Experimental studies on rats have shown that eating food not only leads to satiety but also affects memory. High-calorie foods activated the vagus nerve, which triggered the release of acetylcholine in the hippocampus, helping to fix memories of feeding locations. However, in animals that were fed fast food from an early age, this chain was disrupted, resulting in impaired spatial memory.

The connection between the gut and the brain regulates appetite, and recent research has demonstrated that the vagus nerve, which maintains this link, also influences higher cognitive functions. Until now, it was unclear how the signal of satiety is transmitted to the hippocampus and how this process is related to memory.

During the experiment, rats had optical fibers implanted in the dorsal hippocampus, and researchers recorded fluorescent signals during acetylcholine release. To test signal transmission, one group of animals had their vagus nerve severed, another group had neurons in the medial septum responsible for acetylcholine synthesis destroyed. A third group was fed fast food from day 26 of life for a month, after which they were switched to a standard diet for further testing.

Spatial memory was assessed using a table with 18 identical holes around the edge, with a sweet treat hidden under one of them. For orientation, contrasting geometric shapes and a plush unicorn were used as visual cues.

The results showed that consuming high-calorie food caused a sharp increase in acetylcholine levels in the hippocampus, but only when the rats ate calorie-rich products such as sucrose solution or corn oil. No such reaction was observed when they drank water with artificial sweetener.

During the final test, the food was removed from the maze. In healthy animals, an acetylcholine surge occurred when they found the correct hole by memory, using visual markers for guidance. Rats with disrupted signal transmission or damaged neurons failed the task and showed no increase in acetylcholine levels. Without the signal from the stomach, they could not recall the correct location and checked the holes at random.

Fast food consumption had an effect on the brain similar to surgical intervention. Rats raised on chips also did not release acetylcholine after eating, and their memory did not recover even after returning to a healthy diet.

The study demonstrated that the process of eating influences cognitive functions, not just the regulation of hunger and satiety. The "gut-brain" axis, through the vagus nerve, medial septum, and hippocampus, controls spatial memory formation, while unhealthy food disrupts this system and leads to memory impairment.