There is little doubt among mainstream health professionals that being truly obese is a health hazard. But what about the borderline plump? The mildly pudgy? All those people with BMIs between 25 and 29, who according to the charts are overweight?

Government health agencies often lump all degrees of overweight together, noting for instance that over 60 percent of Americans are too heavy. But more than half of these people — roughly one-third of all Americans and 800 million people worldwide — are overweight but not obese.

Riding to the mall one Saturday, Joanne Ikeda's younger sister turned to her and asked, out of nowhere, "Do you know that I am now overweight?"

From heart-sinking personal experience, millions upon millions of people can imagine exactly what led up to that admission. She had stepped on the scales and noticed a number a little north of usual. So she looked it up on a body mass index chart. And — No! — she was officially too heavy.

Not obese, not even close. But her BMI was 26, a full, leaden point above the carved-in-stone cutoff for being overweight.

Many who are overweight but not obese are like Ikeda's sister, Irene Pakel: 55 years old, 5-foot-3, weight in the mid-140s, maybe 10 pounds or so into the overweight category. Does she have a weight problem, one that might even shorten her life?

To many in the field, the answer is clear.

"Over 99 percent of experts throughout the world are convinced by overwhelming data that being overweight is a huge problem indeed for the majority of the world," says Dr. Philip James, chairman of the International Obesity Task Force (search) in London.

But as co-director of the Center for Weight and Health at the University of California at Berkeley (search), Ikeda has a professional, though somewhat contrarian, opinion: Not likely.

"She's very physically fit," says Ikeda. "She goes to Curves everyday after work. To look at her, you would say, 'Here is someone who is not even slightly chubby.'"

Ikeda has a weight problem of her own. Her BMI is 33, which makes her officially obese. Does that bother her?

"Not really," she said. "What matters is my metabolic fitness."

Some in the world of diet and health — and as James says, they are a minority — feel too much is made of the lower end of the BMI scale, that perhaps even the entire category called overweight causes much anxiety for nothing.

"This is so ludicrous," says Ikeda. "Why are we doing this to ourselves? I think it has a lot to do with the dieting and pharmaceutical industry and the pressure to medicate every condition."

"A completely phony category" is what University of Colorado attorney Paul Campos calls overweight. He is author of "The Obesity Myth" and argues that the real health problem is too little exercise, not too much weight.

"There is no basis in the medical literature to draw the conclusion that having a BMI between 25 and 29 an is an independent health risk," he says. "It is quite preposterous to make that claim."

Such out-of-step pronouncements are fighting words in the field of nutrition and health, where many consider the evils of fat to be beyond question.

Former New England Journal of Medicine (search) Editor Jerome Kassirer remembers the indignation six years ago when he wrote a skeptical editorial calling the data linking weight and ill health "limited, fragmentary and often ambiguous."

"We got flack from just about everybody except the fatties," he remembers, although he hasn't seen anything since to change his mind.

One of the most persistent doubters is Steven Blair of the Cooper Aerobics Center (search) in Dallas. His research buttresses the idea that fitness is more important than fatness.

Following 25,000 Cooper patients for eight years, he found that it is better to be fit and fat than skinny and sedentary. In fact, overweight people who have good stamina on a treadmill test live just as long as equally fit people who are not overweight.

"If you look at people in the overweight category, many have none of the conventional risk factors, like elevated blood pressure and cholesterol," says Blair. "Should they be stigmatized and treated? For what?"

Many obesity experts concede Blair has a point: People who are overweight but fit probably do escape many of the consequences of their size. But they often add: So what?

"It may be true, but the fact is, overweight people are not fit," says Dr. Xavier Pi-Sunyer, head of obesity research at St. Luke's-Roosevelt Hospital Center (search) in New York City. "The average American is incredibly sedentary."

Not always, says Blair. About half of the oversize people who get physical exams at Cooper in fact are physically fit, based on their treadmill tests. While hardly a random sample — these people are overwhelmingly white, educated and well off — Blair contends plenty of overweight Americans are just like him.

"I'm a short fat guy," says Blair, whose BMI is about 32. "Would I rather be a short thin guy? Sure. But I'm not. I run everyday and eat a healthy diet and do what I can."

Until a few years ago, government agencies generally agreed that concern about weight begins when a man's BMI hits 28 and a woman's 27. That's 152 pounds for a 5-foot-3 female. But in 1997, the World Health Organization (search) adopted a new standard. BMIs between 25 and 29.9 were now "preobese."

The next year, an expert committee of the U.S. National Institutes of Health came to the same conclusion and called the new category "overweight." Suddenly an extra 35 million Americans were judged to have weight problems.

"We felt it makes sense that if someone's BMI is between 25 and 30, they should not gain more weight," says Pi-Sunyer, who headed the U.S. committee. "We don't feel those millions of people should be trying desperately to lose weight. At a BMI of 30, the risk for both disease and early mortality is so great that people should begin to lose weight."

The 25 cutoff was chosen, he said, because of evidence that the risk of diabetes, high blood pressure and high cholesterol all seem to increase around that point.

The studies backing this up are mostly reviews of large population groups that look for ties between increasing weight and the risk of various diseases and death. The individual studies may have flaws, but taken together, many contend they draw a convincing picture of weight being on a continuum from good health to ill.

"Some say it's all made up, that there's no risk to being moderately overweight," says Dr. Lawrence J. Cheskin, director of the Johns Hopkins Weight Management Center (search). "But there is more and more data to show a very definite dose effect. The heavier you are, the greater your risk."

Some believe the ideal body weight is actually around a BMI of 20 or 21, or a willowy 115 pounds for that 5-foot-3 woman. Others put it closer to 24 or 25. "Everybody agrees that if your BMI is 28, you are at increased risk," says Dr. Steven Heymsfield of St. Luke's-Roosevelt.

This view is backed by studies showing mortality inching up gradually when BMIs reach the high 20s, then climbing more sharply through the 30s and beyond.

One of the most recent, published last year in the Journal of the American Medical Association (search), concludes that white men and women lose about an average year of life if their BMIs top 26 or 27 by the time they reach middle age. However, for reasons that are not easy to explain, this may not be true for blacks. They actually seem to live a year or so longer if overweight but not obese.

While some question the scientific rigor of the mortality estimates, there is less disagreement that common health problems increase among people who are overweight but still below the obesity threshold of a BMI of 30.

For instance, a woman with a BMI of 26 is twice as likely as one who is 21 to develop coronary heart disease. She is twice as likely to get high blood pressure. And she is eight times as likely to get diabetes. Of course, a very thin woman has only a tiny risk of these diseases, so a risk that is double or triple may still be small.

Nevertheless, national health surveys show that about a quarter of people who are overweight but not obese have metabolic syndrome, a particularly worrisome combination of high blood sugar, high blood pressure, low HDL and high triglycerides.

Diabetes is an especially important concern, even for those nowhere near rotund. In fact, the ideal size for avoiding this disease appears to be around a BMI of 22. The risk goes up 25 percent with each unit of BMI after that, and it appears to be especially great for those with pot bellies, even small ones.

Of course, just because surveys of thousands of people show a clear link between rising BMIs and bad health doesn't mean that any particular individual is in trouble with a BMI that hits 25. Many lucky people carry their extra pounds with no ill effect, especially if they have normal blood sugar, healthy blood pressure and no worrisome cholesterol signs.

Many experts contend the biggest hazard of being overweight is what almost inevitably comes next.

"Fat people get fatter," says Dr. Peter McCullough, head of the weight control center at Beaumont Hospital in suburban Detroit. "It is very clear that those who are overweight will become obese over time. People need to understand they have to get this under control."