First RCT data: Lack of fiber in ultra-processed food may cause weight gain

By Niamh Michail

- Last updated on GMT

© GettyImages/moniaphoto
© GettyImages/moniaphoto

Related tags Ultra-processed food Obesity Nutrition Fiber

Eating a diet high in ultra-processed foods causes weight gain because the natural food matrix has been changed to remove fiber, according to the first randomized controlled trial (RCT) in this area.

Previous studies have shown associations between eating ultra-processed foods and obesity, and suggesting this may be due to the ‘unholy trinity’ of salt, sugar and fat content; its availability and convenience; or additives and preservatives.

“As compelling as such theories may be, it is important to emphasize that no causal relationship between ultra-processed food consumption and human obesity has yet been established," ​wrote the study authors in Cell Metabolism​.

Therefore, their recently published study - although small in size - is significant because it is the first randomized controlled trial to investigate the effects of eating a diet high in processed foods.  

Commenting on the study in The Conversation,​ Richard Hoffman, lecturer in nutritional biochemistry at the University of Hertfordshire, said the study was "firm evidence at last"​ that ultra-processed foods cause weight gain. 

Weight gain

During the month-long study, 10 weight-stable adults followed an ultra-processed diet for the first two weeks while 10 ate an unprocessed diet. After two weeks, they swapped diets. Individuals could eat as much as they liked of the three daily meals.

The meals were well-matched for total calories, energy density, macronutrients, fiber, sugars, and sodium. However, the ultra-processed and unprocessed meals differed substantially in terms of added sugar (54% versus 1%, respectively), insoluble to total fiber (77% versus 16%, respectively), saturated to total fat (34% versus 19%), and the ratio of omega-6 to omega-3 fatty acids (11:1 versus 5:1).

By the end of the month, individuals on the ultra-processed diet had consumed around 500 more calories than those on the unprocessed diet, even though both diets were matched calories, sugar, fat, fiber, and macronutrients. They also gained weight – almost one kilo – while those on the unprocessed diet lost weight.

“In conclusion, our data suggest that eliminating ultra-processed foods from the diet decreases energy intake and results in weight loss, whereas a diet with a large proportion of ultra-processed food increases energy intake and leads to weight gain," ​the scientists wrote.

'Firm evidence at last'

Participants did not report stronger cravings for the ultra-processed foods than the unprocessed ones.

One of the key differences, however, was the time taken to eat food; individuals ate the ultra-processed meals much more quickly, meaning they consumed more calories before feeling full. This was the case even when the scientists matched the diets for macronutrients, by supplementing the ultra-processed drinks with fiber.

The reason behind the weight gain may be due to the loss of the original food matrix, according to Hoffman.

An interesting message emerging from this and other studies seems to be that to regulate calorie intake, we must retain food structure, like the natural food matrix of unprocessed foods.

“This obliges us to eat more slowly, allowing time for the body’s satiety mechanisms to activate before we have eaten too much. This mechanism does not operate with ultra-processed foods since the food matrix is lost during manufacture,"​ Hoffman wrote.

Benefits of reformulation are 'unclear'

The authors of the study say it is "unclear​" if reformulating ultra-processed foods could remove their negative impact on health while retaining their palatability and convenience.

“Until such reformulated products are widespread, limiting consumption of ultra-processed foods may be an effective strategy for obesity prevention and treatment.

“Such a recommendation could potentially be embraced across a wide variety of healthy dietary approaches including low-carb, low-fat, plant-based, or animal-based diets. However, policies that discourage consumption of ultra-processed foods should be sensitive to the time, skill, expense, and effort required to prepare meals from minimally processed foods—resources that are often in short supply for those who are not members of the upper socioeconomic classes.”

Different diets, same message

According to the study authors, avoiding ultra-processed foods can be applied to many different diets.

“The perpetual diet wars between factions promoting low-carbohydrate, keto, paleo, high-protein, low-fat, plant-based, vegan, and a seemingly endless list of other diets have led to substantial public confusion and mistrust in nutrition science.

“While debate rages about the relative merits and demerits of various so-called healthy diets, less attention is paid to the fact that otherwise diverse diet recommendations often share a common piece of advice: avoid ultra-processed foods​.”

Source: Cell Metabolism
Available online ahead of print,
“Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomized Controlled Trial of Ad Libitum Food Intake”
Authors: Hall et al.

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