Sometime today, your brain will probably be besieged by contradictory demands.
Your better judgment will tell you NO NO NO. Your surroundings will tell you YES YES YES. A nerve reaching in from your gut will insist NO NO NO. Signals from your mouth will make some brain circuits cry YES YES YES.
And in the end, your brain will make an executive decision and probably order your mouth to say:
"Pie? Sure, I'd love some."
Putting on a Thanksgiving feast is complicated enough, but it's nothing compared to how your body decides how much of it to eat.
Scientists have learned a lot in the past few years about what controls appetite. In a nation where many people eat too much, such discoveries often hit the front pages because they might lead to better weight-loss drugs. This year alone brought widely reported evidence that one natural hormone boosts appetite and another dampens it, for example.
Such research also sheds light on that Thanksgiving Day war of impulses in your brain.
Psychology and biology
Your eagerness to eat on that day -- or any other day -- is an "exquisite interaction between our psychology and our biology and our environment," says Barbara Rolls, nutrition professor at Penn State University.
It all starts long before anyone buys the turkey.
Everybody needs to eat, of course, but eat too much and you get too fat. So your body has a regulatory system that works over the long haul for controlling how much fat you carry.
As many Americans can attest, it doesn't always work. But in fact, scientists say, in general it works very well at matching the number of calories you eat to the number you burn off over the course of a week or more.
In Western cultures, on average, people put on a half-pound to a pound per year over much of their adult lives, which translates to an excess intake of just 10 to 20 calories a day, physicians David Cummings and Michael Schwartz of the University of Washington in Seattle note in a forthcoming review of the biology of obesity.
That excess is about the equivalent of a Ritz cracker a day.
Of course, some people put on more weight and others less. But the Ritz cracker example illustrates a remarkably precise regulation of body weight, considering how variable people's diets and activities are from day to day, Schwartz says.
How does the body achieve that? So far scientists have identified two hormones, insulin and especially leptin, that tell the brain how much fat you're carrying.
Leptin, made by fat cells and discovered only in 1994, raised hopes of being a powerful weight-loss drug until studies in people proved disappointing.
Leptin and insulin appear to influence how sensitive the brain is to the body's satiety signals, Schwartz said. If you've put on fat, their levels go up, making it easier for the brain to hear your body's signals to stop eating, so you eat less, he said. If you're underweight, low levels have the opposite effect.
Unfortunately for most of us, this system appears much better at keeping us from losing weight than gaining it. That's probably because it evolved at a time when famine could appear anytime. In that environment, weight gainers are rewarded with survival, while "the finicky guys are dead," Cummings said.
So this long-term system is already humming when the Thanksgiving party gets under way, and all sorts of short-term hunger signals go to work.
Much -- maybe most -- of your hunger to start eating and keep eating a particular meal comes from psychological cues rather than any biological signal that your body needs energy right away, scientists say.
"When you and I sit down to eat, it isn't because our blood sugar is low or because some hormone is too low or too high," says Stephen Woods, director of the obesity research center at the University of Cincinnati Medical Center.
"It's because it's convenient. It's a break between assignments, or it's because it's a time of day that we feel comfortable eating. We've been rewarded and had good meals at this time in the past. Or because somebody prepared a meal for us at that time."
That's not all, especially at a ritualistic meal like Thanksgiving's. Just being in a convivial crowd makes you eat more. You want to please the cook. You revel in happy memories of past feasts. People urge you to take another helping, perhaps to justify their own.
The tradition to pig out is hard to resist, especially if a few drinks have undermined your restraint.
As with any meal, huge portions encourage you to eat more. And the variety in the foods does the same thing, because people can tire of one kind of food but remain ready to eat another, Rolls said. She recalled a 14-course pasta meal in Italy. Everybody got sick of pasta, but then they readily dived into dessert.
More generally, tasty foods make brain cells release chemicals that make you feel rewarded and may suppress feelings of being full, said researcher Bart Hoebel of Princeton University. Result: you eat more.
Then there's the hormone ghrelin. Discovered only two years ago, it is produced mostly by the stomach. Its bloodstream levels start rising a couple hours before each meal and peak about the time you finally pick up a fork, apparently spurring you to eat, Cummings said.
Even this biological signal might be turned on by psychological factors, Schwartz said, although Cummings said new, unpublished research argues against that.
A fluctuation in blood sugar levels has also been proposed as a hunger signal, but scientists disagree on whether that's true in everyday life.
As you eat and eat, your body mounts a campaign to get you to stop.
Tiny structures in your stomach sense that this organ is stretching, and send a message -- basically "'AAAI@semovm.semo.edu" -- to the nearby vagus nerve, which extends up to your brainstem. The vagus nerve is also hearing from a hormone called cholecystokinin, or CCK, that pours out of the small intestine to say, "Stop eating." At least half a dozen substances from the intestine reinforce the message, Cummings said.
One messenger from the gut made news last summer: PYY3-36, a hormone that enters the bloodstream and goes to the brain. It was found to make people eat about one-third less than usual over 24 hours when injected into volunteers.
With all these satiety signals banging on the pipes, you'd think the brain upstairs would get the message pretty quick.
But in your heart, you know better.
These signals pressure the brain to stop the meal, "but they're not absolute," Woods said.
"It doesn't matter how many satiety factors we've got. When they roll out the pumpkin pie on Thanksgiving, I'm going to get hungry, particularly if they put whipped cream on it."
Even with ordinary meals, the deck is often stacked against the body's satiety messages, said obesity expert Dr. F. Xavier Pi-Sunyer of the St. Luke's-Roosevelt Hospital Center in New York.
American meals tend to be so brief -- around 10 minutes -- that it's not clear the signals have enough time to restrain somebody who's wolfing down two cheeseburgers, a shake and a sack of potato chips, he said.
Nonetheless, we do stop eating eventually. The signals grow strong enough to restrain us, Pi-Sunyer said, and people follow their habits for whatever portion sizes have satisfied them in the past.
While scientists say they have plenty to learn about how the body controls eating, they're optimistic that their findings are revealing targets for drugs that can help people lose weight.
For example, since obese people already have high levels of the appetite-quelling hormone leptin, scientists can look for drugs that will make their brains pay attention. Or they could try to meddle with the chemical messages in the brain that promote or suppress eating. And ghrelin, PYY3-36 and similar bodily signals are obvious leads.
"We're just in our infancy in trying to define what's out there to curb our appetite," said researcher Sarah Leibowitz of Rockefeller University in New York.
In the meantime, Schwartz sees hope for resisting that Ritz-cracker-a-day pressure to gain weight slowly over the years.
"I believe this is not something that is inevitable," he said. "The genetics don't determine you're going to gain this weight. It's an interaction between your biological system and the environment in which you live. So I think it's very reasonable for people to think they can avert further weight gain by adopting a healthier lifestyle."