The Real Reason Processed Foods Make You Gain Weight

Spoiler: There’s nothing magical about the carbs, fat, sugar, or salt content that make it easier to pack on the pounds.

  • Processed food has been linked to obesity, but researchers were not sure if there was actually something about the meals that led to weight gain.
  • Now, in a new study published in Cell Metabolism, researches found that even if calories and nutrients like carbohydrates, fat, sugar, sodium were matched, people still ate more of processed food than they do of unprocessed food.
  • People ate about 500 calories per day more, adding up to a weight gain of about two pounds over two weeks on the processed diet.

It’s probably one of the most well-known tenets in the wellness industry: Lay off the processed stuff and eat more whole foods if you want to lose weight.

The link between processed foods and obesity is well-established, but what hasn’t been quite as clear is if there is something about processed foods that actually causes weight gain or obesity.

Researchers set out to answer that question with the first randomized, controlled study—often described as the gold standard for determining cause-effect relationships, where a variable is tested on one group, and the other group serves as a control—on calorie intake and weight gain on processed and unprocessed diets.

In the study published in Cell Metabolism, researchers split a group of 20 participants in one of two groups: The first group ate an unprocessed diet of three meals per day and snacks that was provided to them for two weeks, while the second ate processed food for the same amount of time. Examples of an ultra-processed breakfast consisted of Honey Nut Cheerios, whole milk with added fiber, a packaged blueberry muffin, and margarine. An unprocessed breakfast included a Greek yogurt parfait with strawberries, bananas, walnuts, salt, and olive oil, and apple slices with fresh-squeezed lemon.

After two weeks on their first diet, participants switched and ate the opposite diet for another two weeks.

The researchers made sure that the calories and nutrients like carbohydrates, fat, sugar, salt were matched in each meal. They instructed the participants to eat as little or as much at each meal as they desired.

At the end of the study, people ate significantly more if their meals were ultra-processed—around 500 more calories per day—than they did if they were given unprocessed meals.

In fact, on the processed diet, they consumed 54 times the added sugar and 1.8 times more saturated to total fat.

The added calories on the processed diet contributed to a two-pound weight gain. On the flip side, people on the unprocessed diet ended up losing about two pounds over the course of the two weeks. Additionally, body fat mass increased by almost 1 pound when eating the ultra-processed diet.

Participants reported liking both options, which eliminated taste as a factor for why people consumed more calories on an ultra-processed diet.

Researchers speculate that the reason more calories were eaten overall is that people tend to chow down on the ultra-processed meals faster, leading to more food consumed. They found that differences in calorie intake were not associated with reported differences in appetite, taste of the food or familiarity with one diet.

“It is possible that ultra-processed foods are easier to chew and swallow, softer, and that this could delay the satiety signals,” study author Kevin Hall, Ph.D., a NIDDK senior investigator, told Runner’s World. “More studies are needed to determine if the consistency and density of the foods, how easy they are to eat, are important drivers of the total amount of food consumed.”

A few things to take into account: The cost of the ultra-processed food provided in the study was significantly less than the unprocessed meals—$106 versus $151 for the week. That supports the cost issue many people give when explaining what makes them reach for processed food rather than cooking their own. And add that to the convenience factor: In this study, both meals were provided to the participants, meaning all they had to do was sit down to a healthy meal, while in the real world, they would have to take time to prep it themselves. Those are two reasons that can help explain why reaching for the fast food egg sandwich is more enticing for breakfast than scrambling your own.

Cost—both monetarily and in time—are valid concerns, but they don’t sentence you to a lifetime of processed junk. Making time to meal prep breakfast or lunch for the week can be one step to helping decrease the amount of ultra-processed foods you eat in a week, and can save on time and on takeout cost.

Or if you are pressed for time, look for unprocessed foods that are convenient, like store-roasted rotisserie chicken and frozen veggies you can steam with it. Taking small steps will be beneficial to your health and help these changes seem less daunting.

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