Bioconversion of β-carotene to vitamin A across life stages using stable isotope methodologies: a protocol for the systematic review

Vitamin A deficiency (VAD) remains a persistent health concern in the Philippines. Studies using stable isotope methods for assessing β-carotene bioconversion to vitamin A in the vulnerable Filipino population are limited. This review protocol aims to map the global evidence in quantifying β-carotene bioconversion efficiency across diverse population age groups and settings. Following the Preferred Reporting Items for Systematic Review and Meta-Analysis Protocols (PRISMA-P) 2015, studies published in English between 1 January 2013 and 31 January 2026 will be systematically searched. The databases to be queried include Embase, Web of Science, Scopus, ProQuest, and CINAHL, MEDLINE, and Academic Search Ultimate via the EBSCOhost platform. Citation tracking will also be conducted to identify additional relevant studies. A team of reviewers will conduct a database search, screening, extraction, and procedural analysis. Deduplication will be done using Covidence® software. At least two independent reviewers will perform these tasks. A different reviewer will resolve any conflicts between them. Inclusion criteria are as follows: (a) human participants—apparently healthy or nutritionally at-risk populations, any age group and setting; (b) reporting the use of stable isotope techniques; (c) studies that compare various methodological protocols that use stable isotope techniques and examine the variability of factors across different life stages. Findings will be narratively synthesized in accordance with the Synthesis Without Meta-analysis guidelines, and a descriptive summary will be provided. This review focuses on quantifying β-carotene bioconversion efficiency using stable isotopes in diverse populations. The results aim to guide the selection of suitable isotopic methods for assessing intestinal β-carotene bioconversion efficiency in vulnerable populations. This is particularly relevant in resource-limited settings such as the Philippines. Potential limitations of this review as the research team operating within a developing country context include restriction to English-language publications. Also, the anticipated heterogeneity in study designs, isotope methodologies, and outcome measures may limit direct comparability of findings. Nonetheless, the structured synthesis approach is intended to provide a transparent and rigorous overview of the current evidence base and identify priorities for future research. PROSPERO CRD420261279500