In the 15th century it was the plague, in the 19th century it was tuberculosis, in the 21st it is... obesity? Last year, the prestigious Journal of the American Medical Association devoted an entire issue to what researchers called the "obesity epidemic" in the United States, including studies about annual deaths attributable to obesity and the "disease burden" associated with being overweight. It was enough to scare you right out of Dunkin' Donuts -- for today, anyway -- and it got the predictable front-page coverage.

Some interesting things happen when health officials make pronouncements like this. Though the information is important to our health, it also can result in trouble when people overreact to this news. How do they overreact? They begin to diet. And since most people still define a diet as a limited period of severe hunger and self-deprivation, they eventually go off the diet, usually in the form of a binge. Or they obsess, then go ahead and eat the "bad" stuff anyway -- guiltily, furtively -- which does not lead to satisfaction.

Worst-case scenario: They develop eating disorders, a condition that is, not entirely coincidentally, also often called epidemic these days. Or, if not full-fledged anorexia or bulimia, they can develop so-called subclinical disordered eating: chronic dieting behaviors that, while not life threatening, can severely damage the quality of one's life.

"I weigh more than my friends"

Here's one way that can happen. A 10-year-old girl (in this case, my daughter) gets a physical. She's weighed and measured. Her (young, recently educated) doctor starts talking about how her weight gain outpaced her height gain this year, asks about her diet and cautions me, as her (increasingly outraged and irritated) mother, to watch her intake of sweets and junk food. I try to deflect his comments as my daughter listens avidly. She walks out saying, "I weigh 79 pounds! That's so much! That's more than my friends" -- sentiments she repeats several times over the next few weeks.

Reality check: My daughter has a slender, athletic build. Like many children, she often grows in a pattern that goes out, then up, but the fact that she's not even remotely overweight is observable to the naked eye. Her medical chart records the fact that there's no obesity problem lurking in her family history. And there are these facts to consider: By age 9, more than 50 percent of girls have embarked on their first weight-loss diet; by age 17, four out of five healthy-weight young women think they are too fat.

Obviously, not every one who's cautioned about her weight is going to develop an eating disorder. But this tale of modern doctoring illustrates how new medically sanctioned fat paranoia can distort one's view of reality -- especially when the physician isn't fully informed about the dangers of inappropriately focusing on weight loss. (This doctor had only, after all, to use his eyes to see that my daughter didn't have a weight problem. And he could've inquired about her physical activity, nutrition habits and mental health, rather than stressing chart measurements.) It also shows what happens in the culture at large when one looks only at numbers on a scale or chart and not at an individual: You don't get the whole picture.

Fat doesn't mean unhealthy

For example, researchers like Steven Blair, M.D., at the Cooper Institute for Aerobic Research dispute the idea that overweight automatically means unhealthy. Blair, himself a rather stocky fellow, has conducted several well-designed studies that show that fit but overweight people can be just as healthy and long-lived as fit, normal-weight people. It was the unfit -- whether 120 pounds or 200-plus -- who were getting sick and dying off. (FYI, the fit subjects were not severely obese.)

Another study, this one published two years ago by the New England Journal of Medicine, JAMA's competitor as the nation's other top medical journal, suggested that the extra risk of dying associated with being fat was relatively modest -- much more modest than expected. There was no extra risk to being moderately overweight. ("Moderately overweight" is now defined by experts as a Body Mass Index of 25-27, while obese -- a different health issue entirely -- is a BMI of 30-plus.) This was a huge study, the largest ever conducted on the health risks of being overweight (more than 324,000 people were analyzed), but the press coverage was minimal -- it showed up on page 19 of The New York Times. After all, what a nonstory: Being fat won't kill you!

Want to gain weight? Go on a diet

The American propensity for shoveling in low-quality food while moving around as little as possible is cause for alarm, true. Ironically, there seems to be a parallel in the greater cultural expectation of slenderness for everyone -- for both aesthetic and health reasons -- and the statistical rise in obesity. In one very straightforward way, the deck has been stacked: Two years ago health officials at the National Heart, Lung, and Blood Institute introduced new, stricter weight charts that instantly transferred 29 million Americans from a "healthy weight" category into the "overweight" category.

In a more subtle way, years of observation have made clear that traditional low-cal weight-loss diets -- still the way most people try to slim down -- often backfire. Temporarily and drastically lowering calorie intake slows down metabolism, produces intense cravings and feelings of deprivation, and often results eventually in weight gain. Many weight-loss experts now agree with David M. Garner, Ph.D., a clinical psychologist with the River Centre Clinic, an eating-disorder treatment center in Sylvania, Ohio, that "if you want to gain weight, go on a diet."

Do 2 extra pounds eventually become 20?

There is a slippery-slope fear behind the overzealousness exhibited by my pediatrician, the idea that 1 or 2 extra pounds inevitably become 10 or 12 and then 20 or 30. While it is true that a gradual weight gain is common with age -- a phenomenon that some researchers say may actually be beneficial, providing a margin of nutritional safety in case of illness -- it is not remotely a given that every overweight person becomes obese. (However, the condition known as creeping obesity, the gradual weight gain over decades, is without question a health risk.) Common sense reminds us that we all know people who have been pudgy their entire lives, who work out and eat well and have round, endomorphic physiques that are usually echoed elsewhere in their families. We also all know thin people who are sedentary and a stranger to most fruits and vegetables. They usually have thin relatives, too.

Here's a concept: What if the public-health officials whose warnings about fitness sound so dire focused more on the process rather than on the end point? The research just isn't there to say that every overweight person is unhealthy, so let's encourage the habits that lead to good health -- exercising on a regular basis, eating fruits and vegetables in their natural state -- rather than encouraging a singular physical condition (thinness) that tells us very little about someone's actual chance of living robustly or keeling over of a heart attack at 52. The evidence of Steven Blair, among others, tells us that if you act healthy you become healthy, regardless of numbers on a scale. Excellent news for every 25-year-old -- or 10-year-old -- tempted to diet herself down to someone else's idea of perfection.