Junk food and inactive lifestyles may not be the only reasons so many Americans are overweight.

Some researchers now believe that chemicals in the environment may be reprogramming babies’ metabolisms. Chemical exposure, they say, may help explain the dramatic rise in obesity, even among young children.

“If you talk to medical professionals, you hear the same old story — eat less fat and exercise,” said Bruce Blumberg, a biologist at the University of California, Irvine. “There’s obviously something else going on.”

Blumberg will be the keynote speaker at a day-long conference Friday at Colby College in Waterville.

“Chemicals, Obesity and Diabetes: How Science Leads Us to Action” is a project of the Environmental Health Strategy Center, the Colby Environmental Studies Program and the college’s Goldfarb Center for Public Affairs and Civic Engagement. It will begin at 8:30 a.m. at the Ostrove Auditorium in the Diamond Building, and the public is invited.

Blumberg is one of the leading researchers into what he has termed “obesogens” — industrial chemicals in plastics, food packaging, pesticides and cosmetics that may be affecting how babies make fat cells and regulate body weight.

“It’s a factor that’s never been considered before, and the danger about obesogens is if it happens early in life, when the woman is still pregnant, or in the early years, when a child is developing, those effects may be permanent and difficult to reverse,” Blumberg said.

The environmental link doesn’t mean that the other factors — including diet, exercise and heredity — aren’t important, he said. But it would help explain why people, even young children, have become so much heavier.

One-third of Maine kids and two-thirds of Maine adults are now considered to be overweight or obese.

The chemicals that Blumberg and others are studying are known as endocrine disrupters because of their ability to interfere with hormone systems. They include bisphenol-A, the plastic additive that’s now being phased out of children’s products in Maine, and tribuytyltin, a disinfectant and fungicide.

Studies on cell cultures and lab mice suggest that exposure to the chemicals can change the metabolic process, stimulating the formation of fat cells and altering appetite regulation. A small number of studies on humans also indicate a link between exposure and obesity, although more research is needed to clarify the connection and determine how much of a factor the chemicals may be.

“It’s early, but I think it’s opening some eyes,” Blumberg said. “Certainly six or seven years ago, the reception was, ‘Are you crazy? Everybody knows eating too much makes you fat.’“

Some in the medical community still consider the theory a distraction from the real culprits.

“His work is not without controversy,” said Dr. Michael Dedekian, a pediatric endocrinologist with Maine Medical Center and clinical director of the Countdown to a Healthy ME obesity treatment clinic in Portland.

“It’s very new, so like anything in science, it needs to be reproduced and confirmed. But I think there’s something to it,” Dedekian said. “We think of (obesity) as not eating right and not exercising, and it’s more complicated than that.”

While Blumberg advises people to avoid plastics and eat organic foods, Dedekian said it’s still too soon to give that advice to parents as a way to prevent obesity. “We just don’t know yet,” he said.

Chemical exposure may well turn out to be a contributing factor, said Heidi Kessler, the school nutrition manager for Let’s Go, an obesity prevention program of the Barbara Bush Children’s Hospital at Maine Medical Center.

But, Kessler said, that wouldn’t change the fact that kids now take in 200 more calories a day than they did 20 years ago, or that 25 percent of their diet is now sugar-based beverages, or that they no longer walk to school.

“While chemicals may play a role and it’s really important for us to investigate that role, there’s also no doubt that our other lifestyle factors play a significant role,” she said.

To register for Friday’s conference or for more information about it, go to: www.preventharm.org.

Staff Writer John Richardson can be contacted at 791-6324 or at: jrichardson@pressherald.com