Health gains from achieving optimal body mass index in Australia: a simulation study

Background: We estimated the health gains and health inequality impacts for the Australian population alive in 2021 (n = 25.0 million) in the next 20 years and over their remaining lifespan, from shifting everyone above a BMI of 25 kg/m² to 25 kg/m² compared to the BMI distribution in 2021 persisting into the future. Methods: National Health Survey 2017–2018 was used to estimate BMI distributions by sex, age and, socio-economic status (Socio-Economic Indexes for Areas; SEIFA). A proportional multistate life table linking BMI to 19 associated diseases and allowing for time lags and competing morbidity and mortality, was used to estimate the future stream of health adjusted life years (HALYs) gained from eradicating high BMI. Findings: Undiscounted health gains in the first 20 years and lifetime of the population were, respectively, 2.00 million (95% uncertainty interval 1.70–2.32) and 20.4 million (17.0–24.2) (at a 3% annual discount rate, HALY gains were 1.37 and 5.77 million, respectively). Reductions in the incidence of cardio metabolic diseases contributed 61% (95% UI: 54%–68%) of the undiscounted health gains in the first 20 years, musculoskeletal diseases contributed 26% (20%–32%) and cancer 5% (3%–8%). HALY gains in the first 20 years and lifetime, per person alive in 2021, were 2.5 (2.4–2.5) and 1.9 (1.9–2.0) times higher for the most compared to the least deprived SEIFA quintile. Interpretation: The total theoretical envelope of health gains, and health inequality reductions, through eradication of BMI is substantial. Our modeling infrastructure can be used to estimate the health impacts and cost effectiveness of many actual interventions. Funding: No funding was received for the study.