BPA-free plastic products may not be as safe as you think. The chemicals that substitute for bisphenol A (BPA) may still have similar effects in the body, new research suggests.

The study, just published in Endocrinology, found that a common BPA substitute called bisphenol S (BPS) can encourage the formation of human fat cells. The researchers sampled undifferentiated human cells called preadipocytes from the hip, thigh, or abdomen of female donors. The cells were then exposed to various levels of BPS. The researchers found that the chemical triggered fat cell formation in a way comparable to BPA.

"Our research indicates BPS and BPA have comparable effects on fat cells and their metabolism," Ella Atlas of Health Canada said in a press statement.

The study joins a small but growing body of research that suggests BPA-free plastic products may not be so different from the products they replaced. In another study, published last month, researchers at UCLA exposed zebrafish embryos to BPS and found the chemical had a similar impact on the embryo as BPA—the embryos grew faster than normal and had premature birth.

“These recent findings speak to the flaw in our regulatory system, which holds a chemical as innocent until proven guilty,” Leonardo Trasande, professor of environmental medicine and pediatrics at New York University, tells mental_floss. (Trasande wasn’t involved in the new studies.) “BPS is only one of the numerous chemicals that have replaced BPA. To solve the problem, we need to test chemicals proactively—before introducing them into products, not after.”

The industrial chemical BPA is commonly used to manufacture many types of plastic, including the kinds used in children toys and food packaging. BPA has a chemical structure that resembles that of the hormone estrogen. In recent years, concerns have been raised about BPA’s potential to leak into food and act like hormones in the body. Many studies have found links between exposure to BPA and health outcomes such as obesity in teenagers and reproductive problems in adults. Others have shown BPA causes changes in the bodies of lab animals

In response, many manufacturers started phasing out BPA from some consumer products following the widespread concerns. But they replaced BPA with chemicals such as BPS, which only has a slightly different chemical structure.

“If you Google BPA and BPS, you can see their striking similarities,” Trasande says. 

But the case is far from closed; we still don’t know whether exposure to low levels of BPA has health effects in human adults, according to the Centers for Disease Control and Prevention. Currently the U.S. Food and Drug Administration regards BPA as safe at the current levels it occurs in contact with food and drink, and it's still found in many consumer products, including water bottles, food containers, contact lenses, and water pipes.

However, in 2012, the agency banned the use of BPA in all baby bottles, sippy cups, and infant formula packaging because the manufacturers had already started phasing out BPA use and replacing it with alternatives.

Several countries, including Canada, have stricter rules on BPA than the U.S. does, but mainly as a precautionary measure.

If you want to lower your exposure to BPA—and its substitutes—try glass, stainless steel, and food-safe ceramic containers.