The research, which was conducted by University of California bio-engineers in San Diego, involved the digestion of fresh human breast milk and nine different infant formula products before comparing free fatty acid (FFA) levels.

According to the study, FFAs created during infant formula digestion “may contribute​” to NEC, which occurs most commonly in premature infants. The condition has long been linked to those fed formula rather than breast milk by scientists.

The study, Digested formula but not digested fresh human milk causes death of intestinal cells in vitro: implications for necrotizing enterocolitis,​ examined the “mechanisms”​ of NEC and the apparent “protection”​ offered by breast milk.

Cytotoxic to intestinal cells​

“Premature infants fed formula are more likely to develop necrotising enterocolitis (NEC) than those who are breastfed, but the mechanisms of intestinal necrosis in NEC and protection by breast milk are unknown,”​ said the study.

“We hypothesized that after lipase digestion, formula, but not fresh breast milk, contains levels of unbound free fatty acids (FFAs) that are cytotoxic to intestinal cells.”​

To test their theory, the researchers digested multiple term and pre-term infant formula products and human milk.

They then tested for FFA levels, and whether they killed off three types of cells involved in necrotising enterocolitis – epithelial cells that line the intestine, endothelial cells that line blood vessels, and a type of white blood cell called neutrophils.

They found, overwhelmingly, that the digestion of formula led to a cellular death – or cytotoxicity – while breast milk did not.

Digestion of the nine tested infant formula products caused death in 47% to 99% of neutrophils – the most common type of white blood cell. While only 6% died as a result of milk digestion.

“FFA-induced cytotoxicity may contribute to the pathogeneses if NEC,”​ it added.

The researchers will next determine whether these results are replicated in animal studies, and whether intervention can prevent FFAs from causing intestinal damage or death from NEC.

The study was published online in the journal Paediatric Research.