Recognizing hunger

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!

Growling and rumbling: Your stomach speaks

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 (see Chapter 2 for more about digestion). 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.

Getting that empty feeling

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. (See Chapter 8.) 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.

What meal is this, anyway?

Breakfast and lunch leave no doubt. The first comes right after you wake up in the morning; the second, in the middle of the day, sometime around noon.

But when do you eat dinner? And what about supper?

According to Webster's New International Dictionary of the English Language (2nd edition, 1941 — 15 pounds, including the new binding I put on when the old one crumbled after I

dropped the darned thing on its spine once too often), dinner is the main meal of the day, usually eaten around midday, although (get this) some people, "especially in cities," have their dinner between 6 p.m. and 8 p.m. — which probably makes it their supper, because Webster's calls that a meal you eat at the end of the day.

In other words, dinner is lunch except when it's supper. Help!

Knowing when you're full

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 14-1). For example, the hypothalamus releases neuropeptide Y (NPY), a chemical that latches onto brain cells and then send out a signal: More food!

Figure 14-1:

Your hypothalamus is in charge of your appetite!

Figure 14-1:

Your hypothalamus is in charge of your appetite!

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.

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