Hunger would seem to be a fairly straightforward instinct: Depending on how much you eat, you either will or you won't be hungry afterward.
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As it turns out, our relationship to food may not be so simple. In recent years, a growing body of research has shown that our appetite and food intake are influenced by an array of factors besides our biological need for energy, including our eating environment and our perception of the food in front of us.
Studies have shown, for instance, that eating in front of the TV (or a similar distraction) can increase both hunger and the amount of food consumed. Likewise, people's food intake appears to be susceptible to their surroundings, such as the eating behavior—and body size—of their dining companions. Even simple visual cues, like plate size and lighting, have been shown to affect portion size and consumption.
A new study, published in the journal PLoS ONE, adds a new wrinkle by suggesting our short-term memory also may play a role in appetite. Several hours after a meal, the study found, people's hunger levels were predicted not by how much they'd eaten, but rather by how much food they'd seen in front of them—in other words, how much they remembered eating.
Here's how the experiment worked: Researchers in the U.K. showed 100 adults a bowl containing either a small (10-ounce) or a large (17-ounce) serving of tomato soup, and asked them to eat the whole portion. However, half of the participants ate more or less than their eyes led them to believe, thanks to a concealed tube that imperceptibly refilled or drained the bowl.
Immediately after the meal, the participants' hunger levels depended on the amount of soup they had consumed. Those who had eaten the large serving were more likely to report feeling full—a predictable response to the signals sent out by the stomach and gut following a meal, the researchers say.
Two to three hours later, however, the participants' feelings of fullness were related only to the perceived amount of soup consumed. Regardless of how much soup they'd actually had, those who believed they consumed 17 ounces reported being less hungry, on average, than those who thought they consumed 10 ounces.
This time-based disparity suggests the memory of our previous meal may have a bigger influence on our appetite than the actual size of the meal, says Jeffrey M. Brunstrom, the lead author of the study and a professor of experimental psychology at the University of Bristol.
"Hunger isn't controlled solely by the physical characteristics of a recent meal. We have identified an independent role for memory for that meal," Brunstrom says. "This shows that relationship between hunger and food intake is more complex than we thought."
These findings echo earlier research that suggests our perception of food can sometimes trick our body's response to the food itself. In a 2011 study, for instance, people who drank the same 380-calorie milkshake on two separate occasions produced different levels of hunger-related hormones depending on whether the shake's label said it contained 620 or 140 calories. Moreover, the participants reported feeling more full when they thought they'd consumed a higher-calorie shake.
"We've known for many years that our eyes are bigger than our stomach, but it may be more accurate to say that our eyes tell our stomachs a story," says Susan Albers, a clinical psychologist with the Women's Health Branch of the Cleveland Clinic, in Wooster, Ohio, and the author of Eating Mindfully.
"We eyeball portion sizes and then create a story or interpretation of what we see," Albers continues. "Thoughts like 'that's a small portion' or 'that was a huge slice' encode the memory into our minds in a certain way that have a significant impact on the way we eat later."
What does this mean for our eating habits? Although it hardly seems practical to trick ourselves into eating less than we think we are, the new findings do underscore the benefits of focusing on our food and avoiding TV and multitasking while eating. Such distractions, Brunstrom says, may "inhibit the formation of a memory for a recent meal."
So-called mindful eating strategies can fight distractions and help us control our appetite, Albers says.
"Take three seconds to look closely at what you are eating," she says. "Think for a moment about the words that spring to mind: small, large, filling, et cetera," she says. "Taking a moment to consciously and thoughtfully encode this meal into your memory will make it more likely that you will remember what you've eaten later."
Many of our decisions about food are made "on autopilot," Albers adds. "To gain better control, we have to start being more mindful of what we eat and actively remembering it later."