The Delboeuf Illusion: Common Weight-Loss Trick Debunked in Study

Eat off a small plate to trick your brain into feeling full—it’s a common weight loss tip espoused by so-called diet and fitness experts online. But it’s probably false, according to research. 

The study examines a phenomenon known as the Delboeuf illusion in relation to food. The original illusion shows how our brain can become confused by the size of objects depending on their context. It features two filled circles of the same size, but with one appearing smaller because of its surroundings.

When it comes to dieting, the idea is that by piling up a small plate with food, we will convince our brain we have eaten a large amount. In turn, we will feel more satiated. But if we place the same amount of food on a larger plate, we will feel as though we’ve eaten less—making us prone to overeating later on. 

This theory has been presented in a number of studies to explain our portion sizing. A 2014 study published in the International Journal of Obesity found that due to the Delboeuf illusion, participants overestimated the diameter of food on plates with wider rims compared with those with thinner rims.

Food-plate-salad-stock Eating from smaller plates is commonly believed to prevent overeating, but new research suggests otherwise. Getty Images

Similarly, in a 2012 study researchers asked participants to serve themselves soup in bowls of different sizes. They found individuals poured more soup in larger bowls, on average, than in smaller bowls. The team behind the study, which was published in the Journal of Consumer Research, surmised that the Delboeuf illusion explained the different behavior.

But in a paper published recently in the journal Appetite, researchers Noa Zitron-Emanuel and Tzvi Ganel at Israel’s Ben-Gurion University of the Negev called this apparent association between the Delboeuf illusion and food into question.

To test their hypothesis, the team tested the Delboeuf illusion on participants who had eaten an hour before the experiment or hadn’t eaten for three hours before, according to Scientific American.

Related: 5:2 and other intermittent fasting diets could raise risk of diabetes, scientists warn

They then presented the participants with Delboeuf illusions, including pizzas on trays and images of inedible objects, such as black circles inside circles and hubcaps on tires.

When the participants were asked to compare pizza sizes, the deprived group guessed more accurately than the individuals who weren’t hungry. This indicates that attempts to trick our brains into eating less are futile.  

Aisling Pigott, a dietitian and spokesperson for the British Dietetic Association who was not involved in the research, told Newsweek the study was relatively small but highlighted the complexity of weight loss and management.

"We often hear of people offering ‘magic answers’ to weight loss, but how we eat and perceive our food is much more complex than plate size alone," she said. "The link between deprivation, hunger and portion size makes sense and explains why we are more likely to overeat when we are feeling restricted or ‘dieting.’" 

She went on to describe the language commonly used when talking about losing weight as "unhelpful." 

"Try and view your changes in a positive way," she said. Instead of using terms such as cut downreduce or restrict, some alternatives are enjoy and appreciate.

When it comes to staying satiated, regular meals containing a sensible balance of carbohydrates and proteins, while considering how much energy you burn in a day, can help stabilize appetite and prevent overeating, said Pigott. Planning ahead for snack times to ensure you have healthy foods is therefore important.

Susana Martinez-Conde, a professor of ophthalmology, neurology, physiology and pharmacology at SUNY Downstate Medical Center, who was not involved in the study, wrote in Scientific American the “data indicate that our attempts to ‘trick’ ourselves into eating smaller portions by using the Delboeuf illusion are regrettably doomed to failure in situations when we feel hungry—as we’re prone to do while trying to stick to a diet.”

This article was updated to include a comment from dietitian Aisling Pigott.