A new study released Thursday provides the first evidence that processed foods aren't just bad for our health, but actually cause people to eat more and gain weight.

Previous research has found that processed foods are linked to higher rates of cancer and early death.

Scientists consider food to be processed if it's made with additives and preservatives for flavor and freshness, like sugar, salt, or oil.

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They're quick, easy, cheap, and really bad for us.

Processed foods are under fresh scrutiny this week after a groundbreaking study from the National Institutes of Health found that people on ultra-processed diets ate more calories and gained more weight than they did when offered the same amount of nutrients from less processed food.

The finding suggests there's something different about how quickly our bodies take in processed foods and how those foods interact with key hormones that help regulate our appetites.

But this is far from the first time that processed foods have been linked to dangerous outcomes. Other researchers have connected packaged and ready-made foods with more cancer cases and more early deaths.

This mounting evidence raises a somewhat tricky question: What exactly designates a certain food as processed? After all, a chicken-salad sandwich prepared at home may still qualify as a processed meal, as could a cheesy quesadilla.

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To answer that question, scientists and nutrition experts often use a four-tiered system called NOVA that classifies everything we eat as one of these four categories: unprocessed or minimally processed, processed culinary ingredients, processed foods, and ultra-processed food and drink products.

Unprocessed foods include edible parts of plants (fruits, vegetables, seeds, roots, etc.) or animals, as well as fungi and algae. These can be fresh, frozen, or even fermented — the important distinction is that they have not been treated with additives, injected with salt, or rubbed with oil until they're about to be eaten. Examples include dry beans; grains like rice; fresh or dried mushrooms; meat and dairy products; seafood; plain yogurt; nuts; and spices.

Processed culinary ingredients involve a step up in production. These are ingredients made from unprocessed foods, like vegetable oils, butter, and lard. This category also includes extracted food, like honey from combs, sugar from cane, and syrup from maple trees.

Processed foods are items that get infused with ingredients like sugar, salt, and fat to help keep them edible longer. Canned fruits, fermented breads (which most breads are, as they're made with yeast), alcohol, cheese, pickles, and salted nuts all make this list.

Finally, there are ultra-processed foods. These items are designed to be ready to eat and ready to heat at a moment's notice. To make that possible, these foods are often made in a factory, broken down from their whole or fresh form and treated with thickeners, colors, glazes, and additives. They may be fried before they're packed in cans or wrappers. They might contain high-fructose corn syrup, protein isolates, or interesterified oils (replacements for trans fats, which are now widely banned). Examples of ultra-processed foods include packaged granola bars, carbonated soft drinks, candy, mass-produced breads, margarine, energy drinks, flavored yogurt, chicken nuggets, and hot dogs.

These are the items researchers are referring to when they say that ultra-processed foods are linked to more cancer cases, early deaths, and weight gain.

Of course, these items also tend to be more convenient and cheaper than less processed food, since they're less perishable.

Via Flickr

"Ultra-processed food has a lot of advantages in terms of its convenience," Kevin Hall, the lead author of the NIH study, told Business Insider. "It's cheap. It sticks around for a while. You don't have to have all the fresh ingredients on hand, which might spoil. You don't have to have all the equipment to prepare these meals from scratch."

But experts, including Hall, say that if you can afford it, cutting back on ultra-processed food is a good strategy for maintaining a healthy weight and staying disease-free.

"You can't just tax them and make them more expensive and less convenient for people," he said. "You also have to support access and availability to unprocessed meals."