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The problem with most health food is that nobody likes it — not least your own stomach. New research suggests that the stomach signals less satisfaction after eating “health food,” regardless of the actual fat and calories consumed. In contrast, foods that people perceive as indulgent and sinful produce a greater sense of fullness and gratification, even if they are actually much lower in calories.

For their study, Yale psychologists led by Alia Crum recruited 46 participants who were told that the researchers were testing the body’s response to two milkshakes that were designed with varying nutritional content. In reality, the two milkshakes were exactly the same, but one was described as being high-fat and containing 620 calories; it was labeled “indulgent” and offering “decadence you deserve.” The other shake was described as low-fat with only 140 calories. Its label promised “guilt-free satisfaction.”

Participants were asked to taste the milkshakes one week apart, so they were unable to directly compare the experiences. Each milkshake actually contained 380 calories.

Blood samples showed that gut levels of ghrelin — a hormone that rises in response to hunger and falls with fullness — declined rapidly when participants believed they were consuming a sumptuous treat. When participants thought they were getting health food, however, ghrelin levels stayed stable, meaning that their bodies did not signal the appropriate feeling of fullness after drinking the shake.

The research offers a possible clue as why diets fail so often: when we believe that we will be deprived by eating food that is lower in calories, our guts will psych us out with more hunger and less satiety.

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It also presents a conundrum for food manufacturers: accurately labeling health food as such may make it less satisfying, but how do you tell consumers that the products have fewer calories without evoking this effect? Maybe food makers’ annoying habit of shrinking packages without telling us may actually serve a good purpose after all.

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The current research was published in the journal Health Psychology.