PHYSICAL SIDE OF EATING AND HUNGER: ENTER THE ENDORPHINS

One of the most fascinating avenues of biochemical research over the past few years led to the discovery that the body manufactures its own natural painkillers. These substances became known as endorphins, from endo (meaning “arising from within”) and morphine. Evidence suggests that endorphins play a direct role in regulating appetite and affect other functions such as pain relief, memory-even blood pressure.

Like a lock that accepts only a certain key, cells of the central nervous system have receptors that accept and respond only to particular endorphins. For example, it appears that one receptor involved in stimulating appetite (called the “kappa” receptor) is designed to work only with the substance known as dynorphin. Dynorphin molecules fit into the kappa receptors. Nerve impulses then travel to the hypothalamus, which interprets and relays them as hunger cues. Appetite is thus stimulated, particularly for sweet-tasting foods.

Scientists soon found that rats given morphine (which increases endorphin activity) and allowed to choose from among the macronutrients tended to increase their fat intake while ignoring the carbohydrates. The investigators then administered drugs known to block the opiate receptors. Doing so, they found, tended to suppress eating. They found the same effect in humans. They then theorized that use of opiate blockers (also known as opiate antagonists) suppresses appetite by producing feelings of fullness or satiety.

But what triggers the release of endorphins in the first place? Further experiments found that food deprivation or stress can play a role. In other words, if you are under stress-pressure at work or school, for example-your body secretes endorphins to control the damage. Some of those endorphins ease any physical pain you might be feeling, while others work to stimulate your appetite, especially for sweet foods. You may respond to these signals from your hypothalamus by gobbling down a slice of pie, for example.

But why should eating sugar help relieve stress? Why doesn’t lettuce (to pick a food at random) have the same effect?

As it turns out, the very act of eating sugar stimulates the opiate-releasing process even further. Not only does sugar make the body release more endorphins, it also enhances the ability of the receptor to bind with the substance-like oiling a lock to make a key work better. Thus, eating sweet foods does indeed relieve stress, producing feelings of relaxation and contentment by enhancing the amount of natural painkillers floating around inside the body.

You might have spotted the flaw in this otherwise tidy little system. Here’s the problem: When a person is under stress, the body releases endorphins that stimulate the appetite for sweet foods. The person then eats a candy bar. The sugar in turn stimulates further secretion of endorphins, triggering greater appetite and leading to more consumption of sweets. Where does the cycle end?

As you might have suspected, it may not end. Theory has it that some people with bulimia might be caught up in the vicious cycle represented by the opiate-receptor feedback loop.

A bulimic will typically skip meals in the belief that doing so will keep her weight under control. But skipping meals only defers appetite; it doesn’t eliminate it. Eventually the urge to eat becomes overpowering, leading to a binge. Food deprivation itself can trigger the release of endorphins, which in turn stimulate appetite. Thus a bulimic who deprives herself of meals causes her body to produce a powerful natural appetite stimulant.

Many a patient reports that her binges occur in times of stress. Research has shown that stress also cranks up the endorphin system. Finally, when a binge leads to consumption of high-sugar items-ice cream, cookies, candy-even more opiates are released, stimulating appetite even further, and the vicious cycle kicks into high gear. For many, the only thing that disrupts this process is a drastic measure: self-induced vomiting.

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