Ineffective delivery of diet-derived microRNAs to recipient animal organisms.

Cross-kingdom delivery of specific microRNAs to recipient organisms via food ingestion has been reported recently. However, it is unclear if such delivery of microRNAs occurs frequently in animal organisms after typical dietary intake. We found substantial levels of specific microRNAs in diets commonly consumed orally by humans, mice, and honey bees. Yet, after ingestion of fruit replete with plant microRNAs (MIR156a, MIR159a, and MIR169a), a cohort of healthy athletes did not carry detectable plasma levels of those molecules. Similarly, despite consumption of a diet with animal fat replete in endogenous miR-21, negligible expression of miR-21 in plasma or organ tissue was observed in miR-21 -/- recipient mice. Correspondingly, when fed vegetarian diets containing the above plant microRNAs, wild-type recipient mice expressed insignificant levels of these microRNAs. Finally, despite oral uptake of pollen containing these plant microRNAs, negligible delivery of these molecules was observed in recipient honeybees. Therefore, we conclude that horizontal delivery of microRNAs via typical dietary ingestion is neither a robust nor a frequent mechanism to maintain steady-state microRNA levels in a variety of model animal organisms, thus defining the biological limits of these molecules in vivo.