We know that due to their high calorie content, we need to eat less junk food, such as chips, industrial pizzas and sugar-sweet drinks. These "ultra-processed" foods, as they are now called by nutritionists, contain a lot of sugar and fat, but is that the only reason why they cause weight gain? An important new test from the American National Institute of Health (NIH) shows that there is much more at work here than just calories.
Studies have already found a link between junk food and weight gain, but this link has never been investigated with a randomized controlled trial (RCT), the gold standard of clinical trials.
In the RCT of the NIH, 20 adults of about 30 were randomly assigned to a diet of ultra-processed foods or a "control diet" of unprocessed food, both eaten as three meals plus snacks throughout the day. Participants were allowed to eat as much as they wanted.
After two weeks on one of the diets, they were switched to the other for another two weeks. This type of crossover study improves the reliability of the results because each person participates in both research arms. The study found that participants ate an average of 500 more calories per day while consuming the ultra-processed diet compared to eating the unprocessed diet. And they arrived on the ultra-processed diet – almost a kilo.
Although we know that ultra-processed foods can be quite addictive, the participants reported that finding the two diets was just as tasty, without being aware of a greater appetite for the ultra-processed food? than for unprocessed food, despite consuming 500 calories more of them per day.
Unconscious over-consumption of ultra-processed foods is often attributed to snacking. But in this study, most of the excess calories were consumed during breakfast and lunch, not as snacks.
Slow food, no fast food
A crucial indication why the ultra-processed foods caused a higher calorie consumption may be that participants ate the ultra-processed meals faster and therefore consumed more calories per minute. This can cause excessive calorie intake before the signals from the body for satiety or fullness have the time to kick in.
Dietary fiber is an important satiety factor in unprocessed foods. Most ultra-processed foods are low in fiber (most or all of them are lost during manufacture) and are therefore easier to eat quickly.
In anticipation of this, the NIH researchers equalized the fiber content of their two diets by adding a fiber supplement to the ultra-processed diet in beverages. But fiber supplements are not the same as fiber in unprocessed food.
Fiber in unprocessed food is an integral part of the structure of the food – or the food matrix, as it is called. And an intact food matrix slows down how quickly we consume calories. For example, it takes us much more time to chew a whole orange with an intact food matrix than to swallow the equivalent calories as orange juice.
An interesting message that emerges from these and other studies seems to be that to regulate calorie intake, we must preserve the food structure, such as the natural food matrix of unprocessed food. This requires us to eat more slowly, giving us time to activate the body's satiety mechanisms before we have eaten too much. This mechanism does not work with ultra-processed food, because the food matrix is lost during production.
Finding time for a meal of unprocessed food that is eaten slowly can be a real challenge for many. But the importance of seated meals is an approach that is vigorously defended in some countries, such as France, where a succession of small courses makes for a more relaxed and pleasant way of eating. And it can also be an important antidote to the weight gain caused by a quick meal of ultra-processed food.