An eating disorder is a psychological illness that leads you to eat either too much or too little. Indulging in a hot fudge sundae once in a while is not an eating disorder. Neither is dieting for three weeks so that you can fit into last year’s dress this New Year’s Eve.

The difference between normal indulgence and normal dieting to lose weight versus an eating disorder is that the first two are acceptable, healthy behavior while an eating disorder is a potentially life-threatening illness that requires immediate medical attention.

Eating too much

Although many recent studies document an alarming worldwide increase in obesity, particularly among young children, not everyone who is larger or heavier than the current American ideal has an eating disorder. Human bodies come in many different sizes, and some healthy people are just naturally larger or heavier than others.

An eating disorder may be present, though, when:

• A person continually confuses the desire for food (appetite) with the need for food (hunger).

• A person who has access to a normal diet experiences psychological distress when denied food.

• A person uses food to relieve anxiety provoked by what he or she considers a scary situation — a new job, a party, ordinary criticism, or a deadline.

Traditionally, doctors have found that treating obesity successfully is difficult. However, recent research suggests that some people overeat in response to irregularities in the production of chemicals that regulate satiety (your feeling of fullness).

This research may open the path to new kinds of drugs that can control extreme appetite, thus reducing the incidence of obesity-related disorders such as arthritis, diabetes, high blood pressure, and heart disease.

Bingeing, purging, and starving

Some people relieve their anxiety not by eating but by refusing to eat or by regurgitating food after they’ve eaten it. The first kind of behavior is called anorexia nervosa; the second, bulimia. Anorexia nervosa (voluntary starvation), the eating disorder that sidelined Mary-Kate Olsen in 2004, is virtually unknown in places where food is hard to come by.

It seems to be an affliction of affluence, most likely to strike the young and well-to-do. It’s nine times more common among women than among men. Many doctors who specialize in treating people with eating disorders suggest that anorexia nervosa may be an attempt to control one’s life by rejecting a developing body.

In other words, by starving themselves, anorexic girls avoid developing breasts and hips, and anorexic boys avoid developing the broad wedge-shape adult male body. By not growing wide, both hope to avoid growing up. Left untreated, anorexia nervosa can end in death by starvation. A second form of eating disorder is bulimia.

Unlike people with anorexia, individuals with bulimia don’t refuse to eat. In fact, they may often binge (consume enormous amounts of food in one sitting: a whole chicken, several pints of ice cream, a whole loaf of bread). But bulimic people don’t want to keep the food they eat in their bodies.

They may use laxatives to increase defecation, but the more common method they use for getting rid of food is regurgitation. Bulimic people may simply retire to the bathroom after eating and stick their fingers into their throats to make themselves throw up. Or they may use emetics (drugs that induce vomiting). Either way, danger looms.

The human body is not designed for repeated stuffing followed by regurgitation. Bingeing may dilate the stomach to the point of rupture; constant vomiting may severely irritate or even tear through the lining of the esophagus (throat).

In addition, the continued use of large quantities of emetics may result in a life-threatening loss of potassium that triggers irregular heartbeat or heart failure, factors that contributed to the 1983 death of singer Karen Carpenter, an anorexic/bulimic who — at one point in her disease — weighed only 80 pounds but still saw herself as overweight.

One symptom of anorexia and/or bulimia is the inability to look in a mirror and see yourself as you really are. Even at their most skeletal, people with these eating disorders perceive themselves as grossly fat.

As you can see, eating disorders are life-threatening conditions. But they can be treated. If you (or someone you know) experience any of the signs and symptoms just described, the safest course is to seek immediate medical advice and treatment.

For more info about eating conditions, contact the National Eating Disorders Association, 603 Stewart St., Suite 803, Seattle, WA 98101; phone 800-931-2237; e-mail info@NationalEatingDisorders.org; Web site www.nationaleatingdisorders.org.