By measuring the telomeres of chromosomes, scientists have discovered that girls who are under stress age more rapidly than those who aren't.

Ian Gotlib, a psychologist at Stanford University, and his colleagues from Stanford, Northwestern University and the University of California, San Francisco studied healthy girls at risk of developing depression due to a disorder in their family.

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When stressed out, the bodies of the girls reacted by releasing much higher levels of cortisol than normal. Cortisol is the stress hormone.

For the research study, the team recruited girls aged 10 to 14 with a history of depression in their family and compared them to similar girls but without the disease in their ancestry.

The participants also had shorter telomeres, equivalent to the telomeres of adults six years older. The telomeres are found at the end of the chromosomes, capping them.

Every time a cell undergoes mitosis, the caps get a little shorter, making the telomeres similar to a biological clock recording the age of an individual.

When exposed to stress, telomeres also shorten. Researchers found associations in adults between shorter telomeres and frequent infections, chronic diseases and premature death.

Gotlib was amazed by the shortening of the telomeres among the participants.

"I did not think that these girls would have shorter telomeres than their low-risk counterparts -- they're too young," he said.

Next, Gotlib had to figure out what came first: premature aging, stress or depression.

Scientists recorded the response of the girls to stress tests, asked them to count from a hundred in multiples of seven and interviewed them regarding stressful events. The team measured the cortisol levels of the girls before and after the test. They also analyzed samples of DNA for the length of telomeres.

Prior to the study, "No one had examined telomere length in young children who are at risk for developing depression," Gotlib said.

When healthy, the 12-year-old girls with a high-risk of developing depression had markedly shorter telomeres, a sign of premature aging, than the girls who did not.

"It's the equivalent in adults of six years of biological aging," Gotlib said, but "it's not at all clear that that makes them 18, because no one has done this measurement in children."

TagsStress, girls, Little girls, Cortisol, Telomere, Stress Test, psychology, depression