Objectives Economic evaluations of lifestyle interventions, which aim to prevent Diabetes/Cardiovascular Disease (CVD), have not included dementia. Lifestyle interventions decrease dementia risk and extend life expectancy, leading to competing effects on healthcare costs. We aim to demonstrate the feasibility of including dementia in a public health cost-effectiveness analysis, and quantify the overall impacts accounting for these competing effects. Methods The School for Public Health Research

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... (SPHR) Diabetes Prevention Model describes individuals' risk of type 2 diabetes, microvascular outcomes, CVD, congestive heart failure, cancer, osteoarthritis, depression and mortality in England. In Version 3.1 we adapted the model to include dementia using published data from primary care databases, health surveys and trials of dementia to describe dementia incidence, diagnosis and disease progression. We estimate the impact of dementia on lifetime costs, and Quality Adjusted Life Years (QALYs) gained of the NHS Diabetes Prevention Programme (NHS DPP) from an NHS/PSS perspective with three scenarios: (1) No Dementia, (2) Dementia Only, (3) Reduced Dementia Risk. Sub-group, parameter and probabilistic sensitivity analyses were conducted. Results The lifetime cost savings of the NHS DPP per patient were £145 in the (1) No Dementia scenario, £121 in the (2) Dementia Only scenario and £167 in the (3) Reduced Dementia Risk scenario. The QALY gains increased by 0.0006 in (2) Dementia Only and 0.0134 in (3) Reduced Dementia Risk. Dementia did not alter the recommendation that the NHS/DPP is cost-effective. Conclusions Including dementia into a model of lifestyle interventions was feasible but did not change policy recommendations or modify health economic outcomes. The impact on health economic outcomes was largest where a direct impact on dementia incidence was assumed, particularly in elderly populations.