We know that we should eat less junk food, such as potato chips, industrially produced pizzas and sugary drinks, because of their high calorie content. These "ultra-processed" foods, as nutritionists now call them, are high in sugar and fat, but is that the only reason they cause weight gain?
A major new test from the US National Institute of Health (NIH) shows that there is a lot more work than calories.
Studies have already found a link between junk food and weight gain, but this link has never been studied with a randomized controlled trial (RCT), the absolute reference of clinical studies.
In the NIH RCT, 20 adults, approximately 30 years of age, were randomly assigned to either an ultra-processed diet or a "control" diet of unprocessed foods, both of which were consumed in three meals -cas during the day. Participants were allowed to eat as much as they wanted.
After two weeks, one diet was replaced by another for two weeks. This type of cross study improves the reliability of the results since each person participates in both parts of the study.
The study found that on average, participants consumed 500 more calories a day when they consumed the ultra-processed diet, compared to the unprocessed diet. And on the ultra-processed diet, they gained weight – almost a kilogram.
Although we know that ultra-processed foods can be addictive, participants said they found the two diets as appetizing, without worrying more about their appetite for ultra-processed foods than for unprocessed foods, though 39, they consume 500 calories a day.
Unconscious overconsumption of ultra-processed foods is often attributed to nibbling. But in this study, most excess calories were consumed during breakfast and lunch, not as a snack.
Slow eating, not fast food
A crucial clue to why ultra-processed foods have resulted in higher calorie consumption is that participants ate ultra-processed meals faster and consumed more calories per minute. This can lead to excessive caloric intake before the body's satiety or satiety signals have time to react.
Dietary fiber is an important factor of satiety in unprocessed foods. Most ultra-processed foods contain little fiber (most or all are lost during manufacturing) and are therefore easier to eat quickly.
Anticipating this, NIH researchers equalized the fiber content of their two diets by adding extra fiber to the ultra-processed beverage diet. But fiber supplements are not the same as fiber in unprocessed foods.
Fiber in unprocessed foods is an integral part of the food structure – or the food matrix, as it is called. And an intact food matrix slows the speed with which we consume calories. For example, it takes us much longer to chew an entire orange with its intact food matrix than to gobble up the calorie equivalent of the orange juice.
An interesting message from this and other studies seems to be that to regulate caloric intake, we need to maintain the food structure, such as the natural food matrix of unprocessed foods.
This forces us to eat more slowly, which leaves time for the satiety mechanisms of the body to activate it before having eaten too much. This mechanism does not work with ultra-processed foods because the food matrix is lost during manufacturing.
Finding time for a meal consisting of unprocessed foods eaten slowly can be a real challenge for many. But the importance of seated meals is an approach vigorously defended in some countries, such as France, where a succession of small dishes ensures a more serene and pleasant food.
And this can also be an important antidote for weight gain caused by taking a quick meal of ultra-processed foods.
Richard Hoffman, Lecturer in Nutritional Biochemistry, University of Hertfordshire
This article is republished from The Conversation under a Creative Commons license. Read the original article.