Emerging evidence in humans suggests a typically Western high-fat, high-sugar 'junk food' diet can quickly undermine your brain's appetite control.
After indulging in a week-long binge of waffles, milkshakes and similarly rich foods, researchers in Australia found young and healthy volunteers scored worse on memory tests and experienced a greater desire to eat junk food, even when they were already full.
The findings suggest something is amiss in the hippocampus - a region of the brain that supports memory and helps to regulate appetite. When we are full, the hippocampus is thought to quieten down our memories of delicious food, thereby reducing our appetite.
When it's disrupted, this control can be seriously undermined.
Over the years, extensive research on juvenile mice has found the function of the hippocampus is very sensitive to 'junk food', but this has only recently been observed in young and healthy humans.
In 2017, after a week of Western-style breakfasts of toasted sandwiches and milkshakes, researchers found participants performed worse on learning and memory tests which are typically dependent on the hippocampus.
Now, in this latest study the team has found that not only do such high-fat, high-sugar diets impair memory in humans, they also appear to directly affect our ability to control our appetite.
"As this is an emerging area, and with much still to be learned about how these processes inter-relate, our conclusions are of course tentative," the authors admit.
Nevertheless, the foundations for claims like this are quite extensive, especially amongst the animal literature. What's more, the results, even for a small sample size, are compelling.
After just one week, the authors say the changes they saw in appetite control were "strongly correlated" with hippocampal-dependent learning and memory measures.
"More broadly," they write, "this experiment, alongside those from the other animal and human studies cited here, suggest that a Western-style diet causes neurocognitive impairments following short-term exposure."
In the study, researchers randomly told more than 100 young, lean and healthy participants to either start a week-long fast food diet or continue their usual eating habits. Scoffing all those rich foods seems to have been hard for some - by the follow-up session at the end of the month, eight people had dropped out, leaving a sample of 102 participants.
On the first day and the last day, participants were given a toasted sandwich and a milkshake in the lab. But for the rest of the study period, participants in the junk food group were instructed to eat two Belgian waffles at least four times a week, and two fast food meals on at least two occasions per week.
Before and after each breakfast in the lab, participants were also given a test on their desires. First, they were given six food samples and asked to rate them on a scale of how much they would like to eat them at that particular moment.
Then, they were asked to consume each food and rate how much they liked it and how much more of it they could eat right then.
Not only did this diet correlate with a clear weakening of appetite control, the authors found it was also linked to a decrease in learning and memory scores designed to test hippocampal function.
Three weeks later, when the group returned for follow-up testing, the differences had disappeared, as several animal models have previously suggested they would.
While this might indicate briefly impaired function of the hippocampus, the true mechanisms at play are a mystery, and the authors admit there are other possible explanations.
Rachel Batterham, a researcher in obesity, diabetes and endocrinology who was not involved in the study, told The Guardian that while the new data adds support to previous animal studies, we really need more research beyond mere correlation.
"The mechanisms at work remain to be elucidated and will require further research with the application of more sophisticated neuroimaging methods," she said.
The study was published in the Royal Society Open.