Breastfeeding is the ideal nutrition for a newborn’s integral necessities. It seems crucial therefore to know its composition in order to provide suitable infant formula when required. Of these, polyamines (with lactation and the microbiota being its intestinal source) are involved in the development of gut epithelium and immunity. Safety concerns limit human intervention studies. Therefore, we have studied whether the different amounts of polyamines supplied by breast milk (varying among mothers) or infant formulas feeding, up to day 30 postpartum, determine the polyamine content in newborn faeces. Independent samples (68) of breast milk from 59 healthy Caucasian woman (day 0, 1, 3, 5, 7, 15 and 30 postpartum) who had natural deliveries after week 38, same day newborn faeces, when available (81 from breast milk and 55 from infant formula fed) and 6 infant formulas were determined by HPLC. In breast milk, polyamines and isoamylamine (a primary amine), with interindividual variations, increased over time with a higher content of spermidine (no other amines were present). Overall, they were much higher than in infant formula. By the 2nd week after birth the content of polyamines, cadaverine and tyramine, but not isoamylamine, in the faeces of infant formula were higher than in breast milk fed. Cadaverine and tyramine could be used to predict the type of newborns feeding used. The differences in the content may be related to distinct colonisation of amines-producer bacteria, to which polyamines could help establish. Further studies are required to determine the clinical utility of these findings.