Despite the convenience and deliciousness of fast food, we all know it's something we shouldn't be eating too much of – if we care about our dietary health, that is.

But now a new study suggests there's another reason we should be careful about how much deep-fried takeaway we're consuming, and it's not just because of the unhealthy ingredients that fuel obesity. A link has been found between the consumption of highly processed food and exposure to a potentially harmful group of chemicals called phthalates, which are used in the manufacture of plastics and adhesives.

According to the US Centres for Disease Control and Prevention (CDC), the effect of phthalates on human health is unknown. But harmful effects seen in the reproductive systems of lab animals suggest it wouldn't be a bad idea to lower your consumption of fast food (in case you needed more convincing).

Researchers from George Washington University (GWU) collected data on 8,877 people who took part in the US National Health and Nutrition Examination Survey between 2003 and 2010. These participants gave detailed information about the food they had consumed in the last 24 hours and provided researchers with a urine sample, which was analysed to reveal the breakdown of two specific phthalate chemicals: DEHP and DiNP.

Phthalates like DEHP and DiNP are used in products to lubricate plastics and make them more flexible and durable. There's now more than 20 different types of phthalates in common use, and they appear in a huge range of things, including building products, cosmetics, footwear, sporting goods, and toys.

And it appears they're also widely involved in the processing of food – and fast food in particular – according to the results from the study.

"People who ate the most fast food had phthalate levels that were as much as 40 percent higher," said researcher Ami Zota from GWU's Milken Institute School of Public Health. "Our findings raise concerns because phthalates have been linked to a number of serious health problems in children and adults."

All up, about one-third of the participants in the study reported consuming highly processed or fast food in the day leading up to their urine sample collection, and these people showed significantly higher levels of phthalates than those who didn't.

People who consumed low amounts of fast food on that particular day showed 15.5 percent higher levels of DEHP than non-consumers of fast food, while those who consumed high amounts of fast food were 23.8 percent higher. In terms of DiNP, low consumers had 24.8 percent higher levels than non-consumers, and high consumers had 39 percent higher levels.

While phthalates aren't intentionally added to fast food, it's possible for the chemicals to be absorbed into foods during processing, the researchers suggest. They could end up being leached from a number of sources before fast food gets to you, such as by coming into contact with food packaging, beverage cans, receipt paper, lid gaskets, milk tubing, food gloves, and conveyor belts, among other things.

Of course, lots of the food we eat these days is exposed to processing in some way or other, but fast food in particular is known for being highly processed, packaged, and handled. What's more, the researchers say foods that are high in fat – including dairy and meat – could be more prone to phthalate contamination, due to some phthalates like DEHP being lipophilic (drawn to fats at a molecular level).

In addition to meat items, the team found grain food was the most significant contributor to phthalate exposure, including bread, cake, pizza, burritos, rice dishes, and noodles.

The researchers say their findings are just the latest scientific evidence to point out why we should limit our intake of junk food. There are, of course, plenty of alternatives to burgers, fries, and chicken wings.

"People concerned about this issue can't go wrong by eating more fruits and vegetables and less fast food," said Zota. "A diet filled with whole foods offers a variety of health benefits that go far beyond the question of phthalates."

The study has been published in Environmental Health Perspectives.