Introduction
Low back pain is the number one cause of disability [
1]. It is also most commonly experienced among musculoskeletal pains for all age groups [
2,
3]. As a whole, musculoskeletal pains impact individuals’ diseases and functional status such as depression [
4], dementia [
5], falls [
6], and disability [
5]. Based on the systematic review of the prevalence of low back pain in the adult populations, the estimated one-year prevalence was 38.0% ± 19.4% and more likely to be higher in the older populations [
2].
Socioeconomic inequalities in health among older populations have emerged as a global concern [
7,
8]. Recent studies have reported that such inequalities were observed not only in diseases but also in symptoms, including musculoskeletal pains [
9‐
12]. Various studies reported socioeconomic inequalities in the risk factors of low back pain [
13‐
16] such as depression [
17], obesity [
18], and smoking [
18]. However, the results of previous studies on socioeconomic status (SES) and low back pain have been inconsistent. A recent large-scale cross-sectional study from the United States reported that the lowest income levels are significantly associated with low back pain compared with the highest income levels [
12]. On the other hand, another cross-sectional study from France reported that there was no association between educational attainment and low back pain [
19]. The difference in results might be explained by the different aspects of SES indicators; income is a proxy of the present SES and education is a proxy of the past SES. Seldom studies have investigated the associations between various SES factors and low back pain. Here, we conducted a cross-sectional study to determine the association of past and present SES with low back pain among older Japanese people.
Discussion
To the best of our knowledge, our study was the first to reveal the association of past and present SES with low back pain in the older population. We found that participants with low SES, as measured by education, past occupation, income, subjective economic situation, and wealth, were more prone to experience low back pain compared with those with high SES. Moreover, these results showed that there was a socioeconomic gradient in low back pain; people with lower socioeconomic background were more likely to suffer from pain. Therefore, low back pain is a problem for not only of the deprived people, but also a problem for the whole society. Expectedly, the associations of SES with low back pain dramatically attenuated when depression was adjusted for.
Regarding present SES, a cross-sectional study from the United States found lower-income levels to be associated with low back pain in the general population [
12]. This study also indicated that associations between income and low back pain were stronger among males than among females [
12]. The findings of our study are also in line with those of this cross-sectional study. We found that older individuals with a lower income level were more likely to suffer from low back pain. This association was strongly observed among older males.
We also newly elucidated the association between other present SES, as represented by wealth or subjective economic situation, and low back pain. Accordingly, we found that participants with a lower level of both wealth and subjective economic situation were more likely to experience low back pain, when separately analyzed.
Our further analyses which included all SES factors showed that the impact of more difficult subjective economic situation remained significant while the effects of other SES indicators were attenuated (see Table
3). Recently, subjective economic situation has been focused upon as a new SES indicator representing the perceived relative deprivation of individuals [
38,
39]. A cross-sectional study from Germany showed that subjective economic situation mediates associations between objective SES indicators (education, occupation, and income) and depressive symptoms in adults [
39]. Moreover, the study reported that the association of subjective economic situation with poor mental health was stronger than that of other SES indicators [
39]. Our findings have the same context with these results to show that the subjective economic situation had the largest impact. Furthermore, we revealed that present SES was found to be associated with low back pain among participants aged < 75 years as well as ≥75 years. This indicates that present SES-related inequalities persist throughout the life.
According to our understanding, this study is among the first to reveal the associations of past SES, as measured by educational attainment and past occupation, with low back pain among older individuals. We found that participants with the lowest educational level and blue-collared workers were more likely to suffer from low back pain. Furthermore, the association between education/occupation and low back pain was stronger among males than among females. For educational attainment, in contrast to our study, a cross-sectional study from France that interviewed labor population reported that the association of educational attainment with low back pain was no longer statistically significant when adjusting for several lifestyle indicators, including BMI and smoking [
19]. The difference in educational inequalities between studies might be explained as follows: educational inequalities affect health via health literacy [
40], and health literacy is significantly higher in labor generations compared with that in older generations [
41,
42]. Therefore, such differences between studies emerged due to demographic differences. No previous study has investigated the association of occupational inequalities with low back pain among older populations. However, numerous previous studies have indicated that heavy labor—a common issue faced by many blue-collared workers—is a risk factor of low back pain [
43‐
46]. Our study is in accordance with the results of these prior studies. Similar to present SES, associations of past SES attenuated when all status indicators were mutually adjusted (see Table
2, Model 4). Furthermore, the association of educational attainment with low back pain was also observed among participants aged < 75 years as well as ≥75 years, indicating that educational inequalities persist throughout the life.
When considering the mechanism of low back pain, the role of risk factors must be determined. Previous studies have indicated that depression [
13,
14], obesity [
15], smoking [
16], and lower-income level [
12] are risk factors of low back pain, which is partially in accordance with our findings. Consistent with the results of a previous study [
12], present SES as represented by income, subjective economic situation, and wealth were found to be statistically associated with low back pain among older adults. Two possible pathways exist for present SES-related inequalities in health: psychosocial stress and material poverty [
47]. Subjective economic situation is considered to be a result of income level and is considered to represent psychosocial stress rather than material poverty [
47,
48]. Moreover, individuals with lower income levels are more likely to face barriers in accessing medical care [
49]. In our study, among participants with low back pain, medical access to low back pain was significantly different by SES (see Additional file
1: Table S6). This indicated that barriers in accessing medical care would be a proxy for material poverty to account for socioeconomic inequalities in low back pain. A previous study indicated a mutual effect between depression and low back pain [
14]. Additionally, a causal relation between low SES and depression has been previously reported [
31,
32], which supports our idea of depression as an intermediary factor. In addition to depression, numerous earlier studies have reported obesity [
15] to be risk factors of low back pain. In our study, overweight and obesity were associated with low back pain. The associations of obesity somewhat attenuated when depression was additionally adjusted for. Previous studies have reported that such adverse health-related factors were strongly related to psychosocial stress [
38,
39], derived from relative deprivation. Therefore, in addition to depression, obesity might contribute to low back pain through psychosocial stress that is affected by SES. Furthermore, the association of drinking habit with low back pain was not statistically significant in our study. However, previous studies have indicated that alcohol abuse might be associated with low back pain [
24,
25]. We could not identify participants with alcohol abuse; however, alcohol abuse is associated with low SES [
50].
There are several strengths and limitations of our study. First, we examined the association of past and present SES with low back pain. Second, we analyzed a large sample size (
n = 26,037), which is higher than that analyzed in previous studies [
12,
19]. The first limitation of our study is that we were unable to distinguish between acute and chronic pain, which leads to regression dilution bias. In contrast to chronic pain, a previous study has shown that individuals with a higher income level were more likely to experience acute low back pain [
12]. Hence, we believe that our results are under-estimating the associations when considering such biases. Second, the pain questionnaire we used lacked information on degree of pain. There is a possibility that inequalities in low back pain might differ in degree of pain. In fact, in our sensitivity analysis, the associations were emphasized for all models when performing the same regression analysis among participants who experienced low back pain with limitations in daily life (see Additional file
1: Table S5). Future studies should include question about degree of pain. Third, we could not clarify the causal pathway because this is a cross-sectional study. Thus, the probable mediation by depressive conditions is not always consistent. However, we revealed that past SES and present SES were associated with low back pain. Longitudinal or cohort studies are necessary for future studies. Fourth, our study participants were not disabled and were not eligible for the Japanese long-term care insurance system. Future study is expected to investigate association between SES and low back pain among population including those of highly physically limited older people. Fifth, the generalizability of the present results to the entire Japanese population remains unclear. This is because the 30 municipalities investigated in this study were not randomly selected, and the sampling method for residents differed according to the population of the municipality. It was difficult to compare our study population with the entire older population due to lack of demographic characteristics in national survey.
Acknowledgements
The authors would like to thank all the respondents for their cooperation.
Funding
This study was supported by a grant of the Strategic Research Foundation Grant-aided Project for Private Universities from the Ministry of Education, Culture, Sport, Science, and Technology, Japan, 2009–2013, for the Center for Well-being and Society, Nihon Fukushi University, Grants-in-Aid for Scientific Research (22330172, 22390400, 22390400, 22592327, 23243070, 23590786, 23790710, 24390469, 24530698, 24653150, 24683018, 25253052, 25870573, 25870881, 26285138, 26882010, 15H04781, 15H01972, 16H05556, 16 K19267) from the Japan Society for the Promotion of Science. The study was also supported by a Health and Labour Sciences Research Grant, and grants for Comprehensive Research on Aging and Health (H22-Choju-Shitei-008, H24-Junkankitou- Ippan-007, H24-Chikyukibo-Ippan-009, H24-Choju- Wakate-009, H25-Kenki-Wakate-015, H25-Irryo-Shitei-003 [Fukkou], H26-Choju-Ippan-006, H27-Ninchisyou-Ippan-001, H28-Choju-Ippan-002) from the Ministry of Health, Labour and Welfare, Japan, the Research and Development Grants for Longevity Science from AMED (Japan Agency for Medical Research and development, the Personal Health Record (PHR) Utilization Project from AMED, World Health Organization Centre for Health Development (WHO Kobe Centre), (WHO APW 2017/713981), Japan Foundation for Aging and Health Research Support Grant, a grant from The Health Care Science Institute, as well as grants from National Center for Geriatrics and Gerontology. The research funding bodies had no role in the study design, data collection, data analysis, data interpretation, writing, or submitting of the report.