How Exercise Heals the Gut and Mind After a Junk Food Diet

Running fights junk food’s mental toll—healing the gut, balancing hormones, and protecting the brain.

Scientists led by Professor Yvonne Nolan at University College Cork and APC Microbiome Ireland, a world leading Research Ireland Centre, have identified key metabolic pathways that explain how exercise helps counter the harmful behavioral effects of a Western-style cafeteria diet.

Published in the peer-reviewed journal Brain Medicine, the study shows that voluntary running can reduce depression-like behaviors caused by diets high in fat and sugar. These effects appear to involve changes in both circulating hormones and metabolites produced in the gut. The findings offer important clues about how lifestyle strategies could be better designed to support mental health in a time when ultra-processed foods are widely consumed.

Study Design and Experimental Approach

To explore these effects, the researchers studied adult male rats fed either standard chow or a rotating cafeteria diet made up of high-fat and high-sugar foods for seven and a half weeks. Half of the animals in each diet group had access to running wheels. This approach allowed the team to separate the individual and combined influences of diet quality and physical activity on brain function and behavior.

How Exercise Influences Mood

The results showed that voluntary wheel running produced an antidepressant-like effect even when diet quality was poor. This suggests that physical activity may still provide mental health benefits for individuals who regularly consume Western-style diets.

Using untargeted metabolomics, Professor Nolan and her colleagues analyzed caecal contents to assess changes in gut chemistry. The cafeteria diet caused widespread disruption in the gut metabolome, altering 100 of the 175 metabolites measured in sedentary animals. Exercise affected a smaller group of metabolites but helped reverse some of the diet-related changes. Three compounds linked to mood regulation stood out. Levels of anserine, indole-3-carboxylate, and deoxyinosine dropped with the cafeteria diet but were partially restored through exercise.

Behavioral and Cognitive Findings

The team conducted a broad range of behavioral tests to evaluate learning, memory, and emotional responses. While the cafeteria diet alone did not significantly impair spatial learning or recognition memory in adult rats, exercise led to modest improvements in spatial navigation. The researchers also assessed anxiety-like behaviors and found mild anxiety-reducing effects of exercise that occurred regardless of diet.

Hormonal Changes Linked to Diet and Exercise

Blood hormone analysis revealed metabolic shifts that closely matched the behavioral outcomes. Sedentary rats fed the cafeteria diet showed large increases in insulin and leptin levels, but these increases were significantly reduced in animals that exercised. Dr. Minke Nota, the study’s first author, explains that this hormonal rebalancing likely played a role in the protective effects of exercise against behavior changes caused by poor diet.

The researchers also observed complex interactions between diet and exercise involving other metabolic hormones. Exercise raised circulating glucagon-like peptide 1 (GLP-1) levels in rats fed standard chow, but this response was weakened in animals on the cafeteria diet. In contrast, exercise increased peptide YY (PYY) levels only in cafeteria diet-fed rats, pointing to possible compensatory mechanisms that help stabilize metabolism under dietary stress.

Fibroblast growth factor 21 (FGF-21) increased sharply in response to the cafeteria diet regardless of whether the animals exercised, while glucagon levels declined with the diet. Together, these findings highlight the intricate hormonal responses triggered by lifestyle factors and their potential influence on brain function.

Diet Quality and Brain Plasticity

One of the most striking findings involved brain plasticity. The cafeteria diet blocked the usual exercise-related increase in adult hippocampal neurogenesis (formation of new neurons), measured through doublecortin-positive cells in the dentate gyrus. In rats fed standard chow, exercise strongly boosted neurogenesis across the hippocampus, a brain region essential for emotion and memory. This result suggests that poor diet quality may limit the brain’s ability to fully benefit from physical activity at the cellular level.

Gut Metabolites and Brain Performance

Correlation analyses revealed links between specific gut metabolites and behavioral outcomes. Several caecal metabolites, including aminoadipic acid and 5-hydroxyindole-3-acetic acid, were negatively associated with cognitive performance. These relationships were consistent across experimental conditions, indicating fundamental connections between gut metabolite profiles and brain function.

An accompanying editorial by Professor Julio Licinio and colleagues underscores the clinical importance of these results, noting that “exercise has an antidepressant-like effect in the wrong dietary context, which is good news for those who have trouble changing their diet.” The editorial emphasizes that this work helps explain why exercise continues to benefit mental health even when improving diet is difficult.

Implications for Future Research and Treatment

The findings raise important questions about how lifestyle interventions should be sequenced. While exercise alone can improve mood regardless of diet quality, achieving the full benefits for brain plasticity may depend on nutritional status. This insight could shape future programs designed to balance practicality with biological effectiveness.

The study also has limitations. All experiments were conducted in male rats, even though sex differences in metabolic and brain responses to diet and exercise are well known. In addition, the seven-week study period may not reflect longer-term changes that develop with extended exposure. Future research, including female animals, longer study durations, and dose-response analyses, will help clarify these interactions.

The results also point to new opportunities for targeting specific metabolites in mental health treatment. Exercise helped protect levels of anserine, indole-3-carboxylate, and deoxyinosine, suggesting these compounds could serve as biomarkers or potential therapeutic agents for mood disorders. The strong links between gut metabolites and behavior further support growing interest in the microbiota-gut-brain axis as a focus for mental health interventions.

Overall, this peer-reviewed study represents a major step forward in understanding how diet and exercise interact to shape mental health. It shows that physical activity can deliver antidepressant-like effects even when diet quality is poor, challenging traditional views of how metabolism and mental health are connected. By combining advanced metabolomic tools with detailed behavioral and neurobiological testing, the researchers produced findings that expand scientific knowledge and point toward real-world applications. The rigor of the peer-review process strengthens confidence in the results and positions this work as a foundation for future studies that may ultimately benefit people living with mood disorders.

References:

“Exercise mitigates the effects of a cafeteria diet on antidepressant-like behavior associated with plasma and microbial metabolites in adult male rats” by Minke H.C. Nota, Sarah Nicolas, Sebastian Dohm-Hansen, Erin P. Harris, Tara Foley, Olivia F. O’Leary and Yvonne M. Nolan, 21 October 2025, Brain Medicine.
DOI: 10.61373/bm025a.0116

“Exercise as metabolic medicine: Movement counters diet-induced behavioral despair via gut-brain signaling” by Julio Licinio, Ma-Li Wong and Nicholas Fabiano, 21 October 2025, Brain Medicine.
DOI: 10.61373/bm025d.0122

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