110 “lean and healthy” students in their 20's were recruited for the study and split into two groups that each followed a different eating regime:
While all participants consumed a toasted sandwich and milkshake on the first and last day of the experiment, the first group chowed down on Belgian waffles and a main meal, drink and dessert from a fast-food chain over the other six days. The second group otherwise maintained their usual daily diet.
Both before and after the study period, the participants were asked to rate whether they liked or craved sugary foods. Interestingly, the ratings were higher among those who had been eating junk food – even when they were satiated. This group also performed worse in memory and learning tests.
“Junk food may then act to undermine self-control by increasing desire,” the researchers explained in a statement.
“When we see cake, chocolate or crisps, for example, we remember how nice they are to eat. When we are full the hippocampus normally suppresses 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.”