Switching from a healthy diet to a western diet (high fat, high added sugar) for a little as one week can significantly impair cognitive function and encourage people to eat more even when they’re full.

Disruption in the hippocampus, a region that is known to have a major role in learning and memory, seems to be the likely cause.

It’s not the first time something like this has been suggested. Research in the past found that when animals are fed a Western-style diet (rich in saturated fat and added sugar), they show impairment in memory and learning tests. There is a growing body of evidence suggesting that the same conclusion applies to humans and that hippocampal lesions can deregulate a person’s appetite.

Psychologists at Macquarie University in Sydney wanted to put this to the test and enlisted 110 young, lean students, aged 20 to 23, who generally ate a healthy diet.

Half of the students were randomly assigned to a junk food diet for an entire week, while the other half carried on with their normal diet.

The participants in the Western-style diet group had to have a breakfast of a toasted sandwich and a milkshake, high in saturated fat and added sugar, or Belgian waffles, as well as one main meal and a dessert from a popular fast-food chain. Bearing these changes aside, the students were asked to otherwise maintain their normal diet and lifestyle.

At the end of the study, the researchers found that those on the Western-style diet had an appetite for palatable food such as snacks and chocolate even when they were full. They also scored worse on memory tests.

“When we see cake, chocolate or crisps, for example, we remember how nice they are to eat.  When we are full the hippocampus normally supresses these memories, reducing our desire to eat.  We found that lean healthy young people exposed to one week of a junk food diet developed impaired hippocampal function and relatively greater desire to eat junk food when full.  Junk food may then act to undermine self-control by increasing desire,” the researchers stated in a press release.

These results seem to indicate that junk food might cause disruption in the hippocampus, impairing memory and making it harder to resist the temptation to eat even more junk food, which in turn generates more damage to the hippocampus and triggers a vicious cycle of overeating. The more people craved for palatable food when full, the more impaired their hippocampal function was, judging from memory tests.

“More broadly, this experiment, alongside those from the other animal and human studies cited here, suggests that a WS-diet causes neurocognitive impairments following short-term exposure,” the authors concluded.

Western-style diets, characterized by foods high in sugar, salt, and fat, as well as protein from red meat (i.e. burgers, processed meat, ready meals, fries, etc), have been previously associated with the development of obesity-related diseases such as type 2 diabetes, cardiovascular disease, and high blood pressure.

The authors of the new study, which was published in Royal Society Open Science, think that there will come a time when authorities will be pressured to impose restrictions on processed food, similarly to how some policies in place today deter smoking and drinking alcohol.

Another study published last month showed how sugar can trigger changes in the brain similarly to an addiction. After just 12 days of being on a high sugar diet, participants suffered major changes in the brain’s dopamine and opioid systems.