Otherwise healthy children age 0-4 years old who are being considered for acid suppressive therapy for GERD will be eligible for this study. Subjects donate samples before and after being treated with PPIs or H2RAs (must donate at least 2 baseline pre-PPI samples to be eligible for final analysis). 30 total children who complete the study (anticipated 10 who receive lifestyle modification and 20 who receive PPIs or H2RAs). All children will donate 6 stools on or about weeks 0, 4, 12, 20, 38, and 64. The primary outcome will be a significant change in the overall diversity of the colonic microbiome after 8 weeks of PPIs or H2RAs (i.e., from week 12 to week 4), compared to after 4 weeks of lifestyle management.