Changes in Food-Group and Processing-Level Composition of Lunches Consumed Following Implementation of a Universal School Lunch Program: A Quasi-Experimental Study in Saskatchewan, Canada

Background/Objectives: Universal school food programs may reshape lunch composition by changing foods available at school and lunch sources. We assessed whether food-group and processing-level composition changed differently in intervention and control schools following implementation of a universal school lunch program. Methods: Four elementary schools in Saskatoon, Saskatchewan, Canada, were included; two implemented a universal free lunch program and two served as controls. Lunch consumption was measured in May–June 2021 and May–June 2023 using photography-assisted weighed plate waste. Foods were classified using Canada’s Food Guide and NOVA. Outcomes were expressed as grams and percentage of total quantified lunch grams and analyzed using an equal-school-weighted difference-in-differences approach. Results: The analysis included 980 student-period observations. Total quantified lunch grams declined in both groups, with a smaller reduction in intervention schools (DiD = 32.62 g). Vegetable and fruit intake declined less in intervention schools (DiD = 3.52 g), although proportional change did not favour intervention schools (DiD = −3.75 percentage points). Whole-grain foods declined more in intervention schools in grams (DiD = −18.71 g) and proportional contribution (DiD = −8.07 percentage points), a pattern consistent across sensitivity analyses. Ultra-processed foods declined in both groups, with little differential proportional change (DiD = −0.17 percentage points). Conclusions: Implementation was not accompanied by consistent differential improvements in lunch composition. Whole-grain foods showed the most consistent between-group pattern, whereas reductions in ultra-processed foods possibly reflected broader changes across schools. These findings highlight the importance of distinguishing the foods available through school food programs from those students actually consume.