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. 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.” 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? Rumble? Could I make this up?) 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.

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 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.