Study: Junk food can wreak havoc on growing brains, affecting memory

New research finds that eating a high-fat, high-sugar Western diet (read: junk food) during a critical period of brain development can harm long-term memory. The damage is caused by a decrease in a neurotransmitter linked to Alzheimer's disease, and switching to a healthier diet in early adulthood does not reverse the memory damage.

Synonymous with junk food, the Western diet is well-deservedly criticized for its impact on physical and mental health. Broadly speaking, the Western diet is one rich in processed foods, saturated fats, and simple sugars and is associated with excess caloric intake, obesity, and metabolic dysfunction. But what impact does a Western diet have on growing brain function? The teenage brain is a "work in progress." Between the ages of 10 and 24, the brain undergoes significant changes that are largely influenced by factors such as genetics, hormones, sleep, and diet. Previous research has shown that diet, especially a Western diet, is associated with cognitive impairment. A new study from researchers at the University of Southern California (USC) explores how a high-fat, high-sugar diet can damage adolescent brains and affect memory. They studied young and adolescent rats.

"We saw not only in this paper but also in some of our other recent work," said Scott Kanoski, a professor of biological sciences at USC and corresponding author of the study. If these mice were raised on this junk food diet, these memory impairments would not disappear if they were simply fed healthy food, and unfortunately, these effects would persist into adulthood."

Researchers fed rats either a junk food "cafeteria-style" diet to mimic Western eating habits, or standard chow. Rats fed a diet equivalent to a Western diet had free access to high-fat, high-sugar foods, potato chips, chocolate peanut butter cups, and high-fructose corn syrup drinks. Rats consumed their respective chows from postnatal day 26 to postnatal day 56 (i.e., during juvenile and adolescent development). At this point, rats on a Western diet will be switched to a healthy dietary intervention. The purpose of the experiment was to test explicit memory that relies on the hippocampus of the brain. This is the long-term memory of everyday events that occurred at a specific time and place (for example, a memory of a seventh birthday party).

The memory test involved letting the rats explore new objects in different locations. A few days later, the mouse entered an almost identical scene again, only with the addition of a new object. It was found that a Western-style diet led to impairments in explicit memory that persisted even after starting a healthy diet. Rats on the Western-style diet showed greater familiarity with the scene than rats on the control diet, who showed difficulty remembering which object they had seen before and where. A Western diet did not significantly alter the results of memory tests used to assess areas of the brain other than the hippocampus.

Researchers are most interested in understanding how a Western-style diet affects levels of the neurotransmitter acetylcholine, which is critical for memory and learning. Normal memory function in the hippocampus depends on acetylcholine, and levels of acetylcholine tend to be particularly low in the brains of people with Alzheimer's disease. Acetylcholine levels were measured in both groups of mice as they completed memory tests and postmortem studies.

"Acetylcholine signaling is a mechanism that helps them (rats) encode and remember these events, similar to human 'explicit memory', which allows us to remember past events," said lead author Anna Hayes. "This signaling does not appear to occur in animals raised on a high-fat, high-sugar diet."

Changes in the gut microbiome were observed early in the Western dietary intervention but were corrected after the introduction of a healthy diet. Microbiome health was restored, but memory impairments persisted, suggesting that acetylcholine, not the microbiome, was causing these impairments. Interestingly and importantly, the persistent memory impairment caused by the Western diet occurred without effects on body weight and metabolism. This suggests that diet early in life can have long-term effects on brain function, independent of obesity.

While switching junk food to a healthy diet does not alleviate the memory deficits associated with a poor diet, researchers can reverse the deficits using drugs that mimic acetylcholine. Injecting the drug directly into the hippocampus before taking a memory test improved memory performance induced by a Western diet.

The significance of the research results is obvious. Young people, especially teenagers whose brains are in critical stages of development, are at risk of causing long-term damage to brain function if they consume a Western-style diet.

"I don't know how to say this without sounding like Cassandra and doom, but unfortunately, some things may be easier to reverse in adulthood than if they happened in childhood," Karnowski said. It's not that easy to reverse."

• In Greek mythology, Cassandra was the daughter of the Trojan king and queen. The god Apollo was fascinated by her beauty and gave her the power of prophecy. When she rejected Apollo, Apollo cursed her so that no one would believe her (often dire) prophecies, including warning the Trojans not to accept the infamous Trojan Horse as a gift from their Greek rivals.

More research is needed on how to reverse memory problems in adolescence caused by a Western diet.

The study was published in the journal Brain, Behavior, and Immunity.