Longitudinal Evaluation from Birth to Adolescence of Soy Protein–Based Infant Formula Compared with Cow Milk–Based Formula and Breastfeeding: A Comprehensive Summary of Findings

Breastfeeding is widely recognized as the gold standard food for infants. However, many families use infant formulas, including soy-based products, which have not been studied for their long-term safety and developmental effects. The Beginnings Study and the Beginnings Follow-Up Study represent one of the most comprehensive prospective cohorts designed to examine how early infant feeding is related to growth, body composition, cardiovascular, microbiome, and skeletal outcomes, neurodevelopment, and reproductive maturation through adolescence. Conducted in Arkansas, United States, the study enrolled 600 healthy, term infants fed soy-based infant formula, cow milk-based infant formula, or human milk during infancy (200 per group). Of these, 385 participants (73.2%) completed the 6-year visit, and 190 (31.7% of enrolled and 49.4% of 6-year visit completers) participated in the 14-year Follow-Up Study. Breastfeeding was associated with slower weight gain velocity during infancy, and consistent lower BMI, fat mass index, and waist circumference extending into adolescence, than formula feeding. Formula-fed infants had comparable results with breastfed infants for skeletal mineralization, most neurocognitive parameters, and reproductive organ development. However, cardiovascular autonomic measures, including heart rate and vagal tone, differed by feeding group, with some sex-specific effects. Novel contributions included analyses of the gut microbiome and metabolomics profiles in early life, which revealed distinct dietary signatures, as well as neurodevelopmental assessments using electroencephalography, which highlighted transient differences in language-related brain responses among feeding groups. Together, these results demonstrate more similarities than differences between soy-based infant formula and cow milk-based infant formula in health outcomes and support the lasting benefits of breastfeeding. This evidence can help guide health care professionals in infant feeding recommendations and highlight critical windows to prevent obesity and promote lifelong health.