Junk food damages growing brains so severely it affects long-term memory

Eating a fatty, sugary Western diet (read: junk food) during the crucial years of brain development impairs memory long-term, new research has found. Caused by a reduction in a neurotransmitter associated with Alzheimer’s disease, the memory impairment wasn’t reversed by switching to a healthy diet in early adulthood.

Synonymous with junk food, the Western diet has deservedly gotten a bad rap regarding its impact on physical and mental health. Broadly defined as a diet high in processed foods, saturated fats, and simple sugars, it’s associated with excessive caloric intake, obesity, and metabolic dysfunction. But how does eating a Western diet impact the functioning of a growing brain?

Adolescent brains are a ‘work in progress’. Between the ages of 10 and 24, the brain undergoes significant changes that are greatly influenced by factors such as genetics, hormones, sleep, and diet. Previous studies have linked diet, particularly the consumption of a Western diet, with cognitive dysfunction. A new study by researchers from the University of Southern California (USC) has examined how a high-fat, sugary diet damages the teen brain, affecting memory. They conducted their research on juvenile and adolescent rats.

“What we see not just in this paper, but in some of our other recent work, is that if these rats grew up on this junk food diet, then they have these memory impairments that don’t go away,” said Scott Kanoski, professor of biological sciences at USC and the study’s corresponding author. “If you simply put them on a healthy diet, these effects unfortunately last well into adulthood.”

Rats were fed either a junk food ‘cafeteria-style’ diet to model a Western diet or standard chow. Those on the equivalent of the Western diet were given free access to high-fat, high-sugar chow, potato chips, chocolate-covered peanut butter cups, and high-fructose corn syrup beverages. The rats ate their respective diets from postnatal day 26 up to postnatal day 56, representing the juvenile and adolescent periods of development. At this point, rats on the Western diet were switched to a healthy diet intervention. Experiments were conducted to test episodic memory dependent on the brain’s hippocampus. That’s the long-term memory of everyday events that occurred at particular times and places (e.g., memories of your seventh birthday party).

The memory test involved letting the rats explore new objects in different locations. Days later, the rats were reintroduced to a near-identical scene, except for the addition of one new object. The Western diet was found to cause impairments to episodic memory that continued after the healthy diet was commenced. Compared to rats on the control diet, who showed familiarity with the scene, rats on the Western diet showed signs they couldn’t remember which object they’d seen before and where they’d seen it. The Western diet didn’t significantly change the results of memory tests designed to assess areas of the brain other than the hippocampus.

The researchers were most interested in seeing how eating a Western diet affected levels of the neurotransmitter acetylcholine, which is essential for memory and learning. The hippocampus relies on acetylcholine for proper memory function, and acetylcholine levels also tend to be particularly low in the brains of people with Alzheimer’s disease. Both groups of rats’ acetylcholine levels were measured while they were completing memory tests and in post-mortem studies.

“Acetylcholine signaling is a mechanism to help them [the rats] encode and remember those events, analogous to ‘episodic memory’ in humans that allows us to remember events from our past,” said lead author Anna Hayes. “That signal appears to not be happening in the animals that grew up eating the fatty, sugary diet.”

Changes to the gut microbiome were observed early in the Western diet intervention but were corrected when the healthy diet was introduced. That microbiome health was restored but memory impairment persisted suggests that acetylcholine, not the microbiome, drove these impairments. Interestingly – and importantly – the persistent Western diet-induced memory impairments occurred in the absence of an effect on body weight and metabolism. This signifies that diet in early life can have a long-lasting effect on brain function independent of obesity.

While changing from junk food to a healthy diet didn’t mitigate memory deficits related to the poor diet, the researchers could reverse the impairment using drugs that mimic acetylcholine. The drugs were injected directly into the hippocampus before the memory test, improving Western diet-induced memory performance.

The study’s findings have obvious implications. Younger people, particularly adolescents whose brains are undergoing critical development, risk long-term damage to brain functioning if they eat a Western diet.

“I don’t know how to say this without sounding like Cassandra* and doom and gloom, but unfortunately, some things that may be more easily reversible during adulthood are less reversible when they are occurring during childhood,” Kanoski said.

More research is needed to investigate how Western diet-induced memory problems caused during adolescence can be reversed.

* In Greek mythology, Cassandra was the daughter of the king and queen of Troy. The god Apollo, who was enamored of her beauty, gave her the power of prophecy. When she rejected him, he cursed her so that no one would believe her (often dire) predictions, which included warning the Trojans not to accept the infamous wooden horse from their Greek opponents.

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

Source: USC