Similac is the first and only baby formula with 2'-FL Human Milk Oligosaccharides , a unique immune nourishing prebiotic previously only found in breast milk. 2'-FL is backed by more than 15 years of extensive research and Similac Pro-Advance with 2'-FL helps strengthen baby's immune system to be more like a breastfed infant than ever before. Most prebiotics are known to support gut health