Why Do We Want Dessert Even When We Are Full?

Eating to Live or Living to Eat?

It's after lunch, so everybody is full. Then, in comes a luscious chocolate confection. The sight, the smell—even the sound of the word "cake!"—stimulate the reward-and-pleasure circuits of the brain, activating memory centers and salivary glands as well. Those reactions quickly drown out the subtle signals from the stomach that are saying, in effect, "Still digesting down here. Don't send more!" Social cues add pressure and permission to indulge. Soon, everybody is having a slice—or two.

Scholars have understood the different motives for eating as far back as Socrates, who counseled, "Thou shouldst eat to live, not live to eat." But nowadays, scientists are using sophisticated brain-imaging technology to understand how the lure of delicious food can overwhelm the body's built-in mechanism to regulate hunger and fullness, what's called "hedonic" versus "homeostatic" eating.

One thing is clear: Obese people react much more hedonistically to sweet, fat-laden food in the pleasure and reward circuits of the brain than healthy-weight people do. Simply seeing pictures of tempting food can light up the pleasure-seeking areas of obese peoples' brains.

Two Reactions to Cake

Two conferences this week on obesity are each examining aspects of how appetite works in the brain and why some people ignore their built-in fullness signals. Scientists hope that breakthroughs will lead to ways to retrain people's thinking about food or weight-loss drugs that can target certain brain areas.

In a study presented this week at the International Conference on Obesity in Stockholm, researchers from Columbia University in New York showed pictures of cake, pies, french fries and other high-calorie foods to 10 obese women and 10 non-obese women and monitored their brain reactions on fMRI scans. In the obese women, the images triggered a strong response in the ventral tegmental area (VTA), a tiny spot in the midbrain where dopamine, the "desire chemical," is released. The images also activated the ventral pallidum, a part of the brain involved in planning to do something rewarding.

"When obese people see high-calorie foods, a widespread network of brain areas involved in reward, attention, emotion, memory and motor planning is activated, and all the areas talk to each other, making it hard for them to resist," says lead investigator Susan Carnell, a research psychiatrist at the New York Obesity Research Center at Columbia University.

The Power of Cake?
This Power-of-Food Scale helps gauge how vulnerable you are to 'hedonic' eating. Indicate from 1-5 which of the following best describes you:

1 Don't agree at all
2 Agree a little
3 Agree somewhat
4 Agree
5 Strongly agree

___ 1. I find myself thinking about food even when I'm not physically hungry.

___ 2. I get more pleasure from eating than I do from almost anything else.

___ 3. If I see or smell a food I like, I get a powerful urge to have some.

___ 4. When I'm around a fattening food I love, it's hard to stop myself from at least tasting it.

___ 5. It's scary to think of the power that food has over me.

___ 6. When I know a delicious food is available, I can't help myself from thinking about having some.

___ 7. I love the taste of certain foods so much that I can't avoid eating them even if they're bad for me.

___ 8. Just before I taste a favorite food, I feel intense anticipation.

___ 9. When I eat delicious food I focus a lot on how good it tastes.

___ 10. Sometimes, when I'm doing everyday activities, I get an urge to eat "out of the blue" (for no apparent reason).

___ 11. I think I enjoy eating a lot more than most other people.

___ 12. Hearing someone describe a great meal makes me really want to have something to eat.

___ 13. It seems like I have food on my mind a lot.

___ 14. It's very important to me that the foods I eat are as delicious as possible.

___ 15. Before I eat a favorite food my mouth tends to flood with saliva.

Scoring: Add up your responses and divide the total by 15.

1.0 - 2.3: You're unlikely to be preoccupied with food or lose control over eating.

2.4 - 3.6: You're somewhat preoccupied with food but are unlikely to have a problem unless you're significantly overweight.

3.7 - 5.0: You're frequently preoccupied with food and at risk of losing control over your eating. This is especially problematic if you are also significantly overweight.

Source: Walll Street Journal, July 13, 2010.