Objectives: The prevalence of excess weight in Scotland is higher than other UK nations and amongst the highest when compared with European Union countries. We aimed to use historic data to project estimates of the prevalence of excess weight in Scotland using Bayesian age-period-cohort modelling.

Study design: A Bayesian age-period-cohort modelling study.

Methods: Interviewer-validated height and weight measurements were obtained from the Scottish Health Survey (2003-2019) to calculate body mass index (BMI). Relevant socio-demographic attributes were sourced to contribute to modelling and results stratification. Study outcomes were defined as overweight (BMI ≥25 kg/m2) and obesity (BMI ≥30 kg/m2). We estimated historic trends in study outcomes by sex and age-group, and projected future trends to 2040, using Bayesian hierarchical age-period-cohort modelling.

Results: In 2040, we estimate 3.3 million adult cases of overweight, of which 1.6 million are cases of obesity. Projections were more pronounced for obesity, compared to overweight, with an additional 66,000 male cases and 175,000 female cases projected between 2019 and 2040. Between 2003 and 2019, the proportion of male and female cases of both overweight and obesity aged 65 years and above have increased, a trend projected to further intensify as we move towards 2040.

Conclusions: Left unaddressed, we estimate a substantial increase in adult excess weight in Scotland by 2040, particularly for females, compounded by increases in obesity and in the proportion of older cases. These findings are a warning signal of future adverse population health impacts and healthcare service sustainability pressures. Projections are not inevitable and underscore the need to accelerate progress on implementing preventative measures to address the food environment, and on further development of weight management and support services, to improve Scotland’s population health.