A class of chemicals called perfluoroalkyl substances (PFASs) have long been linked with cancer, hormone disruption, immune dysfunction, and high cholesterol. Now comes a new study from Harvard’s Chan School of Public Health, linking them to obesity. A Chan School news release reports that the chemicals have been used for more than 60 years in products ranging from food wrappers to clothing to pots and pans. The latest study analyzed data from 621 overweight and obese people, and tested the effects of four heart-healthy diets on weight loss over a period of two years. When 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 those who gained the most weight back had the highest blood concentrations of PFASs. They also found that the link was strongest among women. The Chan School reports that the study found that higher blood levels of PFASs—known as “obesogens” because they may upset body weight regulation—were linked with lower resting metabolic rate (RMR), or slower metabolism after weight loss. People with a lower RMR, or slower metabolism, burn fewer calories during normal daily activities and may have to eat less to avoid becoming overweight.