MicroRNA in breast milk
MicroRNAs were first found in breast milk in 2010. Their role is still unclear.

MicroRNA (miRNA) is a short, fragile strands of RNA made up of around 22 nucleotides and found inside protective extracellular vesicles called exosomes. According to a 2019 review, more than 1.400 miRNAs have been identified in breast milk so far.

Researchers came up with two theories on the role of miRNAs. The first says that they merely serve as protein packaging for nutrients, such as serum albumin, and are broken down in the gut. Another theory is that miRNAs have a regulatory role and affect an infant’s gene expression. By attaching to matching strands of messenger RNA, which is involved in protein synthesis, miRNA can effectively turn mRNA off and on, and alter what proteins are made.

Researchers suspect that miRNAs play a role in infant development, for example in regulating immune function. If this is true, miRNAs could be added to infant formula, so that babies fed this way could also benefit from these health benefits. However, this requires further research into the role of miRNAs and whether they can survive in the stomach and gut, whether miRNA-containing exosomes can travel beyond the gut to reach other organs through the bloodstream etc.

In a 2017 study miRNA-containing exosomes from breast milk were exposed to acidic conditions that mimic those in the infant gut and were observed how the packages fared when the exosomes were incubated with human cells, the miRNAs made their way into the nuclei of cells, where the molecules could affect gene expression. A research found that miRNAs were not present in the digestive organs or the bloodstream of mice at biologically relevant levels. Another study has already shown that miRNAs in cow’s milk can reach the liver and brain in mice.

Based on the available findings it is possible that miRNAs can withstand acidic conditions. To solve the issue, further clinical trials involving infants would be needed. However, there are ethical issues in conducting these trials.

Adding miRNA to baby formula is unlikely to be easy. Even the oligosaccharides 2′-fucosyl-lactose and lacto-N-neotetraose, which have been shown to have beneficial effects and are also found in breast milk, were only introduced into infant formula after more than a decade. Moreover, an additional difficulty is that miRNA dysregulation has also been associated with certain types of cancer.

If formula manufacturers decide to add miRNAs to their products, it is likely to be of animal origin, as about 90% of the miRNAs found in human breast milk are also present in cow's and goat's milk, and consumers are more likely to accept animal-derived molecules than synthetic ones.


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