2.1. Study Design and Intervention
A randomized, controlled trial with four treatment groups (i.e., factorial design) was conducted as follows: (1) Vitamin D2 (Ergocalciferol) 400 IU/day (The study was carried out according to the guidelines of the Declaration of Helsinki and was approved by the Committees on Ethics, Biosafety, and Research at the National Institute of Public Health (INSP, Cuernavaca, Morelos, Mexico). All supplements were provided free of charge. Parents/caregivers signed an informed consent to participate in the study. The study was registered with Clinical Trials, NCT03544671.