A common grouping of chemicals may be interfering with the body’s natural weight regulation.

Researchers from the Harvard T.H. Chan School of Public Health have found that perfluoroalkyl substances (PFASs) are linked with greater weight gain after dieting, particularly among women. The team also found that higher blood levels of PFASs—known as “obesogens” because they may upset body weight regulations—were linked with lower resting metabolic rate or slower metabolism after weight loss.

“Obesogens have been linked with excess weight gain and obesity in animal models, but human data has been sparse,” senior author Qi Sun, assistant professor in the Department of Nutrition at Harvard Chan School, said in a statement. “Now, for the first time, our findings have revealed a novel pathway through which PFASs might interfere with human body weight regulation and thus contribute to the obesity epidemic.”

PFASs have been used for more than 60 years in various products, including food wrappers, clothing, and pots and pans. Studies have shown that they have contaminated drinking water near industrial sites, military bases and wastewater treatment plants. 

The chemicals have previously been linked to cancer, hormone disruption, immune dysfunction, high cholesterol and obesity.

The research team, which also included scientists from Louisiana State University and Tulane University, analyzed data from 621 overweight and obese participants in the Prevention of Obesity Using Novel Dietary Strategies (POUNDS LOST) clinical trial conducted in the mid-2000s. The trial tested the effects of four heart-healthy diets on weight loss over a two-year period.

The researchers looked at the possible connection between the amount of PFASs in participants’ blood as they entered the study and their weight loss or gain over time. They found that during the first six months the participants lost an average of 14.1 pounds, but regained almost 6 pounds over the next 18 months.

Those who gained the most weight back also had the highest blood concentrations of PFASs, and the link was strongest among women. The women who had the highest PFAS blood levels regained on average 3.7-to-4.8 pounds more body weight than women in the lowest third.

They also found that higher blood concentrations of PFASs were significantly associated with lower resting metabolic rates.

“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,” study co-author Philippe Grandjean, an adjunct professor of environmental health at Harvard Chan School, said 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.”

The study was published in PLOS Medicine.