OTTAWA — Angelo Tremblay is a Canadian obesity researcher who has noticed something funny: whenever he thinks hard, he gets hungry.

This realization sent him straight to the lab, where he found that brain work makes us eat more — at least 200 extra calories at a sitting, compared to when we are relaxing.

It's a clue, he says, into why it's so hard to watch our weight: the modern world has us sitting in cars and at desks, where we don't burn calories. Yet the type of work we do at school or in offices makes us eat more.

The "knowledge-based economy" could make us sick, the clinical physiologist suggests: humans have evolved too fast from farmers and physical labourers into office workers.

"My feeling is that our evolution is maybe not optimal at this time."

The exact mechanism isn't clear yet, but brain work somehow makes our blood sugar levels jump up and down, and this "glycemic instability" triggers our desire to eat.

The eureka moment came when Tremblay was writing grant applications. These are requests for the funding that keeps a lab operating; a scientist has to justify his or her work at length, explaining why a new project is worth tens of thousands of dollars — or more — of public money.

"I am used to dictating my documents to my secretary, and I am used to feeling more hungry after a while, after six to eight minutes," he said.

He ran some quick blood tests on himself as he worked. Bingo: his blood sugar level was bouncing around far more than usual.

Changes in sugar levels, especially sugar lows, are known to cause a person to eat more.

Tremblay tried a more complex experiment on female students at Laval University, where he works. The women went through three preparation periods: once just relaxing for 45 minutes, once doing a battery of computer tasks, and once reading a document and writing a summary of it. After each of these, they had free access to a buffet.

The women weren't burning significantly more energy when they did the mental work. But they ate more at the buffet, compared to when they were relaxing — an average of 202 more calories after working on the document, and 252 more calories after computer tests.

A smaller test on male students showed the same pattern.

The results are published in a journal called Psychosomatic Medicine. (This doesn't mean imaginary illness; it refers to situations where mental processes cause health effects.) It was funded by the Canadian Institutes of Health Research.

The "most striking observation," Tremblay adds, came when he measured the intensity of the mental work. "Those working more strenuously were eating more after the mental tasks." They had higher fluctuations of blood sugar and higher levels of a stress-related hormone called cortisol, indicating that thinking harder makes the body go through bigger physical changes.