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Exercise reverses junk food's damage to gut and mood, new study finds

By isabelle // 2025-10-30

• A junk food diet caused depression-like behavior and altered gut chemistry in rats.
• Exercise reversed the behavioral symptoms and repaired the harmful gut changes.
• The unhealthy diet also caused a severe spike in insulin and leptin levels.
• Physical activity helped normalize these key metabolic hormones.
• However, junk food still blunted the brain's ability to grow new cells from exercise.

Do you ever feel mentally sluggish and down after a stretch of eating junk food? You’re not imagining it, and new scientific research reveals exactly how that processed food damages your system while highlighting exercise as a powerful antidote. Researchers at University College Cork in Ireland have discovered that a junk food diet creates more than 100 harmful changes in the gut chemistry of male rats, leading directly to depression-like behavior. The groundbreaking part? Simple exercise reversed most of this damage. The study, published in the journal Brain Medicine, provides some of the clearest evidence yet of the gut-brain connection. For about two months, scientists fed one group of rats a rotating “cafeteria diet” of high-fat, high-sugar human junk foods like chocolate, peanut butter, and jam alongside their regular chow. Another group ate only healthy food. Within each group, some rats remained sedentary while others had access to a running wheel. The results were dramatic. The junk food diet dramatically altered the gut’s chemical environment, shifting 100 out of 175 measured compounds in the digestive system of sedentary rats. These changes correlated strongly with observable despair. In a standard swim test used to measure depression-like behavior, the sedentary junk food rats quickly gave up and floated passively. Their active, healthy-eating counterparts kept swimming.

The exercise effect

The most compelling finding was how exercise intervened. When the junk-food-eating rats had access to a running wheel, their behavior and gut chemistry shifted dramatically back toward normal. They swam significantly more and floated less. At the molecular level, exercise restored three key gut compounds that the junk food had depleted: anserine, indole-3-carboxylate, and deoxyinosine. These substances are no minor players. Past research has linked them to mood regulation and cognitive function. Their restoration through physical activity, even while the rats continued eating poorly, points to a potent mechanism through which exercise protects mental health. The research suggests that working out helps the gut rebalance what a poor diet throws out of whack.

Hormonal havoc and recovery

The damage from the junk food diet was not limited to the gut. The sedentary rats on the unhealthy diet also suffered from metabolic chaos, with their insulin levels rising to two or three times higher than the control group. Leptin, another key hormone, also spiked. This hormonal dysregulation is a known precursor to metabolic resistance, which is linked to both depression and memory problems. Once again, exercise came to the rescue. The running rats saw their elevated insulin and leptin levels move back toward healthier ranges, despite their continued consumption of junk food.

A limit to exercise's power

Despite these impressive recoveries, the study did reveal a crucial limit to what exercise can fix. The researchers found that the growth of new brain cells in the hippocampus, a process called adult hippocampal neurogenesis, was strongly boosted by exercise, but only in the rats eating a healthy diet. Junk food substantially blunted this brain-building benefit. While exercise nearly doubled new brain cell production in healthy-eating rats, it produced only a modest increase in those eating junk food. This finding delivers a critical message: while exercise can powerfully counteract the mood and metabolic damage of a poor diet, it cannot fully compensate for it when it comes to building new brain cells. Diet quality still matters profoundly for optimal brain health. The research offers a powerful, two-pronged lesson for anyone struggling with diet and mental wellness. Exercise provides a formidable defense against the mental and metabolic consequences of poor nutrition, actively repairing the gut and hormonal systems. However, for those seeking the highest level of cognitive resilience and brain repair, a foundation of healthy food remains non-negotiable. Your workout can clean up the mess, but it works best in a clean house. Sources for this article include: StudyFinds.org GenomicPress.KGLMeridian.com Earth.com

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