Portion sizes vary worldwide, but science isn’t the reason—new study explains why

Despite varied approaches to defining portion sizes, a global study reveals that cultural and economic factors shape dietary recommendations more than scientific methods—raising questions about how to standardize healthy eating worldwide.

Study: A comparison of development methods used to define portion sizes in food-based dietary guidelines around the world. Image Credit: Jose Luis Stephens / Shutterstock

How much food is “just right” for a healthy diet? Food-based dietary guidelines (FBDGs) provide recommendations on portion sizes worldwide, but the methods used to determine them vary widely. In a recent study published in the journal Frontiers in Nutrition, researchers compared different approaches to food recommendations, revealing that geography and methodology significantly influence the suggested amounts for various food groups.

Dietary Guidelines

Dietary guidelines serve as essential public health tools, providing region-specific diet and portion size recommendations. These guidelines aim to improve population health by aligning nutrient needs with realistic food choices. Despite global efforts to standardize dietary advice, countries use different methods to develop their FBDGs, ranging from expert consensus to complex data-driven modeling.

Historically, dietary recommendations were primarily based on expert opinions and reviews of nutritional studies. However, advancements in food consumption data collection and statistical modeling have allowed for more precise, data-based approaches. Some countries incorporate national food surveys to tailor portion sizes to local eating habits, while others rely on international standards.

Despite these efforts, there is no universal method for determining portion sizes, leading to inconsistencies between regions. The study found that while different methodologies exist, only 15 out of 96 FBDGs used data-driven modeling, with the majority relying on expert opinion and literature reviews. This indicates a significant gap in integrating national dietary data into food guidelines. These variations raise questions about the impact of different approaches on dietary adherence and overall public health, also highlighting the need for more coordinated efforts in FBDG development.

Investigating Dietary Recommendation Variations

The present study examined how different countries develop quantitative dietary recommendations, focusing on the methodologies used to determine portion sizes. The researchers collected data from 96 national FBDGs through the Food and Agriculture Organization (FAO) repository and other sources. They categorized the approaches into three main groups — expert consensus and literature reviews, minimal calculations, and data-driven models involving national dietary intake surveys.

To compare the effects of these methods, the study extracted recommended portion sizes for 15 food groups across different regions. The researchers then analyzed whether the methodology influenced the portion sizes suggested in dietary guidelines. Beyond methodological differences, the study also identified economic and cultural factors, such as the cost and availability of certain foods, as major contributors to variation in portion size recommendations.

Data were manually extracted, translated whenever necessary, and analyzed using statistical tests to identify significant differences between methodologies and across global regions, with adjustments for multiple comparisons.

The study aimed to determine whether data-driven approaches produced significantly different portion size recommendations compared to expert-driven methods. It also assessed whether differences in regional dietary habits influenced the suggested portion sizes. By systematically categorizing and comparing methodologies, the study also sought to identify trends that could inform future improvements in dietary guideline development.

Is There a Perfect Portion Size?

The researchers found that, despite differences in methodology, there were no statistically significant variations in recommended portion sizes for most food groups. Of the 96 FBDGs analyzed, 83 relied primarily on expert consensus and literature reviews, while only 15 used data-driven modeling; furthermore, about a third of the guidelines incorporated minor calculations.

When comparing different methodologies, the study revealed that expert-driven and data-driven approaches yielded similar results regarding portion size recommendations. However, when analyzing regional differences, the researchers observed some notable variations. For instance, portion sizes for fish and shellfish were significantly lower in Latin American dietary guidelines compared to those in Europe, likely due to differences in dietary habits, cost, and accessibility rather than methodological differences alone.

The study also highlighted challenges in using dietary intake data, as only eight countries relied on nationally representative food consumption surveys. In many cases, the availability and quality of dietary data were limited, impacting the accuracy of data-driven models. Additionally, while FBDGs aim to provide practical recommendations, cultural factors and existing eating habits can influence adherence, meaning that even well-designed guidelines may not always lead to healthier diets.

One major limitation of the study was the reliance on translated documents, which could have introduced inconsistencies. Moreover, given that only a small proportion of FBDGs used data-driven methods, the findings may not fully reflect the potential benefits of integrating more robust dietary data into guideline development.

Conclusions

The study highlighted the complexities of developing dietary guidelines and the need for greater harmonization in portion size recommendations. Despite methodological differences, the study found broad consistency in portion size recommendations across regions, suggesting a potential for greater harmonization in global dietary guidelines.

The results indicated that while different methodologies produce broadly similar results, regional factors and data limitations still influence dietary guidance. Future studies should focus on improving data collection and exploring how cultural and economic factors shape dietary habits to ensure that FBDGs are both scientifically sound and practical for diverse populations. Additionally, emerging approaches, such as machine learning and metabolomics, could further refine food recommendations and improve the integration of dietary data into national guidelines.