Victim of the Freshman 15? Blame toilet paper.

French fries and pizza might not be the only culprits behind the infamous ‘Freshman 15.’

A new study in PLoS ONE has strengthened the link between bisphenol A (BPA) and weight gain.

It showed that exposure to the chemical results in significantly heightened insulin levels. Over time, increased insulin levels can lead to weight gain and Type 2 diabetes, as the body becomes desensitized to the persistently high concentrations of the hormone.

Modern chemicals with potentially adverse health effects are nothing new—it seems like researchers are warning us about yet another carcinogen every day. But BPA is an especially serious threat for two reasons: it’s nearly everywhere, and even minuscule amounts may impact on your health.

“People are seeing effects of BPA down to 1000-fold below [the threshold],” Frederick vom Saal, an expert in endocrine disruptors at the University of Missouri-Columbia told The Huffington Post. As little as a quarter of a billionth of a gram may have an effect.

Worse, avoiding BPA is even harder than avoiding fries and pizza. It’s in practically everything: toilet paper, cash register receipts, food-can linings and plastics. As a result, as many as 90 per cent of people in developed countries have BPA levels that could be disrupting our hormones.

Exposure to BPA had long been been associated with obesity and diabetes, but the mechanism wasn’t known until now. It turns out BPA masquerades as estrogen—a natural insulin regulator.

Other studies have found potential links to cancer, infertility, heart disease and cognitive problems.

Unfortunately, fetuses are even more vulnerable to the negative effects of BPA, possibly causing them to develop more fat cells or pre-program weight gain later in life.

So to be truly safe, you’d have to make sure your mother avoided it while pregnant. In other words, unless you can time-travel and alter the past Terminator-style, there’s not much you can do.

But if you’ve been putting on pounds since September, at least you can blame junk food and excessive beer a little less. For all you know, it could be your toilet paper that’s making you fat.