title: The relationship between obesity, low back pain, and lumbar disc degeneration when genetics and the environment are considered: a systematic review of twin studies.
abstract: BACKGROUND
  The relationships between obesity and low back pain (LBP) and lumbar disc degeneration (LDD) remain unclear. It is possible that familial factors, including genetics and early environment, affect these relationships.
  
  
  OBJECTIVE
  To investigate the relationship between obesity-related measures (eg, weight, body mass index [BMI]) and LBP and LDD using twin studies, where the effect of genetics and early environment can be controlled.
  
  
  METHODS
  A systematic review with meta-analysis.
  
  
  METHODS
  MEDLINE, CINAHL, Scopus, Web of Science, and EMBASE databases were searched from the earliest records to August 2014. All cross-sectional and longitudinal observational twin studies identified by the search strategy were considered for inclusion. Two investigators independently assessed the eligibility, conducted the quality assessment, and extracted the data. Metaanalyses (fixed or random effects, as appropriate) were used to pool studies' estimates of association.
  
  
  RESULTS
  In total, 11 articles met the inclusion criteria. Five studies were included in the LBP analysis and seven in the LDD analysis. For the LBP analysis, pooling of the five studies showed that the risk of having LBP for individuals with the highest levels of BMI or weight was almost twice that of people with a lower BMI (odds ratio [OR] 1.8; 95% confidence interval [CI] 1.6-2.0; I(2)=0%). A dose-response relationship was also identified. When genetics and the effects of a shared early environment were adjusted for using a within-pair twin case-control analysis, pooling of three studies showed a reduced but statistically positive association between obesity and prevalence of LBP (OR 1.5; 95% CI 1.1-2.1; I(2)=0%). However, the association was further diminished and not significant (OR 1.4; 95% CI 0.8-2.3; I(2)=0%) when pooling included two studies on monozygotic twin pairs only. Seven studies met the inclusion criteria for LDD. When familial factors were not controlled for, body weight was positively associated with LDD in all five cross-sectional studies. Only two cross-sectional studies investigated the relationship between obesity-related measures and LDD accounting for familial factors, and the results were conflicting. One longitudinal study in LBP and three longitudinal studies in LDD found no increase in risk in obese individuals, whether or not familial factors were controlled for.
  
  
  CONCLUSIONS
  Findings from this review suggest that genetics and early environment are possible mechanisms underlying the relationship between obesity and LBP; however, a direct causal link between these conditions appears to be weak. Further longitudinal studies using the twin design are needed to better understand the complex mechanisms underlying the associations between obesity, LBP, and LDD.
authors: Discipline of Physiotherapy, Faculty of Health Sciences, The University of Sydney, PO Box 170, 75 East Street Lidcombe, Sydney, NSW, Australia 2141. Electronic address: adar3900@sydney.edu.au.
authors: Amabile B
authors: AB
authors: Dario
authors: The George Institute for Global Health and Institute of Bone and Joint Research, Kolling Institute, Sydney Medical School, The University of Sydney, Level 13, 321 Kent Street, Sydney, NSW, Australia 2141.
authors: Manuela L
authors: ML
authors: Ferreira
authors: Discipline of Physiotherapy, Faculty of Health Sciences, The University of Sydney, PO Box 170, 75 East Street Lidcombe, Sydney, NSW, Australia 2141.
authors: Kathryn M
authors: KM
authors: Refshauge
authors: Biomechanics and Motor Control Research Group, Science and Technology Faculty-Universidade Estadual Paulista, Presidente Prudente, Sao Paulo, Brazil 19060-900.
authors: Thais S
authors: TS
authors: Lima
authors: Murcia Twin Registry, Department of Human Anatomy and Psychobiology, University of Murcia, Spain 30100; IMIB-Arrixaca, Department of Human Anatomy and Psychobiology, Murcia, Spain 30100.
authors: Juan R
authors: JR
authors: Ordoñana
authors: Discipline of Physiotherapy, Faculty of Health Sciences, The University of Sydney, PO Box 170, 75 East Street Lidcombe, Sydney, NSW, Australia 2141.
authors: Paulo H
authors: PH
authors: Ferreira
mesh_terms: D016022:Case-Control Studies; D059647:Gene-Environment Interaction; D006801:Humans; D055959:Intervertebral Disc Degeneration / Q000453:epidemiology / Q000235:genetics*; D017116:Low Back Pain / Q000453:epidemiology / Q000235:genetics*; D008159:Lumbar Vertebrae / Q000473:pathology; D009765:Obesity / Q000453:epidemiology / Q000235:genetics*; D014430:Twins, Monozygotic
keywords: Body mass index; Body weight; Genetics; Low back pain; Lumbar disc degeneration; Obesity; Twins