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The Impact of Diets High in Fat and Sugar on Memory and Impulsivity in Young Adults

A tray with a burger, chips and a cup of coke.
Credit: Christopher Williams / Unsplash.
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Research has consistently found Western-style diets, which are high in added sugar and saturated fat, damage a section of the brain called the hippocampus, causing poorer memory and increased impulsivity in animals.

Macquarie University Clinical Neuropsychologist Dr Heather Francis and her colleagues have been studying the links between highly processed food, memory and impulsivity in young adults.

Their work has shown that even brief exposure to a Western-style diet can cause a decline in memory performance compared to a control group.

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So far, memory and impulsivity have been examined independently in humans. The new paper, published in science journal PLoS One with authors from Macquarie University, the University of Sussex, and the Centre for Dementia Studies at Brighton and Sussex Medical School, aimed to investigate how they are interrelated.

During the latest study, participants filled out questionnaires on their eating habits, and a combination of questionnaires and different tasks were used to test impulsivity (the ability or lack thereof to say no to something) and everyday memory. Everyday memory covers areas such as forgetting details of what someone has said, asking someone the same question twice, or forgetting where items are kept.

We have consistently found that highly processed food damages the hippocampus. This affects your memory and makes you more impulsive, and therefore more likely to eat more of that same food.

Dr Francis says participants whose diets were high in saturated fat and added sugar reported poorer everyday memory function and greater difficulties with inhibition.

They also performed worse on measures of hippocampal-sensitive memory, including a spatial memory task and a recall task, but were not significantly worse on measures of impulsivity or response inhibition.

“Impulsivity as a personality trait makes you more likely to make poor food choices, but our data also supports the idea that we have a vicious cycle at play,” she says.

“We have consistently found that highly processed food damages the hippocampus. This affects your memory and makes you more impulsive, and therefore more likely to eat more of that same food.”

Memory and appetite

The link between impulsivity and poor food choices seems clear enough: you see food and you are tempted to eat it, even if you know logically you don’t need to. If you are more impulsive, you are more likely to give in.

But the connection between memory and appetite is perhaps less obvious.

Neuropsychologists were able to collect a wealth of information about the hippocampus and its links to memory and appetite regulation from a patient in the 1950s who underwent a temporal lobe resection, also known as a lobotomy.

The botched procedure left Patient HM with a damaged hippocampus, destroying his ability to form new memories.

Unexpectedly, it also left him with impaired eating behaviours. He would eat a meal as normal, but if someone asked him if he was hungry five minutes later, he would not remember having eaten and would readily eat another full meal.

This showed that the physical feeling of a full stomach is not the sole driver of appetite – or that our brains can mediate or even override that feeling.

Patient HM’s case is an extreme one, but it supports the idea that memory is involved in appetite regulation.

The connection between diet and Alzheimer’s disease

There is already a well-established relationship between the Western-style diet and neurodegeneration and cognitive decline in older age.

Long-term international studies of large cohorts of older adults have shown that the Mediterranean diet, which is low in processed foods, saturated fats and added sugar, tends to result in both better memory in middle to older age and a lower risk of developing Alzheimer’s disease.

Tellingly, Alzheimer’s is a disease of the hippocampus.

Dr Francis says the next questions to answer are which bodily mechanisms are causing the impact on memory and impulsivity, and why the effects are already visible in young adults.

“Animal studies indicate neuroinflammation may be the cause, but other possibilities include impaired blood/brain barrier, kynurenine pathway metabolites, and changes to the gut microbiome,” she says.

“These are all risk factors for neurodegenerative conditions such as Alzheimer’s disease.”

Our unhealthy food environment

Dr Francis says she doesn’t like anyone to read study findings like these and feel they are at fault.

“Some of the things at play here are outside the control of individuals,” she says.

“The food environment we live in constantly bombards us with processed food options that are not conducive to healthy eating, and that erodes our ability to regulate our food intake.

“It also comes down to availability and affordability. We take it for granted how easy it is in cities to get fresh fruit and vegetables, but food choices in more rural and remote areas often aren’t there or are prohibitively expensive if they are.

“If it’s cheaper to buy cola than milk, you’re probably going to buy the cola. If it’s $10 for a punnet of strawberries, you’re probably not going to buy the strawberries.

“What studies like this do show is how important it is for governments to potentially step in and make it easier to eat healthily, in much the same way as they have stepped in on smoking.

“It took a long time to amass the evidence against smoking, but once it was there, governments had the impetus to help people make healthier choices.”

Reference: Yeomans MR, Armitage R, Atkinson R, Francis H, Stevenson RJ. Habitual intake of fat and sugar is associated with poorer memory and greater impulsivity in humans. Kubis HP, ed. PLoS ONE. 2023;18(8):e0290308. doi: 10.1371/journal.pone.0290308

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