A review of more than 100 studies found overweight girls tend to reach puberty earlier than their peers do.
While some researchers have argued hitting puberty early in life makes a person at higher risk for metabolic syndrome and diabetes, Emily Walvoord, M.D., pediatric endocrinologist and review author, found such diseases are not linked to early puberty, but obesity.
The review appears online in the Journal of Adolescent Health.
"Early puberty is one of the many outcomes of obesity," said Walvoord, a professor at the Indiana University School of Medicine. "There are clearly other factors we don't understand that have affected the timing."
While childhood obesity is a growing epidemic, she suggested other factors include a possible increase in hormone-disrupting chemicals in the environment and more chronic stress in children's homes.
Mounds of research on the topic exist, but each study parses data differently, leading to scattered results. "It's very hard really to scientifically scrutinize the timing of puberty just by looking at these studies," she said.
Despite the wealth of attention the early start of puberty has received in the last 15 years, Walvoord said many unanswered questions exist, none more troubling than that of the psychological impact. The long-term effect on adolescents is still unclear.
Girls who reach puberty early are more likely to experience negative body image, depression and other mental disorders. However, those mental health issues also link closely to obesity, so it is difficult to untangle cause and effect.
Jane Mendle, psychology professor at the University of Oregon, said those in her field know the psychological effects of early puberty well. "Whether or not that knowledge has been translated in the way it could or should be to the general population, I'm not sure."
Mendle said girls who mature earlier tend to land in social situations they are not psychologically prepared to handle. Because of that, they might develop psychological issues during puberty.
While research on boys and puberty is thin compared to girls, Mendle said her recent work suggests the deciding factor for the emotional influence on boys might be the duration of puberty, while it is timing for girls.
"It could be that with boys by trying to focus on timing we're sort of barking up the wrong tree," she said.