Abstract
Objective
This study aimed to examine the associations of overweight/obesity, physical function and mental function with back pain intensity and disability.
Design
A 14-year population-based study based on Australian Diabetes, Obesity and Lifestyle Study. Body mass index and the SF-36 Survey were used to determine baseline overweight/obesity and physical and mental status respectively. Back pain intensity and disability were assessed using the Chronic Pain Grade Questionnaire.
Results
Participants with overweight/obesity only had higher odds of high-intensity pain and high disability (OR:1.61 95%CI(1.28–2.02); 1.48 (1.12–1.97) respectively), but not low disability (OR:1.22, 95%CI 0.99–1.50), compared to those with neither overweight/obesity nor impaired physical function. However, those with overweight/obesity and impaired physical function had a 9.5 (95%CI 6.32–14.1) and 7.8 (95%CI 5.60–10.8) times higher odds of high-intensity pain and high disability respectively. Compared to people with normal weight and mental function, participants with overweight/obesity only had 1.5-times higher odds of both high-intensity pain and disability (95%CI(1.19–1.97); (1.12–1.99) respectively). Those with overweight/obesity and impaired mental function had 4.2-times (95%CI 2.97–5.93) higher odds of high-intensity pain and 2.9-times (95%CI 2.11–4.16) higher odds of high disability compared with participants without overweight/obesity or impaired mental function.
Conclusions
These findings highlight that among individuals with overweight or obesity significantly impaired physical and mental functions are important in identifying those at risk of having high-intensity back pain and high disability 14 years later. Examining changes in weight and physical and mental health over time is warranted and targeting these factors together may help reduce the huge burden of back pain in the community.
Keywords: Low back pain, Low back disability, Obesity, Overweight, Physical function, Mental function
1. Introduction
Low back pain is a common problem in the community and a leading cause of disability worldwide. According to the Global Burden of Disease Study, low back pain resulted in 64.9 million disability-adjusted life-years in 2017 [1]. The health care expenditure for managing low back pain is high, with approximately AUD$392.9 million/year spent for treating non-serious low back pain [2]. Despite being a common and costly condition, the etiology of low back pain is not completely understood, with almost 85 % of cases classified as non-specific back pain [[3], [4], [5]]. However, changes consistent with osteoarthritis (OA), including multilevel osteophytes and disc degeneration have been shown to be predictive of persistence, severity and recurrence of low back pain [6,7].
Obesity, which is a significant risk factor for OA [8], has reached pandemic proportions [9]. More than 70 % of morbidly obese individuals report low back pain [10]. However, relationship between obesity and low back pain remains unclear. While a recent meta-analysis found that obesity is a risk factor for low back pain [11], previous systematic reviews have found that obesity is a weak risk factor for low back pain, obesity is associated with low back pain in men only, and body mass index (BMI) is not a predictor of low back pain [[12], [13], [14], [15]]. The disparity in these findings suggests that the relationship between obesity and low back pain is complex and other factors, including biological, psychological, and social factors, may be involved in this association.
Impairment in both physical and mental function is commonly experienced in individuals with low back pain and obesity [16,17]. For example, systematic reviews have found that high-intensity back pain and disability are associated with worse physical and mental health-related quality of life [18] and that obesity is also associated with poor health-related quality of life [19]. These findings highlight the potential interplay between obesity, physical and mental impairments and low back pain and the importance of considering the effect of obesity in conjunction with the physical and mental components of health.
Understanding the etiopathogenesis and identifying modifiable risk factors are important in reducing the chronicity and burden of low back pain. Therefore, this community-based study aimed to examine the associations of 1) overweight/obesity and physical function with back pain intensity and disability; and 2) overweight/obesity and mental function with back pain intensity and back disability.
2. Method
2.1. Study sample
Data were obtained from the Australian Diabetes, Obesity and Lifestyle (AusDiab) study. The AusDiab study is a national, population-based cohort study that initially recruited 11,247 participants aged 25 years and older between 1999 and 2000 (baseline) using a stratified cluster sampling method [20]. Participants were followed up from 2004 to 2005 and between 2011 and 2012. Of the total participants, 3472 were excluded as they could not be contacted due to contact refusal, decease, sickness or transfer to a high-care nursing facility. Thus, 7775 participants were invited to participate in the back pain sub-study, of whom 6384 attended and were sent the low back pain questionnaires from February 2013 to October 2014. Of the invited participants, 5058 responded (response rate 65.1 %) and were included in the current study (Fig. 1).
Fig. 1.
Flow diagram of recruited participants.
The AusDiab study was approved by the International Diabetes Institute Ethics Committee and the Monash University Human Research Ethics Committee, and the Back Pain sub-study was approved by the Alfred Hospital Ethics Committee (Project 39/11). Written informed consent was obtained from all participants.
2.2. Demographics, anthropometric, and socioeconomic status measurements
At baseline, trained interviewers obtained demographic information on the date of birth, gender and smoking status using standardised questionnaires [20]. Height and weight were measured to the nearest 0.5 cm and 0.1 kg with a stadiometer and mechanical beam balance, respectively. Participants removed their shoes and wore light clothing during the measurements. BMI (kg/m2) was calculated as weight in kilograms (kg) divided by square height in meters (m2) [20].
Area-level socioeconomic disadvantage was estimated using the Socioeconomic Indexes for Areas (SEIFA) [21]. The index is constructed such that high values reflect areas with high socioeconomic position (relative advantage) and low values reflect areas with low socioeconomic position (relative disadvantage). SEIFA was divided into tertiles and the lowest tertiles indicated the most disadvantaged socioeconomic status. Physical activity was assessed using Active Australia Physical Activity Survey [22,23]. Inadequate physical activity was defined based on the Australian physical activity guidelines, which is no physical activity or moderate to vigorous physical activity of less than 150 min per week [24,25].
2.3. Physical and mental function
Physical and mental functions were measured at baseline using the Medical Outcomes Study 36-item Short Form Health Survey (SF-36-Version 1) [26]. The SF-36 is a self-reported questionnaire that measures different domains of physical and mental function over the previous week. The survey is a multi-item scale that consists of eight domains in which four measure physical function: physical functioning, role limitation - physical, bodily pain and general health; and four measure mental function: vitality, social functioning, role limitation - emotional, and mental health. The scores were obtained through item coding and then summated for a score of 0–100, with lower scores indicating poorer health. The SF-36 has two summary measures: the physical component summary (PCS) and the mental component summary (MCS). We defined normal PCS and MCS as the highest 75 % of the PCS and MCS scores and significantly impaired PCS and MCS as the lowest 25 % of these scores [27,28].
2.4. Back pain and disability
Back pain and back disability were measured using the Chronic Pain Grade Questionnaire, a validated and reliable questionnaire for measuring back pain intensity and back disability in population-based surveys [[29], [30], [31]]. The questionnaire measures back pain and disability in the past six months and includes seven questions from which a pain intensity score (0–100) and disability points score (0–6) were calculated. To examine the relationship between pain intensity and disability and various participant characteristics, subjects were classified into three groups based on their back pain intensity score; no pain (=0), low pain intensity (<50), and high pain intensity (≥50). Participants were also categorised into three groups based on their back disability scores; no disability (=0), low disability (<3) and high disability (≥3) [[29], [30], [31]].
2.5. Statistical analysis
Analysis of variance for continuous variables and chi-squared tests for categorical variables were used to compare the characteristics of participants with different levels of back pain and back disability.
Based on overweight/obesity status (BMI ≥25 kg/m2) and the physical function status (we defined significantly impaired PCS as the lowest 25 % of PCS score), the cohort was divided into four groups: 1) no overweight/obesity and normal PCS; 2) overweight/obesity and normal PCS; 3) no overweight/obesity and significantly impaired PCS; 4) overweight/obesity and significantly impaired PCS [27,28]. Similarly, the cohort was divided into four groups according to overweight/obesity status and mental function status (we defined significantly impaired MCS as the lowest 25 % of the MCS score): 1) no overweight/obesity and normal MCS; 2) overweight/obesity and normal MCS; 3) no overweight/obesity and significantly impaired MCS; 4) overweight/obesity and significantly impaired MCS [27].
Multivariable logistic regression was used to estimate the odds ratio and 95 % confidence interval (CI) for low and high-intensity back pain/back disability according to categories of overweight/obesity and significantly impaired physical and mental function. Each analysis was adjusted for age, sex, smoking status, physical activity, and SEIFA in model 1, with additional co-adjustment for MCS and PCS scores, respectively, in model 2. The significance was determined if p < 0.05. The analyses were performed using Stata 15 (StataCorp LP., College Station, TX, USA).
3. Results
Of the 6384 participants who were sent the low back pain questionnaires, 5058 (65.1 %) responded. These participants were younger (age mean (SD) 48.5 (11.3) vs. 49.5 (14.0), p = 0.001), had lower BMI (mean (SD) 26.7 (4.5) vs. 27.0 (5.0) kg/m2, p = 0.04), more educated (university degree, 1768 (35.5 %) vs. 731 (27.9 %), p < 0.001), had a higher Index of Relative Disadvantage code from the Socioeconomic Indexes for Areas (SEIFA) (in lowest tertile 1585 (32.1 %) vs. 1006 (37.1)%, p < 0.001) compared to those who did not respond [32].
3.1. Back pain intensity
3.1.1. Baseline characteristics
Baseline characteristics of participants with no, low and high-intensity back pain in 2013–14 are presented in Table 1. Of 4983 participants, 3083 (61.8 %) had low-intensity back pain and 1000 (20 %) had high-intensity back pain. Compared to participants with no back pain and low-intensity back pain, those who presented with high-intensity back pain were, at baseline, older, more likely to be women, had higher BMI, more likely to be current smokers, less physically active and from the lowest tertile of SEIFA (p-value range from <0.001 to 0.006). Similarly, compared with the participants with no back pain, those with low-intensity back pain in 2013–14 had lower baseline scores in all the physical and mental function domains of the SF-36 indices; and those with high-intensity back pain scored the lowest in these domains. The baseline prevalence of significantly impaired physical and mental function were high among those who, at follow-up, had low and high-intensity back pain compared to those who had no back pain (impaired physical function; no pain vs. low pain intensity vs. high pain intensity: 7.7 %, 13.5 %, 34.2 %; and impaired mental function; no pain vs. low pain intensity vs. high pain intensity: 15.5 %, 24.3 %, 34.8 %).
Table 1.
Characteristics of participants with no, low and high-intensity back pain (n = 4983).
| Back pain status (2013–14) |
||||
|---|---|---|---|---|
| Baseline data (1999–2000) | No back pain (n = 900) | Low-intensity back pain (n = 3083) | High-intensity back pain (n = 1000) | p |
| Age at baseline (years) | 48.8 (11.1) | 47.4 (10.9) | 51.2 (11.8) | 0.006 |
| Female, n (%) | 497 (55.2) | 1680 (54.6) | 598 (60.2) | 0.008 |
| Body mass index (kg/m2) | 26.0 (4.7) | 26.6 (4.8) | 27.8 (5.4) | <0.001 |
| Current smoker, n (%) | 89 (10.1) | 356 (11.8) | 182 (18.7) | <0.001 |
| Inadequate physical activity, n (%) | 399 (44.6) | 1377 (45.1) | 483 (48.9) | <0.001 |
| SEIFA (in lowest tertile), n (%) | 258 (29.2) | 918 (30.3) | 397 (40.5) | <0.001 |
| Physical function measures (SF-36 indices for physical dimension) | ||||
| Physical functioning | 53.3 (5.0) | 51.5 (6.6) | 47.0 (9.3) | <0.001 |
| Role limitation: physical | 53.1 (7.0) | 51.4 (8.4) | 46.9 (11.3) | <0.001 |
| Bodily pain | 52.6 (7.4) | 50.0 (8.1) | 44.2 (10.1) | <0.001 |
| General health perception | 52.7 (8.0) | 50.6 (8.6) | 46.4 (10.1) | <0.001 |
| Significantly impaired physical function ∗, n (%) | 68 (7.7) | 411 (13.5) | 334 (34.2) | <0.001 |
| Mental function measures (SF-36 indices for mental dimension) | ||||
| Vitality | 47.5 (9.2) | 44.0 (9.7) | 40.1 (11.0) | <0.001 |
| Social functioning | 53.3 (6.6) | 51.8 (8.0) | 47.9 (10.5) | <0.001 |
| Role emotional | 52.4 (7.7) | 51.0 (9.1) | 48.2 (11.1) | <0.001 |
| Mental health | 52.9 (8.5) | 50.4 (9.2) | 47.0 (11.1) | <0.001 |
| Significantly impaired mental function∗, n (%) | 138 (15.5) | 740 (24.3) | 340 (34.8) | <0.001 |
Data presented as mean (SD) or number (%) ∗‘significantly impaired’ denotes the lowest 25 % of physical/mental function.
3.1.2. Overweight/obesity, physical function, and back pain intensity
The risks of low and high-intensity of back pain in 2013–14 according to overweight/obesity status and categories of physical function and mental function in 1999–2000 are presented in Table 2. After adjusting for age, sex, smoking, physical activity and SEIFA (Model 1), compared to participants with normal weight and normal physical function, participants with overweight/obesity only were at increased odds of low-intensity back pain (OR 1.40, 95 % CI 1.19–1.65) and high-intensity back pain (OR 1.66, 95 % CI 1.32–2.08). Those with significantly impaired physical function only were at 2.11-times (OR 2.11, 95 % CI 1.34–3.30) higher odds of low-intensity back pain and 4.18 -times (OR 4.18, 95 % CI 2.54–6.88) higher odds of high-intensity back pain. Those with both overweight/obesity and significantly impaired physical function were at 3.05-times (OR 3.05, 95 % CI 2.10–4.45) higher odds of low-intensity back pain and 11.09 -times (OR 11.09, 95 % CI 7.44–16.52) odds of high-intensity back pain. After additional adjustments for MCS, the findings remained similar (Model 2).
Table 2.
The association between overweight/obesity with physical function or mental function and back pain intensity.
| Low-intensity back pain vs no pain OR (95%CI) | High-intensity back pain vs no pain OR (95%CI) | P for between-group variability (low vs high) | |
|---|---|---|---|
| Overweight/obesity status and physical function | |||
| Model 1 | |||
| No overweight/obesity and ‘normal’ PCSa (n = 1707) | 1 | 1 | – |
| Overweight/obesity and ‘normal’ PCSa, b (n = 2347) | 1.40 (1.19, 1.65) | 1.66 (1.32, 2.08) | <0.001 |
| No overweight/obesity and ‘significantly impaired’ PCSb (n = 235) | 2.11 (1.34, 3.30) | 4.18 (2.54, 6.88) | <0.001 |
| Overweight/obesity and ‘significantly impaired’ PCSb (n = 567) | 3.05 (2.10, 4.45) | 11.09 (7.44, 16.52) | <0.001 |
| Model 2∗ | |||
| No overweight/obesity and ‘normal’ PCSa (n = 1707) | 1 | 1 | – |
| Overweight/obesity and ‘normal’ PCSa (n = 2347) | 1.41 (1.19, 1.67) | 1.61 (1.28, 2.02) | <0.001 |
| No overweight/obesity and ‘significantly impaired’ PCSb (n = 235) | 2.10 (1.34, 3.29) | 3.75 (2.26, 6.23) | <0.001 |
| Overweight/obesity and ‘significantly impaired’ PCSb (n = 567) | 3.01 (2.06, 4.38) | 9.48 (6.32, 14.12) | <0.001 |
| Overweight/obesity status and mental function | |||
| Model 1 | |||
| No overweight/obesity and ‘normal’ MCSa (n = 1479) | 1 | 1 | – |
| Overweight/obesity and ‘normal’ MCSa (n = 2175) | 1.51 (1.27, 1.80) | 1.88 (1.49, 2.38) | <0.001 |
| No overweight/obesity and ‘significantly impaired’ MCSb (n = 463) | 2.07 (1.51, 2.83) | 2.82 (1.91, 4.15) | <0.001 |
| Overweight/obesity and ‘significantly impaired’ MCSb (n = 739) | 2.16 (1.63, 2.86) | 5.12 (3.72, 7.05) | <0.001 |
| Model 2∗∗ | |||
| No overweight/obesity and ‘normal’ MCSa (n = 1479) | 1 | 1 | – |
| Overweight/obesity and ‘normal’ MCSa (n = 2175) | 1.42 (1.19, 1.69) | 1.53 (1.19, 1.97) | <0.001 |
| No overweight/obesity and ‘significantly impaired’ MCSb (n = 463) | 2.30 (1.67, 3.17) | 3.02 (1.99, 4.59) | <0.001 |
| Overweight/obesity and ‘significantly impaired’ MCSb (n = 739) | 2.13 (1.61, 2.83) | 4.20 (2.97, 5.93) | <0.001 |
Model 1: adjusted for age, sex, smoking status, physical activity, Socio-Economic Indexes for Areas (SEIFA).
Model 2: all the variables in model 1 and ∗mental component score of SF 36, and ∗∗physical component score of SF36.
‘normal’ denotes the upper 75 % of physical/mental function.
‘significantly impaired’ denotes the lowest 25 % of physical/mental function.
3.1.3. Overweight/obesity, mental function and back pain intensity
Compared to participants with normal weight and normal mental function, participants with overweight/obesity alone were at a 1.51-times (OR 1.51, 95 % CI 1.27–1.80) higher odds of low-intensity pain and 1.88-times (OR 1.88, 95 % CI 1.49–2.38) greater odds of high-intensity pain. Those with significantly impaired mental function were at 2.07-times (OR 2.07, 95 % CI 1.51–2.83) higher odds of low-intensity back pain; and at a similar magnitude of high-intensity back pain (OR 2.82, 95 % CI 1.91–4.15). Those who had both overweight/obesity and significantly impaired mental function were at 2.16-times (OR 2.16, 95 % CI 1.63–2.86) higher odds of low-intensity back pain; 5.12-times (OR 5.12, 95 % CI 3.72–7.05) higher odds of high-intensity back pain. The above analysis was adjusted for age, sex, smoking, physical activity and SEIFA (Model 1) and remained similar after introducing PCS to the model (Model 2). A significant difference was identified between low-intensity and high-intensity back pain for all analyses (p for between group variability <0.001, low vs high-intensity).
3.2. Low back disability
3.2.1. Baseline characteristics
Baseline characteristics of participants with no, low and high back disability are displayed in Table 3. Of the 4191 participants, 651 (15.5 %) had a low back disability and 481 (11.5 %) had high back disability. Compared to participants with no and low back disability, those with high back disability were older, more likely to be women, had higher BMI, more likely to be current smokers and were from the lowest tertile of SEIFA (p-value range from <0.001 to 0.005). Moreover, compared with participants with no back disability, those with a low back disability had lower scores in all the physical and mental function domains indices of SF-36; participants with a high back disability had the lowest scores in these domains. The prevalence of significantly impaired physical and mental functions was greater among participants with low and high back disability compared to those with no back disability (no disability vs. low back disability vs. high back disability: 10.7 %, 22.2 %, 42.6 % for impaired physical function and 22 %, 32.7 %, 34.1 % for impaired mental function).
Table 3.
Chracteristics of participants with no, low and high levels of back disability (n = 4191).
| Back disability status (2013–14) |
||||
|---|---|---|---|---|
| Baseline data (1999–2000) | No back disability (n = 3059) | Low back disability (n = 651) | High back disability (n = 481) | p |
| Age at baseline (years) | 46.5 (10.7) | 47.7 (11.5) | 51.5 (10.9) | <0.001 |
| Female, n (%) | 1648 (54.0) | 352 (54.2) | 296 (61.8) | 0.005 |
| Body mass index (kg/m2) | 26.3 (4.7) | 27.2 (5.0) | 28.6 (5.8) | <0.001 |
| Current smoker | 334 (11.1) | 97 (15.3) | 108 (23.1) | <0.001 |
| Inadequate physical activity, n (%) | 1361 (44.9) | 311 (48.1) | 239 (50.3) | 0.05 |
| SEIFA (in lowest tertile), n (%) | 888 (29.5) | 212 (33.0) | 194 (41.1) | <0.001 |
| Physical function measures (SF-36 indices for physical dimension) | ||||
| Physical functioning | 52.5 (6.0) | 49.7 (7.6) | 45.2 (9.8) | <0.001 |
| Role limitation: physical | 52.1 (7.9) | 49.2 (10.1) | 45.0 (11.9) | <0.001 |
| Bodily pain | 50.5 (8.0) | 46.8 (8.9) | 43.1 (10.3) | <0.001 |
| General health perception | 51.2 (8.4) | 48.3 (8.9) | 44.7 (10.9) | <0.001 |
| Significantly impaired physical function ∗, n (%) | 324 (10.7) | 142 (22.2) | 201 (42.6) | <0.001 |
| Mental function measures (SF-36 indices for mental dimension) | ||||
| Vitality | 44.7 (9.7) | 41.8 (10.0) | 39.1 (11.5) | <0.001 |
| Social functioning | 52.4 (7.6) | 50.1 (8.8) | 46.3 (11.4) | <0.001 |
| Role emotional | 51.5 (8.5) | 49.1 (10.5) | 47.6 (11.6) | <0.001 |
| Mental health | 50.9 (9.0) | 48.5 (10.1) | 47.2 (11.4) | <0.001 |
| Significantly impaired mental health, n (%) | 666 (22.0) | 209 (32.7) | 161 (34.1) | <0.001 |
Data presented as mean (SD) or number (%) ∗‘significantly impaired’ denotes the lowest 25 % of physical/mental function.
3.2.2. Overweight/obesity, physical function and back disability
The association between levels of back disability (low and high) according to baseline overweight/obesity status and categories of physical and mental function are shown in Table 4.
Table 4.
The association between overweight/obesity, with physical function or mental function and back disability.
| No vs. low back disability OR (95%CI) | No vs. high back disability OR (95%CI) | P between group variability (low vs. high) | |
|---|---|---|---|
| Overweight/obesity status and physical function | |||
| Model 1 | |||
| No overweight/obesity and ‘normal’ PCSa (n = 1442) | 1 | 1 | – |
| Overweight/obesity and ‘normal’ PCSa (n = 1984) | 1.22 (0.99, 1.49) | 1.50 (1.14, 1.99) | <0.001 |
| No overweight/obesity and ‘significantly impaired’ PCSb (n = 202) | 1.70 (1.12, 2.58) | 3.54 (2.30, 5.46) | <0.001 |
| Overweight/obesity and ‘significantly impaired’ PCSb (n = 455) | 3.35 (2.49, 4.52) | 8.49 (6.12, 11.77) | <0.001 |
| Model 2∗ | |||
| No overweight/obesity and ‘normal’ PCSa (n = 1442) | 1 | 1 | – |
| Overweight/obesity and ‘normal’ PCSa (n = 1984) | 1.22 (0.99, 1.50) | 1.48 (1.12, 1.97) | <0.001 |
| No overweight/obesity and ‘significantly impaired’ PCSb (n = 202) | 1.68 (1.10, 2.55) | 3.39 (2.19, 5.24) | <0.001 |
| Overweight/obesity and ‘significantly impaired’ PCSb (n = 455) | 3.25 (2.40, 4.39) | 7.79 (5.60, 10.84) | <0.001 |
| Overweight/obesity status and mental function | |||
| Model 1 | |||
| No overweight/obesity and ‘normal’ MCSa (n = 1239) | 1 | 1 | – |
| Overweight/obesity and ‘normal’ MCSa (n = 1824) | 1.45 (1.15, 1.81) | 1.67 (1.27, 2.20) | <0.001 |
| No overweight/obesity and ‘significantly impaired’ MCSb (n = 405) | 2.11 (1.55, 2.89) | 1.79 (1.19, 2.72) | <0.001 |
| Overweight/obesity and ‘significantly impaired’ MCSb (n = 615) | 2.40 (1.82, 3.16) | 3.53 (2.57, 4.85) | <0.001 |
| Model 2∗∗ | |||
| No overweight/obesity and ‘normal’ MCSa (n = 1239) | 1 | 1 | – |
| Overweight/obesity and ‘normal’ MCSa (n = 1824) | 1.36 (1.08, 1.71) | 1.50 (1.12, 2.0) | <0.001 |
| No overweight/obesity and ‘significantly impaired’ MCSb (n = 405) | 2.33 (1.69, 3.19) | 1.90 (1.23, 2.94) | <0.001 |
| Overweight/obesity and ‘significantly impaired’ MCSb (n = 615) | 2.29 (1.73, 3.04) | 2.97 (2.11, 4.16) | <0.001 |
Model 1: adusted for age, sex, smoking status, physical activity, Socio-Economic Indexes for Areas (SEIFA).
Model 2: all the variables in model 1 and ∗mental component score of SF 36, and ∗∗physical component score of SF36.
‘normal’ denotes the upper 75 % of physical/mental function.
‘significantly impaired’ deotes the lowest 25 % of physical/mental function.
After adjusting for age, sex, smoking, physical activity and SEIFA (Model 1), compared to participants with normal weight and normal physical function, participants with overweight/obesity and normal physical function were at 1.5-times (OR 1.50, 95 % CI (1.14–1.99)) greater odds of high back disability but not low back disability (OR 1.22, 95 % CI (0.99–1.49)). Those with significantly impaired physical function only were at 1.7 times (OR 1.70–95 % CI 1.12–2.58) higher odds of having low back disability and 3.54 times (OR 3.54, 95 % CI 2.30–5.46) higher odds of high back disability. Participants with both overweight/obesity and significantly impaired physical function were at 3.35 times (OR 3.35, 95 % CI 2.49–4.52) higher odds of low back disability and 8.49-times (OR 8.49, 95 % CI 6.12–11.77) higher odds of high back disability. These findings remained consistent with the additional adjustment for MCS (Model 2).
3.2.3. Overweight/obesity, mental function and back disability
Compared to participants with normal weight and normal mental function, participants with overweight/obesity and normal mental function were at 1.45-times (OR 1.45, 95 % CI 1.15–1.81) higher odds of low back disability; 1.67-times (OR 1.67, 95 % CI 1.27–2.20) greater odds of high back disability. Those with significantly impaired mental function only were at 2.11-times (OR 2.11, 95 % CI 1.55–2.89) higher odds of low back disability; 1.79 times (OR 1.79, 95 % CI 1.19–2.72) higher odds of high back disability. Those with both overweight/obesity and significantly impaired mental function were at 2.4 times (OR 2.40, 95 % CI 1.82–3.16) higher odds of low back disability; 3.53-times (OR 3.53, 95 % CI 2.57–4.85) greater odds of high back disability. Each of the above analyses was adjusted for age, sex, smoking, physical activity and SEIFA (model 1). These results remained similar after additional control for PCS (model 2). A significant difference was identified between low back disability and high back disability for all analyses (p for between group variability <0.001; low vs high-intensity).
4. Discussion
This population-based study showed the inter-relationship of baseline overweight/obesity and significantly impaired physical or mental function with back pain and disability 14 years later. Those who were overweight or obese had a considerably greater risk of experiencing back pain and disability 14 years later if they also initially had significantly impaired physical function or mental health. Our findings suggest that in people with overweight or obesity, examining physical and mental function may help in identifying those at greater risk of high-intensity back pain and disability. Moreover, targeting these risk factors together may be important in reducing the burden of low back pain and disability in community-based adults.
We found that being overweight/obese together with physical or mental function impairment resulted in a great risk of high-intensity back pain. This finding is novel as no previous study has simultaneously examined the effect of these risk factors on low back pain. However, it is consistent with findings related to lower limb osteoarthritis and other musculoskeletal conditions. For instance, it has been shown in individuals with hip and knee osteoarthritis that being obese and having a physical function impairment results in a poorer outcome than being obese or having poor physical health alone [28]. Our observation supports the notion that low back pain is complex and multifactorial and that there are multiple etiologic pathways. It is, therefore, plausible that overweight/obesity in conjunction with impaired physical and mental function, increases the risk of high-intensity pain. It is unclear whether underlying structural changes, such as spinal OA also have a role in this relationship. However, our finding demonstrates that identifying subgroups of overweight/obese individuals with impaired physical or mental function may assist in reducing the risk of back pain in the community.
Consistent with our findings for back pain, we found that a combination of overweight/obesity and physical or mental function impairment was associated with a great risk of back disability. Moreover, the effect of having both physical impairment and overweight/obesity on back disability was stronger than that of overweight/obesity or physical function impairment alone. While the effect of having these two risk factors at the same time has not been previously investigated in people with back disability, there is evidence from previous studies that poor physical function is independently associated with a greater risk of back disability, unlike poor mental function. A 7-year cohort study found that the PCS score of SF-36 was a stronger predictor of disability pension among middle-aged people (aged 47–52 years) due to musculoskeletal disease, including low back pain and joint pain, compared to MCS [33]. In chronic conditions other than low back pain, a 7.4-year cohort study reported that having diabetes in conjunction with impairment in either mental or physical function increases the risk of cardiovascular disease and all-cause mortality compared to either diabetes or impaired mental or physical function alone [27]. Taken together, these findings suggest that obese individuals are at higher risk of having chronic debilitating health conditions such as low back pain, if they are living a poor health-related quality of life.
Our results highlight the greater burden of having multiple risk factors together on back pain intensity and back disability. While the relationship between overweight/obesity, poor physical and mental function, and back pain intensity and back disability is complex, several pathways could be involved in the observed associations. There is evidence that obesity, physical impairment and mental impairment are associated with elevated levels of C-reactive protein (CRP), which is a biomarker for systemic inflammation [34,35] and may increase the likelihood of having low back pain [36]. Thus, while each risk factor may contribute independently to inflammation levels in the body, the presence of multiple risk factors may be additive and significantly increase inflammation and levels of pain and disability. It has also been suggested that, in individuals with obesity, fat mass can cause excessive and cumulative mechanical loading on the joints and tissues of the spine [37,38], resulting in a decrease in levels of physical activity and, in turn, an increase in back pain [39]. This is consistent with hip and knee osteoarthritis, where there is a greater burden associated with several risk factors, resulting in inflammatory and metabolic effects and mechanical loading, which result in poorer outcomes [28]. Similarly, individuals who are overweight or obese may be more likely to be depressed or anxious and, therefore, may experience more back pain [40]. Given the relationship between overweight/obesity, physical and mental impairment and their association simultaneously with back pain and disability, reducing one of these risk factors has the potential, in turn, to influence another factor and break the vicious cycle. This is consistent with a recent systematic review that found that eliminating or improving more than one risk factor, such as exercise and education, could reduce the risk of low back pain [41]. Therefore, targeting weight management along with improving physical or mental function may serve as a valuable approach for reducing low back pain and disability in people who are overweight or obese.
Our study had several strengths. It was a population-based study that recruited a large sample from the community, with almost 91 % of our cohort aged between 25 and 65 years. This age group represents the current Australian labour force. They are a very important subgroup of the community, who are young and healthy and represent almost 65.9 % of the current Australian population [42], which makes our results generalisable. Moreover, we used validated and reliable questionnaires to measure back pain intensity and disability, as well as physical and mental function.
This study also had several limitations. The use of the SF-36 may cause misclassification, as having poor physical and mental function cannot be attributed to back pain and may be influenced by other comorbidities such as cardiovascular disease and depression, which are commonly associated with musculoskeletal pain [43,44]. However, the SF-36 is a well-established outcome measure of physical and mental function due to its internal consistency, test-retest reliability, validity and responsiveness in community-based population in Australia [[45], [46], [47]]. While we do not have data on depression which plays an important role in the prediction of obesity, low back pain and physical health and functioning [48,49], we controlled for the MCS score of the SF-36 in our analysis, which was found to identify up to 87 % of depression cases [50]. While we examined the association between baseline weight and physical and mental function with back pain intensity and disability, the role of change in these factors was not investigated and warrants attention in identifying those individuals at risk. Moreover, since back pain and disability were measured at follow-up only, our ability to report on causality was limited; despite that, we were able to find an association between overweight/obesity, impaired physical and mental function with low back pain and disability. Furthermore, selection bias may have affected our results, as participants who responded to the back pain questionnaire were younger, had lower BMI, were more physically active, more educated and of better socioeconomic status compared to those who did not respond. However, we randomly recruited participants at baseline, without mentioning back pain at the time of recruitment, which may minimise this bias.
5. Conclusions
This population-based study showed that having overweight/obesity together with significant impairment in physical or mental function results in greater odds of having high-intensity back pain and disability. These findings highlight that among individuals with overweight/obesity, impaired physical and mental function are important in identifying those at risk of having high-intensity back pain and disability. Targeting these factors may help reduce the burden of this condition in the community.
Author contributions
BA, JES, DJM, and FMC contributed to the conception and design of this study. BA, FMC, YW and DMU contributed to the analysis of data. All authors contributed to the interpretation of data. Article drafts were written by BA and critically revised by all authors. The final version of the article was approved by all authors.
Role of the funding source
The AusDiab study initiated and coordinated by the International Diabetes Institute, and subsequently coordinated by the Baker Heart and Diabetes Institute, acknowledges the support and assistance given by: K Anstey, B Atkins, B Balkau, E Barr, A Cameron, S Chadban, M de Courten, D Dunstan, A Kavanagh, S Murray, N Owen, K Polkinghorne, R Tapp, T Welborn, P Zimmet and all the study participants.
We also acknowledge the funding and/or logistical support from: National Health and Medical Research Council (NHMRC grants 233200 and 1007544), Australian Government Department of Health and Ageing, Abbott Australasia Pty Ltd, Alphapharm Pty Ltd, Amgen Australia, AstraZeneca, Bristol-Myers Squibb, City Health Centre-Diabetes Service-Canberra, Department of Health and Community Services - Northern Territory, Department of Health and Human Services – Tasmania, Department of Health – New South Wales, Department of Health – Western Australia, Department of Health – South Australia, Department of Human Services – Victoria, Diabetes Australia, Diabetes Australia Northern Territory, Eli Lilly Australia, Estate of the Late Edward Wilson, GlaxoSmithKline, Jack Brockhoff Foundation, Janssen-Cilag, Kidney Health Australia, Marian & FH Flack Trust, Menzies Research Institute, Merck Sharp & Dohme, Novartis Pharmaceuticals, Novo Nordisk Pharmaceuticals, Pfizer Pty Ltd, Pratt Foundation, Queensland Health, Roche Diagnostics Australia, Royal Prince Alfred Hospital, Sydney, Sanofi Aventis, sanofi-synthelabo, and the Victorian Government's OIS Program.
BA is supported by a scholarship from Princess Nourah Bint Abdulrahman University (Riyadh, Saudi Arabia). FMC and YW are recipients of the National Health and Medical Research Council of Australia (NHMRC) Investigator Grant (APP1194829) and the NHMRC Translating Research into Practice Fellowship (APP1168185), respectively. JES is supported by an NHMRC Investigator Grant (APP1173952), DJM is supported by a Baker Heart and Diabetes Institute Gender Equity Fellowship and the Alice Baker and Eleanor Shaw Fellowship. DMU was supported by the NHMRC/MRFF Career Development Fellowship (APP1063574). For the remaining authors, no sources of funding were declared.
Declaration of competing interest
None.
Acknowledgments
The authors of this paper gratefully acknowledge Dr Sultana Monira Hussain for her valuable contributions to this research.
Handling Editor: Professor H Madry
Footnotes
This article is part of a special issue entitled: Obesity and Osteoarthritis: Hot Topics published in Osteoarthritis and Cartilage Open.
Contributor Information
Bothaina Alyousef, Email: bothaina.alyousef@monash.edu.
Flavia M. Cicuttini, Email: flavia.cicuttini@monash.edu.
Yuanyuan Wang, Email: yuanyuan.wang@monash.edu.
Anita E. Wluka, Email: anita.wluka@monash.edu.
Jonathan E. Shaw, Email: jonathan.shaw@baker.edu.au.
Dianna J. Magliano, Email: dianna.magliano@baker.edu.au.
Donna M. Urquhart, Email: donna.urquhart@monash.edu.
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