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New research has discovered that consuming a fatty, sugary Western diet (also known as junk food) during crucial years of brain development can lead to long-term memory impairment. The impairment is caused by a decrease in a neurotransmitter that is linked to Alzheimer's disease. Even if a person switches to a healthy diet in early adulthood, their memory impairment may not be reversed.
The Western diet, which is characterized by high consumption of processed food, saturated fats, and simple sugars, is known to cause excessive caloric intake, obesity, and metabolic dysfunction. However, it was unclear how it affects the growing brain.
Between the ages of 10 and 24, the brain is significantly impacted by genetics, hormones, sleep, and diet. A study by researchers from the University of Southern California (USC) examined how a high-fat, sugary diet damages the teen brain, affecting memory. The study was conducted on juvenile and adolescent rats.
The rats were fed either a junk food 'cafeteria-style' diet to represent a Western diet or standard chow. Rats on 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, which is the juvenile and adolescent period 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, which is responsible for long-term memory.
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. Rats on the Western diet showed unfamiliarity with the scene and could not remember which object they had seen before and where they had seen it. In comparison, rats on the control diet showed familiarity with the scene. The Western diet did not significantly change the results of memory tests designed to assess areas of the brain other than the hippocampus.
The researchers were 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 are 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. The researchers found that the signal that helps encode and remember events ("episodic memory") was not occurring in rats 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. However, despite the restoration of microbiome health, memory impairment persisted. This suggests that acetylcholine, not the microbiome, drove these impairments. The study found that diet in early life can have a long-lasting effect on brain function independent of obesity.
Though changing from a junk food to a healthy diet did not mitigate memory deficits related to the poor diet, researchers were able to 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 important implications. Younger people, particularly adolescents whose brains are undergoing critical development, risk long-term damage to brain functioning if they eat a Western diet.
