Data were extracted into separate databases for studies reporting on crops and livestock products (including meat, milk and eggs). Extracted data included all relevant information on study characteristics, methods and results. Data on factors outlined in the conceptual framework on nutrient variability (Figure 1) were also extracted when available. For the first 10 included articles, data extraction was performed in duplicate by two independent reviewers. Extracted data were compared and any inconsistencies noted and corrected as necessary. For the remaining articles, one reviewer entered the data and another checked all entries; any differences were discussed.
