When you open a bag of nacho-flavoured chips or cheese puffs, you probably know that you’re about to indulge in an unhealthy snack.

The dead giveaway? It’s the yummy, spicy, cheesy, neon-orange dust that coats each morsel and gets all over your fingers. Ditto for a frozen pizza and chicken nuggets.

But what about a granola bar? An applesauce pouch? String cheese? Flavoured yogurt? Surely these foods — snacks that millions of children and adults eat every day — are not bad, right?

Well, it turns out that many fall under the category of ultraprocessed foods — depending on their exact ingredients. This type of food has been studied a lot lately, and the results aren’t great.

Ultraprocessed foods represent a relatively new way of categorizing foods. Proposed in 2009 by researchers at the University of São Paulo in Brazil, the system, called , is based not on what kind of food it is — meat, grains, vegetables, etc. — but rather on how processed it is.

NOVA separates foods into four groups, starting with natural and minimally processed foods in the first category to , which use industrial formulations and manufacturing techniques, in the fourth.

“My operating definition for ultraprocessed (foods) is you can’t make it in your home kitchen because you don’t have the machinery and you don’t have the ingredients,” food policy expert Dr. Marion Nestle told CNN Medical Correspondent Meg Tirrell on the Chasing Life podcast recently. Nestle is the Paulette Goddard Professor of nutrition, food studies and public health, emerita, at New York University.

Ultraprocessed foods contain additives such as flavour enhancers, colours and thickeners — basically ingredients you wouldn’t usually use in your cooking. It makes them shelf-stable, easy to prepare (just heat and serve) and in many cases hard to resist. (The food industry pushes back on the NOVA system, saying there is no agreed-upon scientific consensus on the definition of ultraprocessed.)

Due to a confluence of historical, regulatory and economic factors, Nestle said, food companies in the 1980s “did a lot of work on trying to figure out what flavour and texture and colour combinations would be most attractive to people and started producing foods that would make them lots of money.”

She said tens of thousands of new products have hit store shelves since then. “Most of them fail, but the ones that win, win big,” Nestle said.

Before you reach for that can of soda, bag of chips or frozen dinner, why not learn more about what you’re eating? Here are five things to know about ultraprocessed foods:

Ultraprocessed foods are linked to bad health outcomes

Eating a lot of ultraprocessed foods isn’t healthy.

“Now there have been more than 1,500 observational studies — all of them demonstrating a consistent finding, which is that eating ultraprocessed foods is linked to obesity, type 2 diabetes, heart disease, certain cancers, bad outcome from COVID-19, overall mortality,” Nestle said. “Any bad health problem you can think of that’s related to diet is related specifically to ultraprocessed foods.”

The most recent study, published Wednesday in The BMJ journal, analyzed more than 30 years’ worth of data and found that eating ultraprocessed foods was associated with a 4% higher by any cause, including a 9% increased risk of neurodegenerative deaths. Other studies have linked ultraprocessed foods to heart disease, Type 2 diabetes, cancer and mental health disorders such as anxiety and depression.

Nestle pointed out that these studies have been observational and not designed to prove causation — that ultraprocessed foods caused these bad health outcomes.

“You can do that when you have a controlled clinical trial,” she said. “And guess what? We have one.”

Ultraprocessed foods cause weight gain

That one randomized, controlled clinical trial showed that ultraprocessed foods actually caused people to .

These types of studies are not easy or cheap to undertake, which is why they are not done more often. To conduct this one, Dr. Kevin Hall, a senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases, had 20 volunteers spend four weeks living at the National Institutes of Health Clinical Center in Bethesda, Maryland.

For two weeks, they ate a diet made up of 80% healthy ultraprocessed foods (think yogurt and whole wheat bread, not chips and soda). For the other two weeks, they ate a diet that contained no ultraprocessed foods. The diets were matched, among other things, for calories, sugar, fat, fibre and macronutrients. Participants did not know what exactly the study was measuring.

“We basically just asked people, you know, just eat as much or as little of the food that you’d like,” Hall told Tirrell. “You shouldn’t be trying to change your weight, (you) shouldn’t be trying to gain weight or lose weight. Just eat to the same level of appetite as you normally would.”

Researchers found that when the participants were on the ultraprocessed diet, they ate about 500 calories more per day than when they were on the minimally processed one. This difference in calories translated quickly to the scale. Participants gained on average 2 pounds during the two weeks on the ultraprocessed diet and lost 2 pounds on the minimally processed one. And their blood work showed lower markers of inflammation when they were on the latter.

“If you’re not familiar with nutrition research, you have no idea what an important finding this is,” said Nestle, who was not involved in the study. “Five hundred calories is huge.”

Hall said it’s unclear what drives people to consume more calories when they are on an ultraprocessed diet. “One of the things that we’re really interested in now,” he said, “is to figure out what the mechanisms were.”

Ultraprocessed foods are hard to avoid

Ultraprocessed foods are everywhere, and most of us consume them without even realizing it — even when you think you are eating something relatively healthy, such as baked potato chips or a peanut butter and jelly sandwich.

Using data from the National Health and Nutrition Examination Survey, researchers found that ultraprocessed foods make up more than half of American adults’ diets. For U.S. children, that percentage is even higher, at 67%.

Ultraprocessed foods are cheap and convenient

Yup, that’s right: Truly eating “clean” costs more.

“To actually create the minimally processed menu, it was about 40% more expensive than the ultraprocessed menu,” Hall said. “That doesn’t even account for the time that it takes to make the foods, right? So, all those factors probably play a huge role in … the foods that we choose to eat in the real world.”

Not all ultraprocessed foods are bad

Some ultraprocessed foods can provide important nutrients, such as whole wheat bread and yogurt. And others, in Hall’s study, were shown not to increase caloric intake.

“The snacks were neutral in terms of how many calories (the participants) ate,” Hall said. “Which goes to show that not all ultraprocessed foods necessarily drive this effect.”

Hall’s team is conducting a new study to tease out which ultraprocessed foods are harmful and which are neutral, or even healthy.

Americans may soon get more help sorting through the health effects of ultraprocessed foods. The US Department of Agriculture and the US Food and Drug Administration will soon issue new Dietary Guidelines, which are updated every five years. Nestle said that the scientific advisory committee guiding this process has been asked to consider the connection between ultraprocessed foods and poor health outcomes.