Craving food even when we are full could be explained through behavioral conditioning.
Diets often work for only a limited period of time, and once the diet is over, most people relapse into overeating. But why is that?
Researchers now suggest that the answer is behavioral conditioning. Because we have conditioned ourselves not to eat when we feel hungry as part of the diet, this does not mean that the achievement will last outside of the context of dieting.
In fact, the new study - conducted by Mark E. Bouton and Scott T. Schepers, both of the University of Vermont in Burlington - suggest that the actual feeling of hunger or satiety can act as "cues" for eating behavior.
The findings were published in the journal Psychological Science.
The hypothesis: Eating in a context
"Basic research," the authors write, "indicates that after a behavior is inhibited, a return to the conditioning context or simple removal from the treatment context can cause the behavior to return."
In other words, once we move away from the context in which we learned to "be good" - whether that means eating fewer calories, exercising more, or giving up alcohol - we are prone to relapse.
But in dieting, could the actual physical state of feeling hungry (or, conversely, of feeling sated) work as such a context?
As study co-author Bouton explains, "One reason [why diets fail] might be that the inhibition of eating learned while dieters are hungry doesn't transfer well to a non-hungry state."
"If so," he continues, "dieters might 'relapse' to eating, or perhaps overeating, when they feel full again."
The researchers conducted several experiments to test their hypothesis.
Feeling full triggers food cravings
In the main experiment, satiated female rats were placed in a box with a lever that, when pressed, released tasty sweet treats for the rodents.
The animals were conditioned to do this every day for 12 days. This phase of the experiment conditioned the rats to associate feeling full with receiving food.
After that, the researchers put the rodents in the same box, but when they were hungry. For the following 4 days, pressing the lever no longer released food for the starving rodents.
So, in this phase of the experiment, the rats were conditioned to associate being hungry with receiving no food.
The conditions were then repeated. The same rats were placed back into the box and returned to the condition of feeling full. The rodents pressed the lever much more often when they were satiated than when they were hungry.
As Bouton explains, "Rats that learned to respond [to] highly palatable foods while they were full and then inhibited their behavior while hungry, tended to relapse when they were full again."
The researchers performed additional experiments, placing and removing food from the cage both in the first phase and in the second phase of the study.
And the scientists found that the same pattern occurred, independently of the external stimuli - that is, of the food placed in the box.
These findings, the study authors write, "suggest that associations with hunger or satiety stimuli were learned more readily than associations with other potentially useful [external] stimuli."
Overall, the findings confirmed the researchers' hypothesis that internal states of hunger and fullness can act as conditioning contexts.
"A wide variety of stimuli can come to guide and promote specific behaviors through learning. For example, the sights, sounds, and the smell of your favorite restaurant might signal the availability of your favorite food, causing your mouth to water and ultimately guiding you to eat."
"Like sights, sounds, and smells, internal sensations can also come to guide behavior, usually in adaptive and useful ways," they add. "We learn to eat when we feel hunger, and learn to drink when we feel thirst."
"However," the authors conclude, "internal stimuli such as hunger or satiety may also promote behavior in ways that are not so adaptive."