Changes in Body Mass Index on the Risk of Back Pain: Estimating the Impacts of Weight Gain and Loss

Background The relationship between obesity and back pain in older populations is poorly understood. This study aimed to examine (a) the impacts of changes in obesity status on back pain risk and (b) the heterogeneity in the influence of changes in obesity status according to muscle strength. Methods We analyzed 6 868 participants in waves 4 (2008-2009), 6 (2012-2013), and 7 (2014-2015) of the English Longitudinal Study of Ageing. The exposure comprised continuous body mass index (BMI) values, whereas the outcome comprised self-reported moderate or severe back pain. The targeted minimum loss-based estimator was used to estimate the impacts of hypothetical changes in BMI in each wave under 10 scenarios encompassing a 5%-25% hypothetical reduction/increase in BMI. We also performed stratified analysis using handgrip strength at the baseline. Results For the hypothetical reduction scenarios, a 10% (relative risk [RR]; 95% confidence interval [CI] = 0.82 [0.73-0.92], p = .001) reduction in BMI estimated a significantly lower back pain risk compared to the observed data. For the hypothetical increase scenarios, a 5% (RR [95% CI] = 1.11 [1.04-1.19], p = .002) increase in BMI estimated a significantly higher back pain risk. Increased BMI had a higher risk of back pain among those with weak strength when stratified by handgrip strength but not among those with strong strength. Conclusion Our study confirmed that weight gain leads to a greater risk of back pain as well as heterogeneity in the influence of changes in obesity status according to the handgrip strength.