How diet, early chemical exposure, genetic risk, and biological sex combine to shape lifelong memory function remains a great scientific mystery. A new study published in Neurotaxology reveals that consuming a high-fat diet and surviving early exposure to common pesticides alters memory in vastly different ways depending on the animal’s genetic makeup and gender. The results suggest that nutritional advice to protect the brain should be highly individualized rather than provided as a universal rule.
The brain constantly adapts to external forces throughout life. During early childhood, the brain rapidly forms synaptic connections, making it highly sensitive to industrial chemicals in the environment. One of the most common environmental risks comes from pesticides, which are frequently used in agriculture and public health interventions.
Chlorpyrifos is a common organophosphate insecticide frequently used in commercial agriculture. It attacks the nervous system and kills insects, but can also affect humans and other mammals. This chemical blocks an enzyme responsible for removing a particular neurotransmitter called acetylcholine.
Acetylcholine functions as a chemical messenger deeply involved in learning, memory, and muscle movement. When the removal enzyme is blocked, acetylcholine accumulates, leading to severe overstimulation of the nervous system. Although the European Union has banned chlorpyrifos in recent years, it is still widely used in many other countries.
In addition to chemical exposure, daily diet also affects brain function in animals. Researchers often use high-fat diets in laboratory settings to reproduce metabolic diseases and study declines in brain function in rodents. Fat-rich diets are frequently associated with immune system inflammation and memory loss, but experimental results are not always the same between different studies.
Beyond diet and pesticides, genetics also play a major role in how the brain ages over time. The APOE gene (apolipoprotein E) is one of the most well-studied genetic risk factors for memory decline in humans. This gene provides instructions for making proteins that help transport cholesterol and other essential fats through the bloodstream and central nervous system.
People naturally have different versions of this gene, known as alleles. The most common version across the human population is APOE3, which works well for most people. However, another version, called APOE4, is widely recognized as a major risk factor for neurodegenerative diseases such as Alzheimer’s disease. The APOE4 version changes the way fat binds and moves through the blood, resulting in metabolic differences that researchers are still trying to understand.
These environmental and genetic factors are typically studied separately, so researchers wanted to understand what happens when an animal experiences all of these in succession. Laia Guardia Escort, a behavioral researcher at Spain’s Rovira y Virgili University, led a team investigating whether certain genes, gender, high-fat diets, and exposure to pesticides influence memory performance in adults. Investigating these combined effects provides a more realistic model of everyday environmental risks.
The researchers used special laboratory mice for their experiments. These mice were genetically modified to carry the human version of the APOE gene, resulting in mice carrying either the human APOE3 or APOE4 alleles. These were bred in parallel with a control group with normal, unaltered mouse genes.
Shortly after birth, the pups were divided into different experimental groups. Between 10 and 15 days after birth, some mice were given a drop of chlorpyrifos orally. This particular time frame was chosen because it reflects milestones in human brain development immediately after birth.
The dose of chlorpyrifos was relatively low. This concentration was not high enough to cause systemic toxicity or immediate behavioral poisoning in animals. The goal was to simulate the low-level exposures that growing children may experience through everyday environmental residues found in unwashed foods.
The mice were then grown under standard laboratory conditions until they reached 3 months of age. At this point, the researchers changed their eating routines. Half of the mice continued to eat a standard lab diet, while the other half were given a highly specialized diet consisting of more than 60 percent fat.
After eight weeks on these diets, the researchers tracked how much weight the mice gained. As expected, the animals that ate the high-fat diet gained more weight than those that casually ate a normal diet. Male mice also generally gained more weight than female mice.
The mice’s genetic makeup also influenced their physical response to dietary intervention. Mice carrying the APOE3 gene gained the most weight compared to other genetic groups. Previous research suggests that the APOE3 version of the protein is simply more efficient at storing dietary energy as body fat, leading to faster weight gain.
To find out how these variables affected memory, the researchers subjected mice to a classic cognitive test called the Morris water maze. This maze consists of a large circular pool of opaque water with a small platform hidden just below the water’s surface. Although mice are naturally good swimmers, they prefer to stand on solid ground and try to find hidden platforms as quickly as possible.
Over the course of a five-day study, the researchers repeatedly placed animals into the pool from different starting positions. Mice had to rely on visual markers placed on the walls of the room to learn the actual location of hidden platforms. The researchers recorded how long it took the animals to move to safety during each swimming maneuver.
The various groups of mice all successfully learned how to navigate the maze. However, exposure to pesticides during childhood altered learning speed in unexpected ways. Indeed, postnatal chlorpyrifos exposure reduced the time taken by APOE3 males and APOE4 females to find the platform. This means that they temporarily performed better than their unexposed counterparts.
Conversely, exposure to pesticides slowed down the learning process in APOE3 females. This stark contrast highlights that environmental toxins can have very different behavioral effects depending on the biological sex and genetic background of the subject. The researchers suspect that the APOE4 females’ temporary memory improvements may be related to how the insecticide interacts with their naturally low baseline levels of acetylcholine.
After the initial learning phase, the researchers removed the safety platform from the pool completely and tested the animals’ spatial memory retention. They then tracked the time the mice spent swimming in specific quadrants of the pool where the platform was. If it takes a long time to search for the correct area, it indicates a strong spatial memory.
Male mice generally showed very good memory and easily recalled the previous location of the platform. There was one notable exception. APOE4 males on a high-fat diet had difficulty recalling locations during the longest memory test.
APOE4 male mice successfully learned the first task. However, after eating a high-fat diet for two months, they seemed to lose their long-term memory of the location. This highlights the unique vulnerability of this genetic group to metabolic challenges.
Memory retention in female mice varied considerably between groups. APOE4 female mice fed a standard diet had no memory of where the hidden platform was. Unexpectedly, exposing APOE4 females to either a high-fat diet or pesticides improved memory retention compared to the control group.
However, when female mice experienced both pesticide exposure and a high-fat diet at the same time, their memory decreased. This combination worked synergistically to reduce brain function. Men did not show this combined vulnerability, suggesting that women’s brains may be more sensitive to the combined stress of early toxins and later metabolic changes.
The researchers also tested the animals’ cognitive flexibility by moving the platform to the other side of the pool and placing a visible marker directly on top of it. Although all groups managed to complete the task, APOE4 mice took the longest to adapt to the new location. They also swim slower than other genetic groups, suggesting that it is generally difficult to change established behavior.
There are several limitations to consider when interpreting these experimental results. The researchers note that they did not track specific anxious behaviors in the pool, such as swimming only at the edge of the pool to find an exit. High anxiety can look like poor memory during water mazes, which can cloud interpretation of swimming patterns.
The study also did not directly measure hormonal differences, such as the specific levels of estrogen circulating in female mice. It’s entirely possible that environmental chemicals like chlorpyrifos bind to hormone receptors in the brain and alter memory performance. Future studies should directly examine the molecular and cellular physiological changes occurring within these specific brain regions.
Scientists still haven’t figured out why certain combinations, such as high-fat diets and pesticides, harm women more than men, while other unique factors seem oddly protective in the short term. The results show that dietary guidelines aimed at preventing memory loss ultimately need to take into account a person’s biological and genetic profile. General health advice may not be equally beneficial to everyone.
The study, “Effects of high-fat diet on spatial learning and memory: The role of gender, APOE genotype, and postnatal chlorpyrifos exposure,” was co-authored by Laia Guardia Escort, Judith Biosca Brüll, Jordi Blanco, María Cabre, Pia Basaure, Cristian Pérez Fernández, Fernando Sánchez Santed, José L. Domingo and María Teresa Colomina.
