Maybe this reference would be helpful re formula effects. Fallot,M.E. et al:
Breastfeeding reduces incidence of hospital admissions for infections in
tinfants. Ped 65(6): 1121.1980. It is an old one but a good one. The gut
aflora changes within 24 hours of intro of supplemental formula or solids
and it would take from two to four weeks for the intestinal environment to
return to it original state. Based on this article I work very hard to see
that no formula passes the lips of any of the breastfeeding babies.