Here’s What Eating Processed Foods for Two Weeks Does to Your Body

Ultra-processed foods—the kinds made irresistible by sugar, fat and salt—are ubiquitous in the U.S., making up as much as 60% of the average American diet. But a small, intensive new study published in the journal Cell Metabolism shows that their low price and convenience comes at a cost to health.

When people ate a highly processed diet for two weeks, they consumed far more calories and gained more weight and body fat than they did when they ate a less processed diet—even though both diets had the same amounts of nutrients like sugar, fat and sodium.

It wasn’t a shock to find ultra-processed foods weren’t healthy—other research has linked them to a higher risk of cancer and obesity. What was unexpected was that sugar, fat and salt didn’t seem to be what was driving people to overeat. “I was surprised by the results,” says Kevin Hall, lead author of the study and senior investigator at the National Institute of Diabetes and Digestive and Kidney Diseases at the National Institutes of Health. “It’s the first trial that can actually demonstrate that there is a causal relationship between something about ultra-processed foods—independent of those nutrients—that cause people to overeat and gain weight.”

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In the study, 20 healthy adults lived for a month in a lab, where all of their meals and snacks were prepared for them. The two meal plans were either highly processed or unprocessed, and everyone ate one—then switched to the other—for two weeks at a time. (Foods like canned ravioli, chicken nuggets, bagels and diet lemonade comprised the ultra-processed diet; the unprocessed diet had salads, scrambled eggs, oatmeal and nuts.)

Both diets contained nearly identical nutrient profiles, with the same amount of sugar, fat, sodium, fiber and more. But the meals had very different effects. When people ate a highly processed diet, they ate about 500 more calories per day than they did on the less-processed diet. They also gained about two pounds over the course of two weeks on the ultra-processed diet—and lost about the same amount on the unprocessed diet.

They ate faster, too, which could be one reason why they gained more weight. “Ultra-processed food tends to be softer, which makes it easier to chew and swallow,” Hall says. “One of the theories is that if you’re eating more quickly, you’re not giving your gut enough time to signal to your brain that you’ve had enough calories and that you’re full and to stop eating. By the time the brain gets that signal, it’s too late—you’ve already overeaten.”

People’s hormones also changed depending on how processed their meals were. Even though people said they felt equally full and satisfied on both diets, the unprocessed diet led to an increase in an appetite-suppressing hormone called PYY and a decrease in the hunger hormone ghrelin. “Both of these hormonal changes that took place, for reasons we don’t fully understand, tend to support our observation,” Hall says. On an unprocessed diet, “people spontaneously reduce their calorie intake, leading to weight loss and body fat loss, without them having to count calories or even intentionally do so.”

Avoiding ultra-processed food isn’t easy, especially financially. In the study, the ingredients for the unprocessed meals cost about 40% more than for the ultra-processed foods, Hall says. But the study provides the latest proof that cutting down on processed foods may be worth the extra price and effort.

Write to Mandy Oaklander at mandy.oaklander@time.com.

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Eating healthy on a limited budget is possible, researchers find

The affordability of healthy food is often cited as a barrier to low-income families eating nutritious meals. A new study published in the Journal of Nutrition Education and Behavior found that with menu planning and access to stores selling items in bulk, the average daily cost for serving healthy meals to a family of four was $25 in 2010 dollars. This cost was consistent with the US Department of Agriculture (USDA) low-income cost of food meal plan, but higher than the cost of the USDA Thrifty Food Plan. The Thrifty Food Plan is the meal plan used by the USDA to determine food assistance benefits.

“This study determined the likelihood that families living in low-income households could create meals that meet the USDA dietary guidelines presented in MyPlate nutrition education materials,” said lead author Karen M. Jetter, PhD, Agricultural Issues Center, University of California Division of Agriculture and Natural Resources, Davis, CA, USA. “In addition to food cost, the other factors considered were access to stores, time for meal preparation, and whether the menus included culturally appropriate foods.”

This research was part of a larger study to train community members in research methods using community-based participatory research principles. This project was conducted in collaboration with Northern Valley Indian Health, Inc., and the Mechoopda Indian Tribe (MIT) of Chico Rancheria where 88 percent of the population surveyed lived in households with an income of less than or equal to $35,000 a year. The menus were created to feed a household with a father, mother, and children ages 7 and 10 with foods the MIT community liked to eat; met USDA guidelines for healthy eating; and had realistic portions. Menus did not rely on processed foods to reduce the amount of fat and salt in the family diet; were varied so the family would not become bored eating the same foods; did not always require hot meal preparation; and were affordable.

By working closely with the MIT community researchers, two-weeks of daily menus were developed using meal plans provided by the MIT community. Although these plans did not meet the nutritional guidelines every day, all categories achieved the recommended levels on average at the end of a two-week period. “These menus showed that a healthy diet on a budget was achieved by balancing daily targets over two weeks, not every day. This focuses healthy eating on balance rather than being deprived,” said Dr. Jetter.

Once the menus were determined, the MIT community researchers visited 13 grocery stores in Chico, CA to ascertain menu costs. The stores visited were within a 10-minute car ride of 76 percent of the MIT community members and were classified as bulk supermarket, general supermarket, discount market, or specialty market such as a local co-op.

Both bulk and general supermarkets had the highest availability of the items needed for a two-week shopping list, whereas specialty and discount markets lacked as many as 52 of the items needed. Bulk and discount market baskets had the lowest average daily cost of $25, while the specialty market had the highest average cost of $39 per day.

One limitation of the study was the focus on the actual cost of food without considering transactional costs such as the time needed to plan menus, develop shopping lists, research store advertisements, and travel to the bulk supermarket that offered the lowest cost. All of these factors influence a family’s ability to sustain a healthy eating plan.

“This research demonstrates that menus that meet USDA guidelines can be purchased by a family of four when shopping at a bulk supermarket, but any reduction in SNAP benefits or increase in food costs would make it difficult for these economically vulnerable families to maintain a healthy lifestyle,” stressed Dr. Jetter.

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