associations of pain with long-term weight loss and weight loss maintenance. It was hypothesized that: 1) pain-related quality of life, frequency of pain medication use, and self-reported pain in the knees and hips would remain lower than presurgical levels but increase throughout the seven years post-surgery; and 2) greater levels of pain would predict less concurrent weight loss and more concurrent weight regain from nadir weight.

Method

Participants and Procedure

The Longitudinal Assessment of Bariatric Surgery-2 (LABS-2) study includes 2,458 adults who sought bariatric surgery at one of 10 hospitals in the United States between the years 2006 and 2009.⁽³⁵⁾ Participants completed assessments 30 days prior to undergoing surgery, six months following surgery, and then annually thereafter through seven years post-surgery (with the exception of year six due to minimal data collection by study design). The LABS-2 protocol was approved by institutional review boards at each hospital, and participants provided written informed consent. The study is registered at https://clinicaltrials.gov/ct2/show/NCT00465829?term=NCT00465829&draw=2&rank=1.

Measures

Weight Outcomes.

Weight data were primarily collected using a standard Tanita Body Composition Analyzer scale (model TBF-310) at each in-person appointment. If such a scale was not available (i.e., due to the necessity of obtaining weight at an alternative site), weight was measured on a non-study scale. If neither scale measurement was available (e.g., the assessment was conducted over the phone), self-reported weight was used at that timepoint. A previous analysis of weights obtained in the LABS cohort revealed minimal differences between measured and self-reported weight.⁽³⁶⁾

Weight loss from surgery was calculated as percent of total body weight loss (%TBWL): [(baseline weight – current weight)/(baseline weight)]* 100. Higher scores indicate greater amounts of weight loss. Weight regain was calculated using the weight regain as a percentage of maximum weight loss (%MWL) formula: [100*(post-nadir weight – nadir weight)] / (pre-surgery weight – nadir weight).⁽³⁷⁾ Higher scores indicate greater amounts of weight regain. Nadir weight was determined using the multi-step process reported by King and colleagues.⁽³⁷⁾

Pain.

The Bodily Pain subscale of the 36-Item Short-Form Health Survey (SF-36)⁽³³⁾ was used as a measure of quality of life resulting from bodily pain. The subscale includes two items that assess magnitude of bodily pain and how that pain interfered with functioning over the past four weeks. Scores on the subscale range from 0 to 100, with higher scores indicating less pain.

Two subscales from the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) were used to assess pain in two specific joints of the body: hip(s) and knee(s). Each pain index was comprised of five items assessing level of pain that occurred during various activities over the past 48 hours. Scores on the hip and pain indices range from 0 to 20, with higher scores indicating greater pain for that specific bodily joint.

The number of days that participants used prescription or over-the-counter pain medication in the past week was utilized as secondary measures of pain specific to the hip(s) and knee(s), respectively.

Statistical Analysis

A series of linear mixed models were conducted to examine the trajectories of pain scores through seven years post-RYGB surgery. This approach of estimating trajectories in pain scores rather than conducting timepoint-by-timepoint comparisons was selected to 1) better capture the fluctuations in rate of change in pain over the course of the study, and 2) model such novel change over an extended period-of-time/follow-up. To estimate such non-linear trajectories (reflecting fluctuation of change), time (visit) was separated into linear, quadratic, and cubic components and entered as fixed effects into each model in addition to a random intercept. The linear time effect indicates whether the initial rate of change in pain increased or decreased following surgery. The quadratic time effect indicates a deflection point, highlighting an acceleration or deceleration in the initial rate of change in pain. Finally, the cubic time effect captures a possible second deflection point occurring most distal from surgery and further indicates acceleration or deceleration of the rate of change in pain through the long-term following surgery.

To explore whether pain impacted weight outcome, a series of linear mixed models were conducted using either weight loss (%TBWL) or weight regain (%MWL) as the dependent variable. Pain was disaggregated to reflect both between-person pain (e.g., those individuals with higher pain experience less weight loss) and within-person pain (e.g., on those occasions in which an individual experiences greater than average pain, they experience less weight loss). To calculate these variables, between-person pain was grand mean-centered and within-person pain was centered on the person-specific trend line following the recommendations of Curran and Bauer.⁽³⁹⁾ In addition to a random intercept, both between- and within-person pain were entered simultaneously as fixed effects into the model in order to test concurrent associations between pain and weight outcomes.

To address the issue of multiple comparisons, false discovery rate (FDR) procedures were conducted.⁽⁴⁰⁾ The FDR significance level (Q) was set to .10.⁽³⁵⁾ The original significance values are reported for all results; however, indication of statistical significance is based on the revised threshold produced by the FDR.

Results

Sample Characteristics and Attrition

Only data from individuals receiving RYGB surgery and who had at least one follow-up assessment in the LABS cohort were utilized, resulting in a sample size of 1702 participants. The sample was composed primarily of Caucasian (n = 1438, 84.5%) women (n = 1358, 79.8%). The average age of the sample was 44.82 years (SD = 11.06). Sample attrition and weight outcome statistics are presented in Supplemental Table 1.

Changes in Pain

Regarding trajectories of change in pain outcomes following surgery (see Supplemental Table 2), SF-36 bodily pain scores initially increased (i.e., improved) in the short-term following surgery. At approximately 24 months post-surgery, a deflection point was detected wherein bodily pain began to worsen (Figure 1a). A second deflection point was detected at approximately 60 months post-surgery, wherein bodily pain scores began to once again slightly improve. Trajectories for both the hip- and knee-pain subscales of the WOMAC followed a similar pattern: pain initially decreased following surgery, began to increase around 24 months post-surgery, and then began to once again slightly improve at approximately 60 months post-surgery (Figure 1b). Trajectories for the number of days in which patients took pain medication to treat a hip(s) and knee(s) were not statistically significant, indicating no meaningful change across the seven years post-surgery.

Figure 1.

Nonlinear trajectories of pain through seven years post-RYGB surgery. Figure 1a depicts the trajectory of scores on the Medical Outcomes Study 36-Item Short-Form Health Survey (SF-36) bodily pain subscales. Figure 1b depicts the score trajectories for the hip and knee subscales of the Western Ontario and McMaster Universities Arthritis Index (WOMAC).

Pain Predicting Weight Loss

Models using pain scores to predict weight outcomes showed that greater within-person pain generally predicted weight outcomes at any given visit (Table 1). Specifically, SF-36 bodily pain had a positive relationship with weight loss, suggesting that individuals who experienced greater pain evidenced less weight loss at that visit. A similar pattern was found for pain in the hip(s) and knee(s), such that higher pain scores on the WOMAC predicted concurrent decreases in weight loss. The number of days that an individual took pain medication to treat a knee(s) was also a significant predictor, with more days of medication use predicting less weight loss. Within-person days of hip pain medication use was not a significant predictor.

Table 1.

Pain Predicting Change in Weight Outcomes Following RYGB Surgery.

	Weight Loss (%TBWL)				Weight Regain (%MWL)
	n	Estimate	SE	p	n	Estimate	SE	p
SF-36 Bodily Pain	1683				952
Between		0.03	0.01	.006		−0.02	0.02	.398
Within		0.29	0.01	< .001		−0.09	0.01	< .001
WOMAC Hip Pain	650				396
Between		0.15	0.08	.079		−0.06	0.17	.731
Within		−0.59	0.06	< .001		0.40	0.10	< .001
Days of Hip Pain Medication	682				308
Between		−0.03	0.13	.801		0.08	0.21	.694
Within		−0.81	0.34	.020		−0.38	0.56	.498
WOMAC Knee Pain	838				497
Between		0.25	0.08	.001		0.09	0.15	.554
Within		−1.02	0.06	< .001		0.33	0.11	.002
Days of Knee Pain Medication	841				384
Between		0.08	0.14	.554		−0.06	0.27	.812
Within		−0.95	0.24	< .001		0.04	0.37	.923

Note: SF-36 = Medical Outcomes Study 36-Item Short-Form Health Survey. WOMAC = Western Ontario and McMaster Universities Arthritis Index. Bolded results indicate statistically significant findings after correcting for the false discovery rate (FDR).

Two between-person effects also emerged as significant predictors of weight loss (Table 1). Worse SF-36 bodily pain, as it occurred across all individuals in the study, was predictive of less weight loss. However, for the WOMAC knee pain subscale, worse knee pain at the between-person level was associated with greater weight loss. All remaining between-person effects were not significant predictors of weight loss.

Pain Predicting Weight Regain

Three significant within-person findings emerged for weight regain: worse pain on the SF-36 bodily pain subscale and the WOMAC hip- and knee-pain measures predicted greater weight regain at each assessment. The remaining within-person effects (e.g., days of pain medication use), as well as all between-person effects, were not significant predictors of weight regain.

Discussion

Past research has shown that pain, a key determinant of quality of life, is commonly experienced by bariatric surgery patients prior to surgery and generally markedly reduced after surgery.^(19–27) However, there is also variability in pain outcomes and less is known about how unresolved or recurring pain after bariatric surgery relates to long-term weight loss and weight loss maintenance. This study is the first to directly evaluate trajectories of pain scores through seven years following bariatric surgery and whether pain relates to poorer weight loss and greater amount of weight regained.

One main study finding is that scores on three of the five subjective pain measures showed a similar post-surgery trajectory from baseline: initial improvement (to two years post-surgery), deterioration of those initial improvements resulting in increased pain (from two to five years post-surgery), and then slight improvement once again (from five to seven years post-surgery). This pattern is not only consistent with that found by King and colleagues⁽²⁸⁾ in the short-term, but also builds on past data by more clearly showing when pain begins to reemerge (i.e., two years post-surgery). Additionally, these data suggest that pain, whether it be residual pain from before surgery or increases in pain occurring in the mid-term following surgery, may begin to slightly decrease in the long-term (i.e., five years post-surgery). This finding highlights the need for a greater focus on pain assessment in bariatric surgery patients and provides guidance on appropriate timing of pain management interventions to prevent the loss of pain improvements following surgery. Although reasons for the slight improvement in pain in the long-term (i.e., five years post-surgery) are not entirely clear, it is possible that the continued worsening of pain may have motivated patients to seek additional treatment for managing pain or that patients learned how to manage certain factors that contributed to pain. Finally, the lack of change in the number of days that an individual took pain medication to treat the hips and/or knees following surgery may be explained by the growing awareness of opioid addiction after surgery and subsequent increased attention towards swiftly addressing and treating opioid misuse among this population.

Another main study finding is that greater pain related to both poorer post-surgery weight loss and greater amount of weight regained in the long-term. To our knowledge, these data provide the first evidence to suggest that pain can hinder the ability to maintain weight loss after bariatric surgery. Specifically, increased pain relative to an individual’s own baseline (i.e., the within-person effect) appears to coincide with less-than-ideal weight outcomes. While individual differences in pain (i.e., the between-person effect) seem to impact weight to a lesser extent, the significant effects for overall bodily pain and knee pain suggest that some specific aspects of pain may be particularly salient predictors of weight loss and weight regain. Although this remains a novel and interesting hypothesis, it should be noted that other possible explanations for the relationship between pain and weight outcomes should be explored in future research. For example, empirical investigation of the overall domain of quality of life suggests that the degree to which quality of life changes post-surgery is related to the magnitude of weight change.⁽⁴¹⁾ Thus, instead of pain impacting weight outcomes, the amount of weight lost or regained over time may subsequently impact the experience of pain post-surgery.

Given the relative consistency of these findings between different aspects of pain and weight outcomes, additional research is warranted to identify mechanisms by which pain undermines weight loss and maintenance. While mechanisms of this relationship could be quite broad (e.g., surgical complications, relapse of medical comorbidities such as diabetes and arthritis), past research points to physical activity and eating behaviors as two particularly promising candidate mechanisms. Pain is related to lower physical activity levels among patients seeking bariatric surgery,^(42–45) which may in turn impact weight outcomes. Research also suggests that people with chronic pain, especially those with comorbid obesity, eat palatable and energy-dense foods to soothe pain,^(12,14,46) thus promoting weight gain. Better understanding the mechanistic roles of physical activity and eating in the relationship between pain and post-surgical weight loss could inform development of post-surgical interventions that target these behaviors in combination with standard pain management modalities to simultaneously improve pain and limit weight regain.

The results of this study should be considered within the context of certain limitations. First, we did not comprehensively examine all specific types of pain and pain medications following bariatric surgery. Additionally, comorbidities specific to pain and weight among bariatric surgery patients (e.g., age, chronicity of obesity, presurgical level of depression) were not considered due to the complexity of the analyses conducted. Future research should seek to examine pain more comprehensively as well as consider possible covariates that might impact the relationship between pain and weight outcomes. Second, some patients underwent hip (n = 140) or knee (n = 541) joint replacement surgeries or engaged in other forms of pain treatment (e.g., for neuropathies, ischemia in the limbs, or amputations) during the follow-up period, which could not be systematically assessed in the current analyses and may subsequently impact within-person results. Some participants were also lost to follow-up and sample sizes decreased over time. Yet, there were 1489 and 995 participants at 60 and 84 months, respectively, making this one of the largest samples to explore the relationship between pain and surgery outcomes. Retrospective questionnaires that rely on patients’ memory to assess pain were utilized, resulting in possible reporting inaccuracies and bias towards the worst pain experienced over the recall period.⁽⁴⁷⁾ Additionally, these retrospective measures cannot assess important temporal pain features such as duration and fluctuations in intensity. Future studies are needed that employ ecological momentary assessment methods to capture real-time data that reflect the dynamics of patients’ pain experience in daily life.⁽⁴⁸⁾ Finally, measures of pain sensitivity (e.g., quantitative sensory testing) were not included, which could provide needed insight into how bariatric surgery alters biological pain processing and if such alterations contribute to reported subjective pain experience.

In conclusion, continued or recurrent pain following bariatric surgery is both common and associated with suboptimal weight loss and weight regain. Research identifying mechanisms underlying persistent pain following surgery and the relationship between pain and post-surgical weight outcomes are needed. Data are also needed which help to identify patients at risk for continued pain following bariatric surgery so that treatment strategies can be developed and implemented to ensure optimal post-surgery outcomes.

Highlights.

• Pain is evident in the long term following bariatric surgery

• Trajectories of pain show varying levels of improvement and deterioration over time

• Greater pain is concurrently associated with less weight loss following surgery

• Greater pain is concurrently associated with greater weight regain