Joint associations of baseline heavy physical work and waist-to-height ratio with hospital-recorded intervertebral disc disorders: a UK Biobank cohort study

Background Occupational heavy physical work and central adiposity are associated with disc-related morbidity, but their joint associations and absolute risks require clarification. Methods We studied UK Biobank participants employed or self-employed at baseline (2006–2010), without known disc disease, using linked hospital and mortality records in England, Scotland and Wales through December 31, 2015. Of 276,917 eligible participants, 270,971 with complete adjustment data formed the primary analysis population. Exposures were baseline heavy-work frequency and measured waist-to-height ratio (WHtR). Cox models evaluated continuous WHtR per 0.05 increase and four categories combining lower/higher work frequency with WHtR <0.5/≥0.5. The primary outcome was first hospital inpatient-recorded intervertebral disc disorders [International Classification of Diseases, Tenth Revision (ICD-10) M51]. The M51.1 branch in the UK Biobank coding hierarchy was a secondary, nested radiculopathy subtype. Five-year risks accounted for competing mortality. Results Primary analyses included 2,072 disc-disorder and 951 radiculopathy-subtype events. Each 0.05 higher WHtR was associated with a disc-disorder hazard ratio (HR) of 1.13 (95% confidence interval [CI] 1.10–1.16); higher versus lower work frequency in the same model had HR 1.47 (1.31–1.65). Spline analyses provided no clear evidence of nonlinearity. Higher work frequency with WHtR ≥0.5, compared with lower frequency and WHtR <0.5, had HR 2.04 (1.75–2.38). Corresponding standardized five-year risks were 0.791% versus 0.389%, a difference of 4.03 (3.01–5.04) per 1,000 people. For the nested subtype, the continuous WHtR HR was 1.14 (1.09–1.19) and the joint-category HR was 2.00 (1.59–2.50), with a risk difference of 1.73 (1.07–2.40) per 1,000. There was no clear evidence of multiplicative or additive interaction. Conclusion Higher baseline WHtR and more frequent heavy work were associated with subsequent hospital-recorded disc disorders. Continuous exposure estimates and modest absolute differences characterize these associations in employed adults. Repeated, task-specific measurements are needed before translating them into preventive or screening strategies.