About 80 per cent of people report that they reach for sweets more when they're stressed.

A new study has found that sugar switches off the stress response by reducing levels of the stress hormone, cortisol.

Scientists from the University of California recruited 19 women who ranged from healthy weight to obese and gave them sweetened beverages three times a day for two weeks.

Some in the group consumed drinks sweetened with sucrose, while the rest were given drinks containing aspartame, an artificial sweetener.

They were given "stressful" timed arithmetic tasks to complete and given negative feedback regardless of their results.

They then had saliva samples and brain images taken.

"At baseline [before the intervention], the two experimental groups did not significantly differ in age, BMI, chronic stress score, and cortisol and nausea responses," the researchers wrote.

After the two-week testing period however, cortisol levels were significantly lower in the sugar group and higher in the artificial sweetener group.

Additionally, after the test period, scans showed decreased activity in the part of the brain related to fear and stress in the sugar group and increased activity in the aspartame group.

While it sheds light on our desire to seek out sweet things in times of stress, it is not an excuse to do so.

Rather, the researchers suggest it is this response that makes sugar such a hard habit to quit.

"These experimental findings support a metabolic-brain-negative feedback pathway that is affected by sugar and may make some people under stress more hooked on sugar and possibly more vulnerable to obesity and its related conditions," the authors concluded, noting that while small, the study's findings are consistent with results from rodent studies.

It may, however, mean that targeting the stress-response is as important as targeting the sugar addiction, for the sake of our health and weight.

The senior author, Kevin Laugero, a nutritionist with the federal Department of Agriculture, told the New York Times: "The finding is intriguing because it suggests that there is a metabolic pathway sensitive to sugar outside the brain that may expose new targets for treating neurobehavioural and stress-related conditions."