In a study of 14 female students attending Laval University in Quebec, researchers found that taxing mental effort caused the subjects to eat more, significantly more. The study, published in Psychosomatic Medicine, was initially intended to determine if more study of the relationship between mental effort and eating was worth further pursuit. But, with only 14 students enrolled, they actually discovered a statistically significant effect of what they call “knowledge-based work” (KBW), and so were able to publish even their initial efforts.

Tests were administered at a set point in the menstrual cycle to control for its effect on diet. The students were given a standard breakfast to start the day and, two-and-a-half hours later, asked to perform one of three activities: relaxing in a chair, reading and summarizing some text, or performing a mentally challenging computer task in the presence of distractions. Each student performed each of the three activities on different days, allowing the test conditions to be compared within individuals.

During the testing, heart rate and blood pressure were monitored, and blood samples taken at regular intervals. When the test was over, surveys were administered that included questions on the perceived challenge of the task as well as how hungry the students were. When all that was finished, the students were given access to an unlimited buffet, and their intake was carefully monitored.

The results were clear-cut. Compared to the instances when they sat around relaxing, the same students ate 848 kiloJoules more food after completing the reading/writing task. When faced with a computer, that number went up to 1057kJ. This happened despite the fact that the brain is pretty active even when a person is not mentally engaged, meaning that the KBW didn’t burn off much more energy than sitting around.

An abstract of the study is available at Psychosomatic Medicine