Teenagers go through a challenging time when their bodies begin to change. Body image issues among teens are widespread, according to a U.K. based study by researchers at the University of College London and London School of Hygiene and Tropical Medicine.

Researchers found that two in three 13-year old girls are afraid of gaining weight, and one in three are upset about their current weight and shape. Girls were more than twice as likely as boys to be "extremely" worried about gaining weight or becoming overweight.

Study researcher Nadia Micali, a psychiatrist and senior lecturer at the University College London's Institute of Child Health, said she was "quite surprised that even at age 13, so many parents reported these behaviors in [their] children."

Micali was also surprised at the "high levels" of thoughts and behaviors in adolescent boys that are typical of eating disorders. The study showed that one in five boys were distressed about their present weight or shape.

But young teens aren't just worrying about their bodies, they're taking steps to lose weight. More than one in four girls in the study, and one in seven boys, restricted their food in the previous three months by fasting, skipping meals or throwing it away. Roughly five percent of boys and girls had binged or overate, while less than one percent of boys and girls had used laxatives or made themselves vomit in order to try and lose weight.

"The more children had unhealthy weight concern behaviors and worried about their weight and shape, the more they weighed two years later," Micali said.