Scientists cruelly observed in January that the more weight you have to lose, the harder it can be to lose it. For people struggling to maintain a healthy weight or drop some pounds, willpower is only a tiny part of the equation, as fat inflammation and scarring can make excess fat especially hard to lose. Now, new research from the 2018 European Congress on Obesity suggests that another, more personal factor could be making success harder for dieters.

In a presentation at the conference, which is ongoing in Vienna, Austria, researchers from the universities of Birmingham and Amsterdam showed evidence that people who used to be obese and have successfully kept their weight off react differently to food than people who are obese or lean. In an experiment, researchers measured people’s unconscious reactions to pizza and chocolate, comparing salivation and heart rate. The 65 people who took part in the study included “20 former obese weight-loss maintainers, 25 individuals with obesity, and 20 never-overweight lean individuals.” Between weight maintainers and currently obese people, there was a significant difference in saliva production and heart rate when they were presented with pizza.

Bear standing in the cold water
Salivation is an unconscious reaction to tasty food. It turns out that obese people might react more strongly than others.

The obese participants had elevated heart rates and salivated more when presented with pizza and chocolate, whereas the successful weight-losers showed the opposite effect. The people who had never been obese showed no significant response to the food. The experiment may sound like a caricature, in which a big person is slobbering over pizza, but the truth is that some foods really do cause an unconscious reaction associated with the internal reward system in our bodies. Fatty and sugary foods cause reactions that aren’t too different from the feel-good reaction that happens when people take drugs. With food, this reaction can include salivation.

In another part of the experiment, participants completed a computer task in which they could win or lose money and food. In this task, each symbol correlated with one of four outcomes — winning food, losing food, winning money, and losing money. Once the participants learned the symbols by trial and error, the researchers measured their ability to seek or avoid them. The successful weight-losers were more likely to learn from food losses than food wins, which the researchers suggest could indicate that these people kept their weight off because they’re less motivated by food.

As with any studies on humans’ responses to food and how those elements interact with obesity, it’s not totally clear whether these physical reactions lead to obesity or whether they’re caused by obesity. But the researchers suspect that it’s the former: People who exhibit a stronger physical reaction to food have a harder time losing weight.

This dynamic has been observed by evolutionary psychologists, who argue that the pleasure of eating once served an important survival purpose for our ancestors who foraged food, it can cause over-eating in modern humans. More research with more participants will be necessary to confirm the results of this study, but the results seem to support this hypothesis.