Unregulated chemicals increasingly found contaminating U.S. water may hamper people's ability to lose weight, according to a study released today.

The study is the first to link the group of chemicals, PFASs [perfluoroalkyl substances], to weight gain and obesity, and suggests that exposure to the toxics may counteract weight loss efforts by slowing down people's metabolism.

The chemicals—already associated with other health problems including cancer, hormone disruption, immune dysfunction and high cholesterol—continue to make headlines as they contaminate waterways across the U.S., including high profile pollution hotspots in Michigan and North Carolina.

People with slower metabolism burn fewer calories and are more prone to weight gain and obesity. Obesity is a global health problem: In the U.S., roughly 37 percent of adults and 17 percent of children are obese.

"These findings suggest that environmental chemicals may play a role in the current obesity epidemic," the authors wrote in the study published today in PLOS Medicine.

The compounds—used for decades in stain and water repellants, packaging, firefighting foams, and some cosmetics—do not readily break down in the environment and can linger in people's body for years.

An estimated 95 percent of people in the U.S. have some level of highly fluorinated chemicals in their blood, according to a 2011 study.

A 2016 study estimated the drinking water for about 6 million people in the U.S. exceeded federal health advisory levels for two of the most common compounds, perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA). The research found 16 million people have some levels of PFASs in their water.

In the study out today, researchers examined weight changes and PFAS levels of 621 overweight or obese people from Boston, Massachusetts or Baton Rouge, Louisiana. The participants were all dieting—there were four different diets—for two years, and the scientists checked levels of the chemicals in people's blood while monitoring weight loss.

After six months, most people lost weight, but over the next year and a half, most people gained some weight back.

Those gaining the most weight back also had the highest levels of PFASs in their blood—the link was especially strong for women: women with the highest levels of the chemicals in their blood regained about 3 to 5 pounds more than women with the lowest exposure levels.

The scientists can't definitively say the chemicals spurred the weight gain—however, previous animal studies on the compounds have shown impacts on thyroid hormones, which are crucial for maintaining proper metabolism.

"One of the major determining factors in how much weight people regain after dieting is metabolism … slower metabolism, you burn less energy," said senior author Qi Sun, a researcher and assistant professor in the Department of Nutrition at Harvard T.H. Chan School of Public Health.

PFASs also interfere with estrogen metabolism, Sun said, which is another potential way it is linked to weight gain and potentially why the researchers saw more profound effects in women.

Just the latest on a growing problem

The study is timely as PFAS concerns rip through communities and officials struggle to contain and explain the pollution.

Many military bases are full of the toxics because of use in firefighting foam. Over the past four decades, firefighting foam containing the chemicals has been used at hundreds of bases across the nation.

In Michigan, state officials found the chemicals in the water of 14 communities, most notably in Plainfield Township, near Grand Rapids, where shoe company Wolverine had a tannery that contaminated groundwater and the nearby Rogue River. This month a nearby drinking water well had PFAS levels at 58,930 parts per trillion—experts said it might be the highest levels of the chemicals found in drinking water anywhere in the world. The state has sued Wolverine.

In North Carolina a newer PFAS compound called GenX, made at a Chemours plant in Bladen County, has been found in more than 250 private drinking water wells near the plant. The state Senate and House are trying to come to agreement on how to address the pollution.

The scientists say this latest study just adds to mounting evidence that PFASs are a major public health concern.

"We typically think about PFASs in terms of rare health problems like cancer, but it appears they are also playing a role in obesity, a major health problem facing millions around the globe," said study co-author Philippe Grandjean, a researcher and adjunct professor of environmental health at Harvard T.H. Chan School of Public Health, in a statement.

"The findings suggest that avoiding or reducing PFAS exposure may help people maintain a stable body weight after they successfully lose some weight, especially for women."