Despite a global surge in plant-based eating, a new review reveals that most studies still lack consistent definitions and validated tools to assess plant-based food diversity, thereby undermining efforts to link these diets to health outcomes.
Review: Diversity of Plant-Based Food Consumption: A Systematic Scoping Review on Measurement Tools and Associated Health Outcomes. Image Credit: Antonina Vlasova / Shutterstock
In a recent scoping review published in the journal Nutrition Reviews, researchers at King’s College London, United Kingdom, conducted a systematic scoping review to investigate the definitions, assessment tools, methodologies, and health outcomes assessed in previous studies on plant-based diet diversity. The review focused on research conducted in high-income countries and identified 43 eligible studies, 88% (38/43) of which were observational in design.
Background
Plant-based foods are any dietary components derived from plant sources, including grains, herbs, nuts, seeds, legumes, fruits, and vegetables. The oils extracted from plants and beverages, which use plant products as their primary constituents, also fall under this category.
Studies on plant-based foods have highlighted their health benefits, including a reduced risk of all-cause mortality associated with the consumption of fruits and vegetables, which has triggered a surge in their global consumption. Reports estimate a 60% increase over just three years, between 2017 and 2020. Several national and international guidelines now recommend incorporating a variety of plant-based foods into a balanced human diet.
Unfortunately, despite decades of research in the field, no standard definition for ‘plant-based food diversity’ exists. The aforementioned dietary guideline recommendations primarily recommend consuming fruits and vegetables, and omit other plant-derived food sources. Furthermore, standardized methodologies and validated assessment tools for measuring plant-based food consumption and subsequent health outcomes are also lacking, potentially resulting in reduced study reliability and complicating comparisons between independent works.
About the review
The present scoping review has four primary objectives concerning previous plant-based foods publications: 1. to identify the definitions of plant-based foods; 2. to identify variations in assessment tools employed; 3. to characterize the methodologies used; 4. to elucidate the findings (health outcomes) of previous works. The review conforms to the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses Extension for Scoping Reviews (PRISMA-ScR) guidelines.
Study data (publications) were obtained from Web of Science and Medline using a custom search strategy, encompassing all publications in these online repositories from the databases' inception until October 2022. Study inclusion eligibility criteria were designed using the Patient, Exposure, Comparators, Outcome, and Study Design (PECOS) approach and included observations or intervention studies reporting measurements, effects, or health-related outcomes of plant-based food consumption. Studies were restricted to high-income countries (as classified by the World Bank) to ensure a homogeneous sample population.
Potential publications were manually screened for eligibility through a cascade of title, abstract, and full-text reviews. Data extraction comprised study design, population characteristics, methodology, dietary assessment tools (if available), diversity assessment tools (if available), and health outcomes. Review syntheses were both qualitative and descriptive in nature.
Review findings
Of the 6,569 publications identified using the custom search strategy, 95 were deemed eligible after screening the titles and abstracts. Full-text screening excluded a further 52, resulting in a final dataset of 43 publications. Included studies comprised cross-sectional studies (40%, n = 17), prospective cohort studies (33%, n = 14), case-control studies (16%, n = 7), secondary analyses of randomized controlled trials (5%, n = 2), non-randomized controlled trials (5%, n = 2), and a case study (2%). Per-study sample cohorts ranged from 63 to 452,269 participants across the USA, Europe, New Zealand, and Australia.
An assessment of the definition of plant-based foods in studies revealed that almost three-fourths (74%) restricted the definition to comprise only fruits and vegetables, 12% included grains, and 7% included cereals. Only one study extended the definition to grains, nuts, seeds, legumes, fruits, and vegetables. None of the studies explicitly included herbs, spices, plant-based oils, or plant-derived beverages in their definitions.
Plant-based food consumption was primarily assessed via food frequency questionnaires (FFQs) (49%), 24-hour dietary recalls (23%), and food diaries (26%). The three dedicated diversity indices (VIC, FAVI, FAVVA) were validated against food and nutrient intakes or dietary assessment tools; however, none were specifically validated for measuring plant-based food diversity itself—a subtle but important distinction. The durations of plant-based food diversity assessments varied substantially between studies, ranging from 24 hours to three months.
Measurements of plant-based food diversity were similarly varied, with 59% of studies restricting their dietary assessments to only plant-based dietary components, while 40% included non-plant-based foods as well. For those studies that included non-plant foods in their diversity assessment, plant-based diversity scores were typically reported separately. Only three studies utilized dedicated dietary assessment tools, and although these were validated against dietary intake or nutrient biomarkers, none were specifically validated for assessing the diversity of plant-based foods. “Due to the variation in definitions, methods of measurement, and reporting of plant-based diversity outlined above, it is challenging to compare levels of diversity and subsequent associations with health outcomes, across studies.”
Conclusions
The present review highlights significant methodological limitations in previous research on plant-based foods, underscored by a lack of consensus in the definitions of ‘plant-based foods’ and ‘plant-based food diversity.’ Glaring methodological limitations, particularly in measurement and assessment, were also observed, which prevented comparisons between studies and reduced the robustness of their findings regarding health outcomes. This review, therefore, calls for the standardization of definitions and methodologies in the field, as well as the development of dedicated assessment tools.
“While the impact of plant-based food diversity has been investigated for several health outcomes, findings are limited by contradicting evidence and the limitations in study designs. These, in combination with the increased interest in plant-based food diversity among the general population, highlight the urgent need to improve assessment and reporting of plant-based food diversity.”