As the obesity epidemic rages in developed economies, explanations have mostly focused on dietary changes and the sedentary lifestyle that time spent on a computer tends to promote. But a preliminary study from a group of researchers in Quebec suggest that working on a computer may have an additional impact on our waistlines: taxing mental effort appears to cause people to eat significantly more food, even though it doesn't burn many more calories than sitting around and relaxing.

The publication, published in a journal called Psychosomatic Medicine, arose from a pilot study that the researchers were performing in order to determine whether a potential connection between mental effort and eating was worth following up on. 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.

The workers recruited from the female student population of Laval University, selecting for individuals with a normal body mass and eating habits. 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.

The authors provide two potential explanations for their findings, both of which may be accurate to varying degrees. The first involves sugar metabolism. The brain is especially reliant on glucose, and the blood tests revealed that both glucose and insulin levels changed during the KBW tasks, while they gently sloped off during the relaxation. The differences weren't consistent—the two KBW tasks sent the levels in opposite directions—but the instability of the levels was large for both of them. The authors suggest that the eating may simply be an attempt to give the body the chance to stabilize blood glucose.

Their other suggestion is that people find KBW stressful. Both the survey results and blood levels of cortisol (a hormone associated with stress) indicated that the KBW tasks took a mental toll. High stress has been associated with increased eating in a variety of contexts, so the upped food intake in this study may simply reflect that.

Clearly, the authors are going to follow this study up with a larger and more diverse student population. But they are able to cite several references that show eating habits change during times of mental exertion, including one that found that university faculty upped their food intake in advance of an National Institutes of Health grant deadline.

What does this mean for an increasingly knowledge-based workforce? The best advice is probably what it's always been: try to mix work with exercise and don't get too stressed.

Psychosomatic Medicine, 2008. DOI: 10.1097/PSY.0b013e31818426fa