Not all ultra-processed foods are bad for you. But if you’re trying to lose weight, it’s still probably better to opt for nutritious minimally processed foods over protein bars and high-fiber breakfast cereals, according to a new study.
The study, published Monday in Nature Medicine, builds on a growing body of research investigating ultra-processed foods, which make up more than half the daily calories consumed by people in the U.K. and the U.S. The rise of the Make America Healthy Again movement has put a spotlight on how ultra-processed foods affect health, and the related question of what dietary changes might help improve rates of chronic disease — though so far the movement’s leaders have largely focused on what nutrition experts say are incremental changes like getting rid of synthetic dyes.
In this study, researchers set out to compare the results of two diets that each followed the U.K.’s nutritional guidelines, giving people their recommended servings of protein, fiber, and fruits and vegetables. But one diet was ultra-processed — think premade lasagna for dinner — while the other featured food prepared by an independent caterer, like spaghetti bolognese. The 50 study participants who completed the trial followed one diet for eight weeks, took a four-week break, and then switched to the other diet for another eight weeks.
In the end, both diets following the nutrition guidelines led to weight loss — but the minimally processed diet led to an average 2% reduction in weight, compared with a 1% reduction for the ultra-processed diet. (The people in the study didn’t know that weight changes were the primary outcome being assessed and weren’t told to change how they ate.) The minimally processed diet was also associated with greater fat mass loss, fewer cravings, and lower levels of triglycerides, a measure of heart disease risk, compared with the ultra-processed diet. The ultra-processed diet, however, was associated with lower levels of LDL cholesterol — the “bad” kind.
The two main takeaways from these results, according to Sam Dicken, one of the study’s co-authors and a research fellow at the University College London: First, following nutrition guidelines really can make a positive difference in people’s health. (Research suggests that just 7% of the U.K. population gets the amount of fiber recommended in dietary guidelines, while about a quarter get enough fruits and vegetables.)
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Second, Dicken said, “when you’re following that diet, minimally processed foods may have greater benefits.”
To Christopher Gardner, a professor at the Stanford Prevention Research Center who was not involved in the study, it’s worth stressing that people did lose weight when they ate ultra-processed foods that were consistent with dietary guidelines. (Before the study’s onset, participants got an average 67% of their calories from ultra-processed foods and did not meet dietary guidelines except when it came to red meat.) To Gardner, that suggests it may not be ultra-processed food itself that’s the problem — “it’s the junk that’s the problem.”
The study’s authors float several possible explanations as to why participants lost more weight on minimally processed foods. “One is calorie density,” said Dicken. Healthier ultra-processed foods still tend to have more calories per bite than whole or minimally processed foods.
Participants may have also eaten more on the ultra-processed food diet because they found the options more appetizing. They rated flavor and taste lower on the minimally processed diet, though they reported that the diets were equally satiating. And industrial processing itself may have been a factor — “breaking down and putting [ingredients] back together again, it may have been a bit of that as well,” Dicken said.
All the food was delivered directly to the participants, allowing researchers to track results in a real-world setting while mitigating one of the most common pitfalls in nutrition research: Unless people are being monitored in a lab, it’s hard to know for sure what and how much they’re eating. Participants also recorded food diaries so researchers could monitor their adherence to the diet. Ultra-processed food researcher Kevin Hall, whose lab experiments at the National Institutes of Health produced some of the best evidence so far on the subject, helped design the study.
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Also noteworthy is the fact that even with food provided, people in the study had a harder time sticking with the minimally processed diet — “which speaks to potential real world challenges that people may face” in steering clear of ultra-processed foods, or UPFs, said Julia Wolfson, associate professor at Johns Hopkins Bloomberg School of Public, via email. She thinks public health messaging should acknowledge that, because of factors like cost, time, and accessibility, “we all consume at least some UPFs … so when you do, try to choose UPFs that are healthy as well.”
This was a small study of mostly women, and people who follow kosher, halal, or vegan diets were not included. The study’s authors note that the fact that people lost weight by eating exclusively nutritious ultra-processed foods doesn’t mean that others would benefit by doing the same if they don’t eat much ultra-processed food currently.
The study’s results may help inform the larger debate about how to improve the food environment, said Dicken. “We need healthier foods that meet dietary guidelines to be more accessible to people.” That means addressing the financial barriers that can make less nutritious ultra-processed foods the more convenient and low-cost option for many.
As for the push to make ultra-processed foods healthier — whether by lowering sodium levels or, per the current craze, adding protein in everything from waffles to ice cream — Dicken said there are limits to what that can achieve. “Food reformulation can have beneficial effects,” he said, “but it’s not going to be the solution.”
STAT’s coverage of chronic health issues is supported by a grant from Bloomberg Philanthropies. Our financial supporters are not involved in any decisions about our journalism.