Store-bought mac and cheese is one of the ultra-processed foods that a recent study included on eating plans for half the participants. The other half ate minimally processed food. (Tom McCorkle for The Washington Post)

For many years, I’ve steadfastly clung to a position for which there has been almost no evidence: Processed food is the root of obesity.

This doesn’t mean that processed food is the sole cause. There’s also the ubiquity of food, changing social mores and what is probably a more sedentary lifestyle (though evidence for that, too, is surprisingly hard to come by). It also doesn’t mean that all processed food is bad. Whole-grain bread and cereal are excellent, and there are good versions of such things as frozen pizza and jarred pasta sauce. Also wine.

What it does mean is that modern industrial food processing — and only modern industrial food processing — has enabled the manufacture of the cheap, convenient, calorie-dense foods engineered to appeal to us that have become staples of our obesogenic diet. By one estimate, nearly 60 percent of our calories come from ultra-processed food.

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In support of this, there is stuff some people call evidence. In general, studies find a correlation between processed food consumption and obesity, but since I dismiss population studies that connect artificial sweeteners to obesity, I obviously also have to dismiss the ones that connect processed food to obesity for the same reason. People who eat a lot of processed food are different from people who don’t. So, yes, they weigh more and are 57 percent more likely to die of heart disease, but they’re also 69 percent more likely to die in an accident, so we can’t take causality to the bank here.

The case is, instead, simply that as our environment changed to surround us with delicious, cheap, convenient, calorie-dense food that’s specifically designed to be irresistible, we couldn’t resist it. And we do know something about the pull of highly palatable food. Even though it’s practically axiomatic, in controlled studies people eat more of foods they like. Still, a controlled trial on ultra-processed food versus minimally processed food would be nice.

Ask and ye shall receive! The very first randomized controlled trial of ultra-processed food.

Kevin Hall, a researcher at the National Institutes of Health, brought subjects into the lab for four weeks and fed them one of two diets: ultra-processed or minimally processed, designed to have the same combination of carbs, fat and protein. The ultra-processed menus included store-bought mac and cheese, chicken nuggets, canned ravioli and frozen pancakes, while on the minimally processed side there was pasta with shrimp, salad with grilled chicken and grains, and oatmeal with nuts and bananas. Subjects were told to eat as much as they wanted. (Each subject spent two weeks on each diet.)

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The result was that, when they ate ultra-processed food, they consumed 500 more calories per day. Five hundred! They also gained a couple of pounds. Notably, though, subjects rated the meals as equally good-tasting. So why the big increase?

“There are several potential hypotheses,” Hall told me. Top of his list? Calorie density. “There were about two calories per gram in the processed food,” excluding drinks, “and in the unprocessed it was closer to one.” People also ate the ultra-processed meals a lot faster. “It might be softer, easier to chew and swallow,” Hall said. And that could mean that satiety signals, which take time, don’t get to your brain until after you’ve overeaten.

This is not a new idea. Penn State nutrition professor Barbara Rolls has been studying it for a couple of decades and wrote the “Volumetrics” series of diet books based on the idea of calorie-density. According to her, diets based on decreasing calorie density — which comes down to eating foods that have more water and less fat — are more effective than any of the diets that manipulate macronutrients.

They have a different mechanism, she explains. “It’s behavioral. It’s visual.” You’re responding to cues about the amount of food you’re eating. So, for example, if you give research subjects the same cereal in two different forms — flaky (higher volume) and crushed (lower volume) — they eat a third more of the crushed version. On average, people tend to decide how much to eat by gauging the amount of food; the more there is available, the more they eat, and volume and weight both play a role, Rolls says. Visual cues matter.

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When the food in question has a lot of calories for its size, that translates to simply consuming more. Diets that focus on carbohydrates or fat, by contrast, tend to focus on how different combinations of macronutrients trigger satiety hormones. But a decision to stop eating is more immediate, Rolls says. You often make it before hormonal cues have a chance to weigh in.

Calorie density is also part of the palatability equation — people just like those foods better, Rolls says. Any lover of baked goods, ice cream and cashews (raises hand) will find this easy to believe.

In a nutshell: The root of obesity is palatability and calorie density, combined with ubiquity and convenience. Satiety hormones and other metabolic machinations have much less to do with it. We’re responding to cues from without, not from within. One new study doesn’t prove it, of course, but it’s the hypothesis that best fits the preponderance of the evidence.

So what do we do about it? It’s a tough problem because, as historian Rachel Laudan, author of “Cuisine and Empire,” points out, “Processed food is what we eat.” We’ve been processing food for the entirety of human history, and it was a huge boon — until it wasn’t. When it freed women from, say, having to bake their own bread, it was a win, but modern food processing is different, and books such as Michael Moss’s “Salt, Sugar, Fat” and Mark Schatzker’s “The Dorito Effect” have detailed just how seriously food manufacturers take the challenge of winning consumer hearts, minds and stomachs.

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I don’t think we’re going back to universal dinner from scratch any more than we’re going back to bread from flour and yeast, and insisting that we should isn’t terribly useful. Are you going to tell a parent (almost always a mom), struggling with a minimum-wage job and an erratic schedule and a picky kid, that she shouldn’t lay off the only chore she can afford to pay someone else to do? I’m sure not.

But the insistence on chopping onions and roasting chicken gives everyone the idea that the choices are either wholesome homemade dinner or lousy processed dinner. How about wholesome processed dinner? Granted, it’s not that easy to find, but a trip through a grocery store will yield plenty of choices. Grab a frozen veg assortment, complete with sauce, and you’re a rice mix and a rotisserie chicken away from feeding your family.

The alternative to lousy processed food isn’t (always) home cooking. Sometimes it’s better processed food, and I asked Mehmood Khan, until recently the head of research and development at PepsiCo, what it would take to get more of it — whether that’s higher in nutrients or fiber, or lower in caloric density. Although he doesn’t pin all the blame for obesity on processed foods — he points to those changing social mores and our increasingly sedentary lives — he acknowledges that convenient, inexpensive processed food is one element. “The most important thing we can do as a society is figure out how to make more nutritious food, more nutrient-dense food in a manner that’s convenient, but to make them affordable and accessible,” he told me. But there’s a caveat. “When you actually do studies and give people options, they tend to always prefer tastier, and don’t like to compromise taste for health.”

I asked him if he could think of an example of a product that was formulated specifically to be more nutritious or healthful, and was a market success. There was a pause. “No,” he said. “No, I can’t.”

It’s on us, people. Sure, there are a few things the government could do (how about a calories-per-gram label?), but we can’t expect those more nutritious processed foods unless we’re willing to buy them.

As always, we get the food supply we demand.

More from Food:

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