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.