Processed foods and weight gain

Education | Emily Baumgaertner 21 May 2019

For four weeks, 20 healthy volunteers checked into a research center hospital and were served a variety of tempting meals: cinnamon french toast, stir-fry beef with broccoli and onions, turkey quesadillas and shrimp scampi.

Researchers scrutinized everything and came away with the first hard evidence to support a long-held suspicion: heavily processed foods could be a leading factor in the obesity epidemic.

The unusual clinical trial compared the volunteers' calorie consumption and weight gain when they ate a diet based on unprocessed ingredients and when they ate meals dominated by ultra-processed foods. Both daily menus had matching amounts of calories, fat, sugar, carbohydrates and salts, and diners said they were equally tasty and satisfying.

Yet the volunteers chose to consume an average of 508 additional calories per day on the ultra-processed diet. After two weeks, they weighed an average of one kilogram more than their counterparts who had dined on unprocessed foods.

The findings, published in Cell Metabolism, will force scientists to rethink the complicated relationship between dietary habits and health.

"I thought it was all about the nutrients," said study leader Kevin Hall, a section chief at the National Institute of Diabetes and Digestive and Kidney Diseases. "There's something other than the sugar and fat that causes people to overeat and gain weight. We don't fully know the mechanism yet, but processed foods aren't just innocent bystanders."

Diets have changed drastically over the past century. Home-grown produce and local poultry have given way to canned vegetables and deep-fried chicken tenders. Doctors have long suspected that changes in food preparation were among the key contributors to the obesity epidemic, but they've struggled to find ways to reverse the trend.

Hall's team recruited 20 people who weren't picky eaters and were willing to spend a month living at the NIH's Metabolic Clinical Research Unit.

The volunteers were given three meals per day and were allowed to refill their plates as much as they wanted. They also had access to unlimited snacks. They were randomly assigned to consume either an ultra-processed diet or an unprocessed one for the first two weeks of the experiment. Then they switched menus for the remaining two weeks.

If volunteers ate everything put on their plates all day long, those on the unprocessed diet would have consumed the same number of calories and nutrients as those on the ultra-processed diet.

In reality, their consumption was different because the researchers served up gargantuan amounts of food - an average of 5,400 calories each day - and participants left different amounts of food on their plates.

Participants said both diets were filling and delicious. That may sound trivial, but it's important for a nutrition study because it helps eliminate the influence of factors like food preference. "I thought it would be a no-brainer that people simply liked the ultra-processed foods better," Hall said. "My first hypothesis went right out the window."

Researchers tracked how much and how fast each person ate, and the contrast was stark.

For instance, when volunteers were served ultra-processed foods, they ate at an average rate of around 37 grams and nearly 50 calories per minute.

But when eating unprocessed foods, they averaged only about 30 grams and about 32 calories per minute.

Hall said the discrepancy could be due to differences in the foods' texture. Ultra-processed foods are generally softer, and people tend to eat soft foods quickly. That means volunteers would have swallowed more food by the time their guts were able to send signals to the brain that eating should stop.

Whatever the explanation, participants gained an average of one kilogram when they ate ultra-processed foods. Luckily for them, they lost an average of 2 pounds when they were on the unprocessed diet.

Blood sugar levels and measures of liver health remained largely the same on both diets. Interestingly, the ultra-processed diet triggered a higher expenditure of energy - but not enough to counteract the additional calories consumed.

Several notable factors could have contributed to the caloric difference between the two diets, the researchers said.

Consider the role of protein. In the unprocessed diet, protein made up some 15.6 percent of all calories served, as opposed to just 14 percent in the ultra-processed diet. If people craved more protein, they probably ate more food in order to find it.

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