(USA TODAY) -- Trying to lose weight raises a person's stress levels, even if the dieter doesn't realize it. The stress comes not only from trying to avoid the refrigerator or fighting the bathroom scales. A new study shows there also may be a physical reason for greater stress while dieting.

The study found that people who restrict calories have an increased level of the stress hormone cortisol, says lead author Janet Tomiyama, a Robert Wood Johnson Foundation scholar at the University of California-San Francisco.

She followed 99 women who were randomly assigned to one of four eating groups:

-Those who were taught how to follow a classic low-calorie diet -- 1,200 calories a day -- and were instructed to keep track of their calories.

-Those who restricted their calories to 1,200 a day by eating pre-packaged food but didn't count calories.

-Non-dieters who counted calories.

-Non-dieters who didn't count calories.

Before and after the study, the women completed surveys on their stress levels, and they had saliva tests to evaluate their bodies' level of cortisol.

Findings reported in the online version of Psychosomatic Medicine:

-Dieters lost an average of 2 pounds in three weeks. Non-dieters gained 2 1/2 pounds.

-Participants who cut calories had higher levels of cortisol than before they started the plan and higher levels than non-dieters in the study.

Tomiyama says cortisol "has lots of other jobs in the body, so we need to figure out exactly what's happening."

Still, "anything you can do to decrease your stress when dieting is a good idea," she says. Or you can "ditch the diet and exercise more."

Copyright 2007 USA TODAY, a division of Gannett Co. Inc.