It turns out that performing mental tasks, like trying to solve problems while working at a computer, stimulates the appetite so much that people tend to eat significantly more calories than they burned while performing the "knowledge-based" tasks.

In a study published in the current issue of Psychosomatic Medicine, researchers found a physiological basis for the spike in appetite. Mental work "destabilizes" the levels of insulin and glucose, two critical components in the body's regulatory and energy machinery, thus stimulating the appetite, said Jean-Philippe Chaput, lead author of the study.

"The brain uses only glucose for energy," unlike the "muscles, which use fat and glucose," Chaput said in a telephone interview. So when the level of glucose, or sugar, becomes unstable, the brain demands more.

According to the research, participants consumed far more calories after performing mental tasks than they consumed after relaxing for the same period of time.

The study is quite small, involving only 14 women, so the results are only tentative, but Chaput said he and his fellow researchers have already embarked on a larger study involving 50 men and 50 women. Only women were used in the pilot study because it has been well established that men and women react differently to stress, and the researchers did not want to cloud the results.

At this point, however, the study indicates that a rapidly changing lifestyle toward "knowledge-based work," like time spent at the computer or trying to solve mental challenges, may be a significant factor in the current obesity epidemic, Chaput said.

"There are a lot of people doing this kind of work now, compared to physical work in the past, so we postulate that it can explain in part" why so many people in so many countries are getting fat, he said.

As the researchers put it in their paper, "knowledge-based work represents the main working modality in a context of modernity." In other words, many are spending fewer calories, but taking more in, because of changes in the work environment.

That's true not only in the world of employment, but in the world of entertainment as well. Many people are spending more time playing computer games, some of which can be intellectually challenging, than playing tennis, for example.

The researchers are members of the university's department of medicine, and they specialize in kinesiology, the mechanics and anatomy of human movement. The 14 women in the study were all students, ranging in age from 20 to 30 years. All were in good health, with no eating disorders, normal weight, and free of food allergies.

During a two-month period they were each required to participate in three 45-minute exercises consisting of relaxing in a chair, reading a document and writing a summary of 350 words, and "a cognitive task consisting of a comprehensive battery of computerized tests."

Each participant was tested separately from the others.

Each "came into the lab at 8 a.m., and we gave each participant a standardized breakfast," Chaput said. "After that we started the exercise at 10:30 a.m., one participant at a time, and they came every two weeks. At about 11:30 a.m. we gave them a buffet type of meal, comprising a lot of food."

The food in the buffet was weighed and analyzed before and after the lunch, so researchers knew precisely how much and which type of food each participant had eaten. Various tests during each exercise also told the researchers precisely how many calories the participants were burning at the time.

The participants burned only about three calories more during each of the two "knowledge-based" experiments than during the 45 minutes when they rested in a comfortable chair. So the caloric expenditures were relatively quite low for mental tasks compared to the period spent relaxing.

But the intake was significantly higher. Participants consumed 203 more calories after the reading experiment, and 253 more calories after the computer tests, than the resting participants. That's an increase of 23.6 percent and 29.4 percent, respectively.

Measurements of glucose and insulin became quite erratic during the mental tests. Glucose soared almost immediately when the participants were reading and then dropped dramatically. It dropped and remained below normal during the computerized tests. Insulin rose slightly during the reading test, and then dropped, and it dropped steadily during the computer tests.

That erratic performance by both glucose and insulin caused the appetite to rise, apparently in response to a need to restore the energy balance, the researchers conclude. The results probably would have been different if men had been included in the study, as they will be in future research. That's because mental challenges provoke stress, and it "is expected to be higher in females than in males," according to Chaput. That is also reflected in many other studies. He said women also tend to eat more following stress than men do, although "we don't know why yet."

Although the first study was limited to college-aged participants, Chaput said other research in his lab suggests that the same findings will apply to children as well as to older adults.

But if mental challenges cause people to eat more food, why aren't nearly all college professors fat?

Simple, Chaput said. Eating is only part of the weight control "package," he added. It's possible to spend a lot of time working out problems at the computer if that is balanced with a reasonable amount of time working out, literally.

Some college professors, of course, are fat. Maybe the skinny ones use their computers on the treadmill.