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Study design for the paired-isotope dilution method in a highly controlled, 90-d feeding intervention with a high provitamin A–containing food, such as vegetables, a fortified food, or a biofortified staple crop. Negative (placebo) and positive (supplemental preformed vitamin A) control groups are recommended. If both control groups are included, a bioefficacy factor can be estimated for provitamin A–containing foods. RID, retinol isotope dilution.
