Six Decades of Methodological Evolution in Portuguese Food Balance Sheets: Implications for Nutrition Surveillance and Food System Monitoring

Background: Food Balance Sheets (FBS) are widely used to assess national food availability, monitor nutrition transitions, and support food and nutrition policies. However, methodological revisions may influence the comparability of food availability estimates and nutrient supply indicators over time. This study aimed to examine the methodological evolution of Portuguese FBS between 1963 and 2024 and to assess its implications for nutrition surveillance and food system monitoring. Methods: A retrospective documentary analysis was conducted using all official Portuguese FBS published by the National Statistics Institute between 1963 and 2024. Methodological characteristics were systematically compared across six reporting periods, including institutional frameworks, data sources, food classification systems, nutritional indicators, food availability equations, food loss estimation procedures, and nutrient conversion methods. Results: Substantial methodological developments were identified over the six-decade period. Portuguese FBS evolved from relatively simple food supply accounting systems towards increasingly standardized frameworks aligned with the Food and Agriculture Organization Supply and Utilization Accounts methodology. Major changes included the incorporation of food losses, animal feed, industrial uses, and seed utilization into food availability calculations; the expansion of reported nutritional indicators from energy, protein, and fat to include carbohydrates, vitamins, and minerals; and the progressive refinement of food classification systems. More recent editions also introduced additional nutritional indicators, enhancing the characterization of food availability and nutrient supply. These developments may have contributed to expanding the nutritional scope of Portuguese FBS, strengthening methodological standardization, and improving alignment with international standards. Conclusions: Over the last six decades, Portuguese FBS have undergone substantial methodological modernization and increasing alignment with international standards. Although FBS remain indirect measures of food consumption, their evolution has strengthened their value for population-level food availability analyses, nutrition surveillance, food system monitoring, and evidence-informed policymaking. The findings also highlight the importance of considering methodological changes when interpreting long-term trends in food availability and nutrient supply.