Overwhelmingly, the digestion of formula led to cellular death, or cytotoxicity - in less than five minutes in some cases - while breast milk did not.
For example, digestion of formula caused death in 47 percent to 99 percent of neutrophils while only six percent of them died as a result of milk digestion.
The study found that breast milk appears to have a built-in mechanism to prevent cytotoxicity.
The research team believes most food, like formula, releases high levels of free fatty acids during digestion, but that breast milk is digested in a slower, more controlled, process.
Currently, many neonatal intensive care units are moving towards formula-free environments, but breastfeeding a premature infant can be challenging or physically impossible and supplies of donor breast milk are limited.
