You see their photos everywhere: gaunt models strutting the latest in fashion, skinny socialites dining in trendy restaurants, svelte young actresses partying till dawn at the newest club. No wonder American women seem obsessed with their weight.

But when is thin too thin? And when does dieting turn into a life-threatening eating disorder?

Over the past few years, the media have been filled with the tragic stories of famous women whose abnormal eating behavior led to serious health problems and even death. For the most part, these women suffered from anorexia nervosa or bulimia nervosa. Together these eating disorders affect approximately 8 million Americans—mostly young women and teenaged girls. At the opposite extreme, excessive overweight (obesity), while rarely deadly in itself, increases your risk of life-threatening medical conditions ranging from breast cancer to heart disease.

All three of these problems represent normal diet gone awry. Anorectic women relentlessly pursue thinness by literally starving themselves for varying periods of time. Victims of bulimia suffer repeated binges of eating followed by purging through self-induced vomiting, laxatives, and similar measures. Seriously overweight women—defined medically as those who are more than 20 percent over their ideal weight—are often plagued by compulsions to eat.

It's important to remember that these disorders are not merely the normal variations in eating we all go through. Dieting is not automatically a sign of anorexia any more than an occasional eating binge such as consuming an entire package of cookies in one sitting means you are suffering from bulimia. If you're 5 or 10 pounds overweight, you may feel "fat," but that doesn't make you medically obese.