Because you need food to live, your body is no slouch at letting you know that it’s ready for breakfast, lunch, dinner, and maybe a few snacks in between. This article explains the signals your body uses to get you to the table, to the drive-through of your favorite restaurant, or to the vending machine down the hall.

People eat for two main reasons. The first reason is hunger; the second is appetite. Hunger and appetite are not synonyms. In fact, hunger and appetite are entirely different processes. Hunger is the need for food. It is:

• A physical reaction that includes chemical changes in your body related to a naturally low level of glucose in your blood several hours after eating.

• An instinctive, protective mechanism that makes sure that your body gets the fuel it requires to function reasonably well.

Appetite is the desire for food. It is:

• A sensory or psychological reaction (looks good! smells good!) that stimulates an involuntary physiological response (salivation, stomach contractions)

• A conditioned response to food

The practical difference between hunger and appetite is this: When you’re hungry, you eat one hot dog. After that, your appetite may lead you to eat two more hot dogs just because they look appealing or taste good.

In other words, appetite is the basis for the familiar saying: “Your eyes are bigger than your stomach.” Not to mention the well-known advertising slogan: “Bet you can’t eat just one.” Hey, these guys know their customers.

Hunger and Satiety Cycle

Your body does its best to create cycles of activity that parallel a 24-hour day. Like sleep, hunger occurs at pretty regular intervals, although your lifestyle may make it difficult to follow this natural pattern — even when your stomach loudly announces it’s empty!

The clearest signals that your body wants food, right now, are the physical reactions from your stomach and your blood that let you know it’s definitely time to put more food in your mouth and — eat!

An empty belly has no manners. If you do not fill it right away, your stomach will issue an audible — sometimes embarrassing — call for food. This rumbling signal is called a hunger pang. Hunger pangs actually are plain old muscle contractions.

When your stomach’s full, these contractions and their continual waves down the entire length of the intestine — known as peristalsis — move food through your digestive tract. When your stomach’s empty, the contractions just squeeze air, and that makes noise.

This phenomenon first was observed in 1912 by an American physiologist named Walter B. Cannon. Cannon convinced a fellow researcher to swallow a small balloon attached to a thin tube connected to a pressure-sensitive machine. Then Cannon inflated and deflated the balloon to simulate the sensation of a full or empty stomach.

Measuring the pressure and frequency of his volunteer’s stomach contractions, Cannon discovered that the contractions were strongest and occurred most frequently when the balloon was deflated and the stomach empty. Cannon drew the obvious conclusion: When your stomach is empty, you feel hungry.

Every time you eat, your pancreas secretes insulin, a hormone that enables you to move blood sugar (glucose) out of the blood and into cells where it’s needed for various chores. Glucose is the basic fuel your body uses for energy.

As a result, the level of glucose circulating in your blood rises and then declines naturally, producing a vague feeling of emptiness, and perhaps weakness, that prompts you to eat. Most people experience the natural rise and fall of glucose as a relatively smooth pattern that lasts about four hours.

The satisfying feeling of fullness after eating is called satiety, the signal that says, okay, hold the hot dogs, I’ve had plenty, and I need to push back from the table.

As nutrition research and the understanding of brain functions have become more sophisticated, scientists have discovered that your hypothalamus, a small gland on top of the brain stem (the part of the brain that connects to the top of the spinal cord), seems to house your appetite controls in an area of the brain where hormones and other chemicals that control hunger and appetite are made (see Figure below).

For example, the hypothalamus releases neuropeptide Y (NPY), a chemical that latches onto brain cells and then send out a signal: More food!

Other body cells also play a role in making your body say, “I’m full.” In 1995, researchers at Rockefeller University discovered a gene in fat cells (the body cells where fat is stored) that directs the production of a hormone called leptin (from the Greek word for thin).

Leptin appears to tell your body how much fat you have stored, thus regulating your hunger (need for food to provide fuel). Leptin also reduces the hypothalamus’s secretion of NPY, the hormone that signals hunger.

When the Rockefeller folks injected leptin into specially bred fat mice, the mice ate less, burned food faster, and lost significant amounts of weight. Eventually, researchers hope that this kind of information can lead to the creation of safe and effective drugs to combat obesity.

Throughout the world, the cycle of hunger (namely, of glucose rising and falling) prompts a feeding schedule that generally provides four meals during the day: breakfast, lunch, tea (a mid-afternoon meal), and supper.

In the United States, a three-meal-a-day culture forces people to fight their natural rhythm by going without food from lunch at noon to supper at 6 p.m. or later.

The unpleasant result is that when glucose levels decline around 4 p.m., and people in many countries are enjoying afternoon tea, many Americans get really testy and try to satisfy their natural hunger by grabbing the nearest food, usually a high-fat, high-calorie snack.

In 1989, David Jenkins, M.D., Ph.D., and Tom Wolever, M.D., Ph.D., of the University of Toronto, set up a “nibbling study” designed to test the idea that if you even out digestion — by eating several small meals rather than three big ones — you can spread out insulin secretion and keep the amount of glucose in your blood on an even keel all day long.

The theory turned out to be right. People who ate five or six small meals rather than three big ones felt better and experienced an extra bonus: lower cholesterol levels.

After two weeks of nibbling, the people in the Jenkins- Wolever study showed a 13.5 percent lower level of low-density lipoproteins (LDL) than people who ate exactly the same amount of food divided into three big meals.

As a result, many diets designed to help you lose weight or control your cholesterol (what? you haven’t got a copy of Controlling Cholesterol For Dummies? Impossible!) now emphasize a daily regimen of several small meals rather than the basic big three. Smart cookies. Low-fat, low-cholesterol, lowcal, of course.

The best way to deal with hunger and appetite is to find out how to recognize and follow your body’s natural cues. If you’re hungry, eat — in reasonable amounts that support a realistic weight.

And remember: Nobody’s perfect. Make one day’s indulgence guilt-free by reducing your calorie intake proportionately over the next few days. A little give here, a little take there, and you’ll stay on target overall.

Your physical and psychological environments definitely affect appetite and hunger, sometimes leading you to eat more than normal, sometimes less.

You’re more likely to feel hungry when you’re in a cool place than you are when you’re in a warm one. And you’re more likely to want high-calorie dishes in cold weather than in hot weather.

Just think about the foods that tempt you in winter — stews, roasts, thick soups — versus those you find pleasing on a simmering summer day — salads, chilled fruit, simple sandwiches. This difference is no accident.

Food gives you calories. Calories keep you warm. Making sure that you get what you need, your body even processes food faster when it’s cold out.

Your stomach empties more quickly as food speeds along through the digestive tract, which means those old hunger pangs show up sooner than expected, which, in turn, means that you eat more and stay warmer and . . . well, you get the picture.

Everybody knows that working out gives you a big appetite, right? Well, everybody’s wrong (it happens all the time). Yes, people who exercise regularly are likely to have a healthy (read: normal) appetite, but they’re rarely hungry immediately after exercising because:

• Exercise pulls stored energy — glucose and fat — out of body tissues, so your glucose levels stay steady and you don’t feel hungry.

• Exercise slows the passage of food through the digestive tract. Your stomach empties more slowly and you feel fuller longer.

Caution: If you eat a heavy meal right before heading for the gym or the stationary bike in your bedroom, the food sitting in your stomach may make you feel stuffed. Sometimes, you may develop cramps.

• Exercise (including mental exertion) reduces anxiety. For some people, that means less desire to reach for a snack.

Severe physical stress or trauma — a broken bone, surgery, a burn, a high fever — reduces appetite and slows the natural contractions of the intestinal tract.

If you eat at times like this, the food may back up in your gut or even stretch your bowel enough to tear it. In situations like this, intravenous feeding — fluids with nutrients sent through a needle directly into a vein — give you nutrition without irritation.

Taking some medicines may make you more (or less) likely to eat. Some drugs used to treat common conditions affect your appetite. When you use these medicines, you may find yourself eating more (or less) than usual.

This side effect is rarely mentioned when doctors hand out prescriptions, perhaps because it isn’t life-threatening and usually disappears when you stop taking the drug.

Some examples of appetite uppers are certain antidepressants (mood elevators), antihistamines (allergy pills), diuretics (drugs that make you urinate more frequently), steroids (drugs that fight inflammation), and tranquilizers (calming drugs).

Appetite reducers include some antibiotics, anti-cancer drugs, anti-seizure drugs, blood pressure medications, and cholesterol-lowering drugs. Of course, not every drug in a particular class of drugs (that is, antibiotics or antidepressants) has the same effect on appetite.

For example, the antidepressant drug amitriptyline (Elavil) increases your appetite and may cause weight gain; another antidepressant drug, fluoxetine (Prozac) usually does not. The fact that a drug affects appetite is almost never a reason to avoid using it.

But knowing that a relationship exists between the drug and your desire for food can be helpful. Plain common sense dictates that you ask your doctor about possible drug/appetite interactions whenever a drug is prescribed for you.

If the drug package the pharmacist gives you doesn’t come with an insert, ask for one. Read the fine print about side effects and other interesting details — such as the direction to avoid alcohol or driving or using heavy machinery.