Who's fat? New definition adopted

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Using the old criteria, the average woman -- with a height of 5 feet, 4 inches (1.6 meters) and weighing 155 pounds (70 kilograms) -- was considered overweight.

Under the new definition, that weight drops to 145 pounds (66 kg). A person at the same height who weighs 175 pounds (79 kg) would be considered obese.

CNN's Anne McDermott explains how the new guidelines affect you 2 min. VXtreme video

Someone who is 5 feet, 10 inches (1.8 meters) tall and weighs 185 pounds (83 kg) was considered overweight under the old guidelines. Now, for the same height, 175 pounds (79 kg) is overweight and 209 pounds (94 kg) is obese.

Or, put another way, 25 million Americans who weren't fat before, are now. Even under the previous standards, more than half of all adult Americans are overweight.

The guidelines are based on Body Mass Index (BMI), a height-to-weight formula that ignores whether the weight is from fat or muscle. It also ignores whether someone has a large or small frame. The weights are the same for men and women.

Dr. Xavier Pi-Sunyer from the National Institutes of Health comments on the new weight guidelines

What factors are figured into the obesity equation AIFF or WAV (264 K / 22 sec. audio) The ramifications of the new weight guidelines "An education campaign ..." AIFF or WAV (315 K / 28 sec. audio)

Under the BMI formula, a reading of 26 or higher is overweight.

Is the change necessary?

The NIH says the changes are necessary because of studies linking extra weight to health problems.

The American Heart Association recently added obesity to its list of major risk factors for heart disease and heart attack, along with smoking, high blood pressure, high cholesterol and a sedentary lifestyle.

Some health experts reject the new guidelines, claiming people who aren't fat are now considered overweight. For example, under the new definitions, many professional athletes would be considered too heavy.

Critics also worry that these lower weights will persuade doctors to start prescribing diet drugs for people who don't need them. Some diet drugs carry health risks, such as an increase in blood pressure.

Correspondents Elizabeth Cohen and Anne McDermott contributed to this report.