The association of plant or animal food origin and processing level with BMI and waist circumference: a prospective cohort study of the UK Biobank.

Background: Body Mass Index (BMI) and waist circumference (WC) gain are key risk factors for cardiometabolic disease and mortality. Although plant-based diets are often promoted for health and sustainability, the rising inclusion of ultra-processed foods (UPF) within these diets raises concerns. This study assessed the associations of food origin (plant vs. animal) and processing level (UPF vs. non-UPF) with BMI and WC increase in a prospective cohort. Methods: A total of 17,374 participants aged 40-69 years from the UK Biobank cohort study were included. Dietary data were collected between 2009 and 2012, and follow-up assessments were conducted between 2012 and 2019. Dietary intake was assessed using a minimum of two and a maximum of five repeated 24-h dietary recalls. Foods were classified by plant or animal origin and processing level using the Nova system. Outcomes were defined separately for BMI and WC as increases of ≥5% and ≥10% from baseline to follow-up. Cox proportional hazards models were applied. Findings: During a mean follow-up of 5.3 years, 2477 cases of ≥5% BMI increase and 4567 of ≥5% WC increase were observed. A 10% higher energy intake from plant-sourced non-UPF was associated with 11% lower hazard of ≥5% BMI increase (HR 0.89, 95% CI 0.85-0.92) and 10% lower hazard of ≥5% WC increase (HR 0.90, 95% CI 0.88-0.93). A 10% higher intake of plant-sourced UPF was linked to increased risk of both BMI and WC increase (HR 1.10 and 1.09, respectively). Substitution analyses indicated that a higher contribution of plant-sourced non-UPF, at the expense of UPF (plant or animal), was consistently associated with a lower risk of study outcomes. Interpretation: Our findings highlight the importance of considering food origin alongside processing, showing that plant-sourced non-UPF may confer protective effects, whereas plant-sourced UPF contribute to adverse weight outcomes. Future research is needed to clarify the behavioural and biological mechanisms behind these findings. Funding: This research received funding from World Cancer Research Fund (WCRF UK), as part of the World Cancer Research Fund International grant programme.