A new study out in the April 6th online issue of Psychosomatic Medicine reports that dieting can actually make you gain weight in the long run. How can this be? The researchers say that it is because dieting increases the stress hormone cortisol, which in turn makes it much harder to shed the pounds.

Dieting raises levels of the stress hormone cortisol, making it harder to lose weight. Exercise can help.

Janet Tomiyama and her team at the University of California, San Francisco wanted to determine what effect calorie restriction had on women’s perceived stress levels and the levels of cortisol in their systems. They divided 121 women into four groups: one group restricted their calorie intake to 1,200/day and recorded their calorie consumption, one group also kept to 1,200 calories/day but did not keep a calorie record, one group wrote down the number of calories they consumed while continuing on their regular diets, and the last group did not track or restrict their calories. The study lasted for three weeks, and the researchers measured the women’s perceived stress levels and cortisol levels at the beginning and end of the study.

The team found that restricting the number of calories taken in caused cortisol levels to rise. And monitoring one’s calories caused the women to feel more stressed – in other words, the perceived stress levels were higher among women who wrote down what they consumed.

Lead author A. Janet Tomiyama sums up the study’s findings in a Medline release, by saying that "[f]or the first time in humans, we are finding out that cutting your calories increases cortisol.” She adds that “[w]e think this may be one reason dieters tend to have a hard time keeping weight off in the long-term.” The results from the study may help explain why so many Americans struggle with losing weight and often gain more weight back than they lost to begin with.

Cortisol is the major hormone that’s released during stressful times – for example, during the “fight or flight” response. And dieting is apparently a condition that’s perceived by the body – and the brain – as a stressful situation. One of the effects that cortisol has, particularly over the long-term, is to make the body “hold onto” weight, rather than lose it. In the case of dieting, it creates a bit of a “Catch-22” for the dieter.

What’s the solution? Adding exercise to one’s routine may do the trick. Not only does exercise help the body burn more calories and increase muscle tone (and muscles burn more calories than fat, even at rest), but it’s also been shown to be an effective way to reduce stress.