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Why 'diet' food is so unsatisfying

When you think you're eating something indulgent, you feel satisfied sooner than when you consume a food that's supposedly better for you, reveals a new study.

Yale University researchers wanted to find out if your frame of mind -- your beliefs and expectations -- while eating a food could influence your body's physiology more than its actual nutritional value. So, they measured levels of ghrelin, a hormone released in the stomach in response to hunger.

When the blood has high levels of ghrelin, the "hunger hormone," it sends signals to the brain to let it know you want food. As you chow down, ghrelin levels decrease, which reduces appetite and makes you feel full.

To see whether ghrelin levels were affected by a person's expectations of a food, they rounded up 46 normal weight or plumper volunteers (ages 18 to 35). Participants were told they would be testing two new milkshakes: One was labeled as a high fat, 620-calorie "indulgent" shake; the other was a no-fat 140-calorie sensible, or "sensi-shake."

The trick was that both were the same 380-calorie french vanilla milkshakes disguised in different packaging.

While volunteers rated the "sensi-shake" as healthier than the "indulgent" shake and had a good-for-you mindset before drinking it, their bodies told a different story. Ghrelin levels were flat or slightly higher while tasting it, suggesting they were not physiologically satisfied with the beverage.

When participants drank the "indulgent" shake, whose label described it as "heaven in a bottle" and the "decadence you deserve," ghrelin levels steeply increased in anticipation of it, followed by a dramatically steep decline after consuming the creamy drink. This indicates they craved the drink more and were more satisfied afterward. Interestingly, though, hunger levels showed little change after either drink. A large drop in ghrelin levels should be accompanied by a large drop in hunger levels.

Researchers had expected that drinking the sensible shake would produce a sharper reduction in ghrelin levels, but the exact opposite occurred.

"The mindset of 'sensibility' or 'restraint' when eating -- no matter what we're eating -- might be compromising our body's physiological response, counteracting our hard work at dieting," says Alia Crum, a clinical psychologist at Yale University in New Haven, Conn., and the study's lead author.

And that's the mindset often adopted when trying to lose or maintain weight. "People should still work to eat healthy," suggests Crum, "but do so in a mindset of indulgence." By this she means believing a food will be enough to indulge your nutritional and hunger needs.

Unhealthy foods that market their healthy virtues (think: multigrain snack chips or chocolate-covered granola bars) may be doubly damaging. Their labels may be misleading and inaccurate but they can also affect people's perception of the food and the body's response to it, explains Crum.

This study appears online in Health Psychology.