The difference between "processed" and "ultra-processed" foods might sound like an issue best left to linguists or hungry English teachers. But for the sake of your health, it's worth understanding.

That's because some of those foods are just fine ­– and too much of others can harm you.

What is the difference? Definitions vary, but the U.S. Department of Agriculture says anything that changes the fundamental nature of an agricultural product – heating, freezing, dicing, juicing – is a processed food.

Which means some can be quite good for you.

"Those little baby carrots that you get in a supermarket – that's a processed food," said Penny Kris-Etherton, registered dietitian and distinguished professor of nutrition at Pennsylvania State University's Department of Nutritional Sciences. So are frozen vegetables, or even broccoli that's been cut into florets.

Ultra-processed food takes things further. Nutritionists started using the term about 10 years ago, and again, definitions vary. One diet classification system called NOVA sums it up as "snacks, drinks, ready meals and many other products created mostly or entirely from substances extracted from foods or derived from food constituents with little if any intact food."

Examples would include packaged chips, soft drinks and candy. "Things that are packaged and pretty much ready to eat with little work at all," Kris-Etherton said. "Things like rice dishes, pasta dishes – all you have to do is add water and put them in the microwave."

Ultra-processed foods, also called highly processed foods, can be cheap, convenient and tasty. But they may have lots of refined carbohydrates, saturated fats and salt. Although the NOVA system authors assert certain additives may be unsafe, any additives used in food must pass safety tests by the Food and Drug Administration.

They also tend to pack a lot of calories into each bite. That means you're likely to eat a lot before you feel full, said Kris-Etherton, chair of the American Heart Association's Council on Lifestyle and Cardiometabolic Health.

A growing pile of research suggests ultra-processed foods – which make up half the diet of U.S. adults, according to recent research – might cause serious health problems. A small 2019 study found people given ultra-processed food ate more and gained more weight than people on a diet of minimally processed food. Other studies have linked ultra-processed foods with obesity, high blood pressure, cancer and death from all causes.

Dr. Dariush Mozaffarian, a cardiologist and dean of Tufts University's Friedman School of Nutrition Science and Policy in Boston, said the basic problem with ultra-processed foods is that some have not been designed with health in mind. Manufacturers prefer to make taste, cost, safety, shelf life and mouthfeel the priority.

When such factors are the goal, thousands of trace nutrients get stripped out, he said. Additives such as emulsifiers and stabilizers are tossed in. Those industrial ingredients are considered safe, but their long-term effects are not known.

Heavy processing may also strip out fiber, altering how the body digests food and affecting friendly gut bacteria, he said.

Mozaffarian stopped short of equating "ultra-processed" with "bad."

"In the modern world, we're going to need processed foods, and even ultra-processed foods, that are healthy," he said. But he wants manufacturers to rethink their goals even as researchers try to pinpoint how such foods affect people.

"From a scientific perspective, we don't have all the shades of grey understood, but we certainly know enough to start making healthier processed foods now."

Meanwhile, how should a careful eater face an ultra-processed world? "What a lot of dietitians will say is, 'Shop the perimeter of a supermarket,'" Kris-Etherton said. "That means stay away from the aisles that do have a lot of the processed food, especially ultra-processed foods."

But you can find healthy choices throughout the store, she said. When choosing packaged bread, skip white bread in favor of something with lots of whole grains. And if you're making a sandwich, choose a lean protein source, a nut butter or an avocado. "What it boils down to is, don't eat it with lunch meats like bologna that are high in salt and unhealthy fats."

Kris-Etherton acknowledged that for most people, "it's really hard not to have any processed foods, let alone any ultra-processed foods, in your diet. But you can still make wise food choices by being aware and reading the nutrition information on the package label."

Mozaffarian said good nutrition is more than just counting calories. Food affects our genes, modifies metabolism, alters brain responses and more. "And those things together, in a really amazingly powerful way," influence how much we eat, how much we weigh and our overall health, he said.

"Humans are not buckets, where we have a hole in the top and a hole in the bottom, and we pour calories in and calories come out. We're complex organisms."

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