New research shows running can reverse many of the brain and hormonal effects caused by junk food diets, restoring mood and gut balance. But for full brain benefits, exercise needs support from a healthy diet.
A new study from University College Cork reveals how running can reverse many of the mental and hormonal effects triggered by a Western-style diet high in sugar and fat. The research, published in Brain Medicine on October 21, shows that physical activity helps restore gut balance and hormone levels linked to mood and stress — even when eating habits are less than ideal.
The Study: How Exercise Outsmarts Junk Food
Led by Professor Yvonne Nolan, the research team studied adult male rats fed either a healthy diet or a “cafeteria diet” — a rotating mix of fatty and sugary foods — for just under two months. Half of each group had access to a running wheel, allowing the scientists to compare how diet and exercise interacted to affect mood, metabolism, and brain health.
The results were striking. Rats that exercised showed fewer depression-like behaviors and more stable hormone levels, even when eating the unhealthy cafeteria diet. “Exercise had an antidepressant-like effect, even in the wrong dietary context,” the authors noted — echoing a hopeful message for anyone struggling to eat well.
Gut Metabolites: The Hidden Link
The team analyzed the caecal contents — the material in a pouch of the intestine rich in gut microbes — to see how diet and exercise changed gut chemistry. Out of 175 metabolites measured, more than half were altered by the junk food diet. Running helped restore several key compounds tied to emotional balance, including anserine, indole-3-carboxylate, and deoxyinosine.
These metabolites are part of the microbiota-gut-brain axis, a communication pathway where gut microbes influence brain function and mood. In short: exercise helped rebalance the gut chemicals that junk food had thrown off.
Hormones Tell the Same Story
Exercise also reshaped the animals’ hormone profiles. Junk food alone spiked insulin and leptin — hormones tied to metabolism and appetite — but running normalized them. Other hormones, like GLP-1 and PYY, also responded differently depending on diet, showing how the body tries to stabilize itself under stress.
“The hormonal rebalancing seen with exercise may be one reason why physical activity helps protect mental health, even when diet quality is poor,” said Dr. Minke Nota, the study’s first author.
The Brain’s Limit: Diet Still Matters
While running improved mood and metabolism, the cafeteria diet blocked the brain’s ability to grow new neurons — a process called neurogenesis — in the hippocampus, the region that regulates learning and emotion. In rats eating a healthy diet, exercise triggered robust neuron growth. In those on junk food, that benefit disappeared.
This finding suggests that while running can lift mood, a poor diet might still limit the brain’s ability to physically renew itself.
Why This Matters
The results highlight a powerful but nuanced message: exercise can buffer some of the brain’s response to an unhealthy diet — but can’t fully erase the damage. It also points to exciting new directions in mental health research. Scientists hope that certain gut metabolites could one day serve as biomarkers or even treatments for mood disorders, offering a new biological link between lifestyle, diet, and emotional well-being.
As the study’s accompanying editorial put it: “Exercise has an antidepressant-like effect in the wrong dietary context — which is good news for those who have trouble changing their diet.”
In a world where fast food and stress are everyday companions, this research offers hope and perspective. Running — or any consistent physical activity — may help rebalance the mind, hormones, and gut, even when our diets aren’t perfect. But for the brain to truly thrive, both movement and mindful eating are key.