Poor diet and lack of exercise might not be the only factors contributing to the obesity epidemic. A new study suggests the environment may also play a role.
"Eating too much and exercising too little are important factors," said Dr. De-Kun Li, a reproductive and perinatal epidemiologist at Kaiser Permanente in Oakland, California. "But they cannot explain the steep increase in the obesity rate the last three decades. We haven't really changed our eating habits and exercise that much."
The environmental culprit, according to the study published Wednesday in the journal PLOS ONE, may be bisphenol-a, a chemical commonly found in plastic and cans.
Li and colleagues studied 1,326 school-age children in Shanghai, China, and measured BPA levels in their urine. In girls ages 9 to 12, higher BPA urine levels were associated with a doubled risk of obesity. And as BPA urine levels increased, so did the girls' obesity risk - measured using their weight in reference to weight distribution in the population.
But strikingly, only girls in this age group were affected, the research showed. Neither girls outside of the 9-12 age range nor boys experienced a risk of being overweight or obese, even with high levels of BPA in their urine.
"Girls seem to be more sensitive to environmental impact, and we don't know exactly why," said Li, the lead study author.
Researchers do know BPA is an endocrine-disrupting chemical. It enters the body and mimics estrogen, the main hormone involved in female development.
When BPA acts like estrogen in young girls, it may accelerate the onset of puberty and cause weight gain -- thus earning its "endocrine-disrupting" title.
"It is biologically plausible that BPA interferes with your normal hormone process - then your body gets screwed up," said Li.
Another study conducted last year among children and adolescents in the United States showed a similar association between obesity and BPA.
"No previous study had examined exposure to BPA with obesity outside of the U.S.," said Dr. Leo Trasande, the author of the U.S. study, who was not involved with the current study. "This study of Shanghai school-age children supports the notion that BPA exposure may contribute to obesity."
BPA is a prime suspect on the growing list of what scientists call "environmental obesogens," chemicals found in the environment that may cause obesity.
And BPA is nearly unavoidable in our everyday lives: it lines the inside of food cans and some plastic containers, entering the body through ingestion.
According to the CDC, nearly everyone in the United States over age 6 has BPA traceable in their urine.
BPA is associated with a variety of other health problems besides obesity. Past studies have tied BPA with low birth weight, asthma, and sexual dysfunction in men.
The Food and Drug Administration last year banned using BPA in manufacturing baby bottles of sippy cups but stopped short of a complete ban.
Li said he hopes this study brings attention to the potential health effects of BPA, and that it spurs action from the FDA.
"It took 50 years from epidemiological research until finally the surgeon general said smoking is bad," said Li. "During that time you can't imagine how many unnecessary deaths there were."
Evidence solidifying BPA's link to obesity is still building, but Trasande and Li say the latest study should at the very least establish BPA as a major health concern for child obesity.