Diets high in fat and sugar appear to harm cognitive function

A new study published in the International Journal of Obesity adds to growing evidence that unhealthy diets don’t just affect the body—they can also harm the brain. Researchers at the University of Sydney found that people who consumed more fatty and sugary foods performed worse on a virtual navigation task that depended on the brain’s memory system. The findings suggest that diets high in saturated fats and refined sugars may impair the hippocampus, a brain region essential for learning and remembering spatial routes.

The study was inspired by previous animal research showing that high-fat, high-sugar diets impair hippocampal function. In rats, these diets have been shown to quickly interfere with spatial memory, even before weight gain or other metabolic changes occur. While some human studies have linked similar diets to cognitive decline, few have directly tested the effects on spatial navigation—a skill that depends heavily on hippocampal function.

“Past research has shown that high fat high sugar diets can impact the brain, particularly a region called the hippocampus,” explained study author Dominic Tran, an Australian Research Council DECRA Research Fellow. “One thing the hippocampus is important for is spatial navigation—the ability to remember places and how to get there. I wanted a way to test spatial navigation that was as close to real life as possible. So rather than administer a memory task on a computer monitor. I used a virtual reality maze to assess spatial memory!”

In the immersive virtual reality task, participants were asked to navigate a maze-like environment, using visual landmarks to learn the location of a hidden target. The task required them to remember and return to that target location later—much like remembering how to get back to your parked car after shopping. By relying on this real-world navigation analogy, the researchers were able to assess participants’ ability to form a spatial map of their surroundings.

The participants were 120 university students in Sydney, but due to issues like motion sickness, incomplete data, and technical problems, only 55 completed the full experiment. The researchers also assessed each participant’s diet using a questionnaire that measured how often they consumed foods high in fat and sugar, such as fast food, desserts, and sugary drinks. They also measured body mass index and working memory to account for general cognitive ability and physical health.

During the training phase of the task, participants were asked to find a treasure chest hidden in a consistent location within the virtual maze. They did this six times, starting from the same place and using visual cues to guide them. Over time, most participants improved at finding the chest more quickly, indicating learning. But when researchers looked more closely, they found that participants who reported eating more high-fat and high-sugar foods didn’t improve as much over time. Their learning curve was flatter, meaning they didn’t get better at the task with practice.

This pattern became more pronounced in the final test phase. In this stage, the planks that formed the maze were removed, leaving only the surrounding landmarks as reference points. Participants had to rely entirely on spatial memory—remembering where the chest had been located in relation to these external cues. Those who reported consuming more unhealthy foods finished farther away from the correct location than those with healthier diets. This difference remained even after adjusting for body weight and working memory, suggesting a specific link between diet and spatial memory.

The results highlight that “eating healthy is important for your physical health and brain health,” Tran told PsyPost.

The findings closely mirror results from rodent studies. In those studies, rats on high-fat, high-sugar diets struggle with tasks that rely on the hippocampus but perform normally on tasks involving other parts of the brain. That pattern appears to hold for humans as well: participants in this study had impaired spatial learning and memory, but their general working memory was unaffected. This supports the idea that unhealthy diets may selectively impair the hippocampus rather than broadly lowering cognitive performance.

One of the strengths of the study was its use of virtual reality, which allowed researchers to simulate real-world navigation more closely than traditional lab-based tasks. Participants had to learn routes from an eye-level perspective and move through a three-dimensional environment, which made the task more comparable to everyday navigation. However, this advantage came with a drawback: nearly 40 percent of participants experienced motion sickness or other issues that prevented them from completing the task.

“I think one of the biggest strengths of the study is in using virtual reality to truly assess people’s spatial navigation ability,” Tran said. “It’s a really sensitive tool that tells us about how the hippocampus is functioning. However, VR was a bit of a double-edged sword as a lot of people got motion sick during the task. I hope improvements in VR technology mean that we can use similar tasks with less cybersickness side effects.”

There are some other limitations to consider. The study relied on self-reported dietary data, which can be prone to bias. The sample size was also relatively small and mostly consisted of young, healthy university students, which may limit the generalizability of the findings. Larger studies with more diverse populations would help confirm whether these effects hold in broader groups, including those with more varied diets and higher levels of dietary risk.

Because the study was correlational, it can’t confirm a cause-and-effect relationship between diet and impaired spatial memory. It’s theoretically possible, though unlikely, that people with poorer spatial skills might be more drawn to high-fat, high-sugar foods. A more plausible explanation, based on extensive animal research, is that these diets impair hippocampal function. That possibility is supported by findings that hippocampal damage in animals leads to similar spatial memory deficits, and that switching from an unhealthy to a healthy diet can reverse those effects. The researchers say that future human studies using diet interventions could help test this causal direction more directly.

“I am really interested in testing if the memory impairments recover and uncovering the conditions around this recovery,” Tran said. “For example, how long does it take to recover and can we speed up recovery.”

The study, “Consumption of a diet high in fat and sugar is associated with worse spatial navigation ability in a virtual environment,” was authored by Dominic M. D. Tran, Kit S. Double, Ian N. Johnston, R. Frederick Westbrook, and Irina M. Harris.