If striving for a perfect diet leaves you feeling stressed and hungry, it might help to focus on balance instead of perfection. Of course, there’s also good reason to avoid a steady stream of junk food: Besides upping your risk of chronic illness, regularly eating foods high in fat and sugar can trigger changes throughout your body, including the gut.
The adult hippocampal neurogenesis, which regulates “anxiety-like behavior, pattern separation, and spatial memory,” is “sensitive to external factors including diet and exercise,” according to the study’s text. In the hopes of better understanding that specific connection, a team of neuroscience and microbiome researchers at University College Cork in Ireland recently conducted an animal study.
The findings, published in October 2025 in the journal Brain Medicine, reveal that regularly eating junk food causes over 100 measurable gut changes in total, and may be linked to low mood and memory problems. However, there was some good news, as regular exercise may counter many of those effects.
For the study, researchers fed 48 adult male rats either a standard diet or a “cafeteria-style” diet for about two months. “The cafeteria diet consisted of several different food items high in fat and/or sugar, with two high-fat and two high-sugar items given each day in rotation in addition to standard chow, for the duration of the experiment,” the study states.
The team then analyzed the contents of the animals’ intestines, where gut bacteria produce hundreds of compounds that influence metabolism and brain function. Out of 175 compounds examined, junk food altered 100, according to the researchers.
Rats on the high-fat, high-sugar diet also showed more depression-like behavior in a swim test, floating passively instead of swimming. The study also found that junk food may divert tryptophan—the amino acid needed to make the “feel good” neurotransmitter serotonin—away from the brain, potentially dampening mood.
Access to running wheels restored both their behavior and gut chemistry to near-normal levels: “Exercise exerted antidepressant-like effects in cafeteria diet–fed animals,” the study authors wrote. While exercise alone changed very little in healthy rats, the team notes that it reversed dozens of those chemical disruptions in the rats routinely fed junk food.
Additionally, rats eating junk food showed spikes in insulin and leptin, hormones that regulate metabolism and appetite. Exercise helped normalize these levels—especially significant since prolonged hormone resistance has been linked to depression and cognitive decline.
Though the study was not conducted with humans, 2024 research published in the journal Nature—with support from the National Institutes of Health (NIH)—confirmed that “almost all human genes known to be associated with diseases have counterparts in the rat genome … confirming that the rat is an excellent model for many areas of medical research.”
While exercise can’t completely erase the impact of an unhealthy diet, the research underscores a powerful interaction between diet, movement, metabolism, and mental health. As one of the study’s authors recommends: “Begin with movement. Walking, cycling, or light exercise can begin to shift your biology in a positive direction. As your body responds, small improvements in diet can then build on that momentum.”
For daily wellness updates, subscribe to The Healthy by Reader’s Digest newsletter and follow The Healthy on Facebook and Instagram. Keep reading: