While it is established that a high-fat/high-sugar "cafeteria" diet induces depressive-like and anxiety-like behavior in rats, this study shows for the first time that voluntary wheel running completely reverses these effects, primarily through restoration of gut-derived metabolites (indole-3-carboxylate, GLP-1), normalisation of insulin and leptin, and rescue of hippocampal neurogenesis.

"Exercise has an antidepressant-like effect in the wrong dietary context, which is good news for those who have trouble changing their diet," stated Professor Julio Licinio in the accompanying editorial.

The research team, led by Professor Yvonne Nolan at University College Cork, employed untargeted metabolomics to reveal that the cafeteria diet dramatically altered the gut metabolome, affecting 100 out of 175 measured metabolites. Exercise showed more selective effects, partially restoring three metabolites linked to mood regulation: anserine, indole-3-carboxylate, and deoxyinosine.

Perhaps most intriguingly, the study found that the cafeteria diet prevented the typical exercise-induced increase in adult hippocampal neurogenesis, suggesting that diet quality may fundamentally alter the brain's capacity to benefit from physical activity at the cellular level.