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"Factors in systemic circulation -- mainly proteins secreted by various organs -- are understood to mediate brain aging in terms of cognition, plasticity, adult neurogenesis, and neuroinflammation," the authors write.

“Part of our strategy is to develop inhibitors that only inhibit FOXO1 in the gut — limiting systemic circulation of the drug so that it doesn’t hit FOXO1 all over the body,” says Lin.

Scientists doubted that diet-derived miRNAs could make it into the systemic circulation of animal hosts at sufficient levels to have a meaningful impact.

The gastrointestinal tract is an important barrier preventing ingested substances crossing into systemic circulation.

However, it also became apparent that the drug produced toxic effects, primarily due to its actions on the cardiovascular system if it reached the systemic circulation, and so enthusiasm for its use gradually waned.