Think You're Thin? Think Again.

Millions of Americans maintaining what they think is a healthy weight are in for some bad news from government scientists, who are changing a widely used standard for what's safe.

A group of experts concluded that the standard body mass index - a way to measure body weight in relation to height - should be lowered. The body mass index (BMI) provides patients and doctors with the most accurate way to determine when extra pounds begin to hold health risks, according to new guidelines released Wednesday by the National Institutes of Health.

The BMI is closely linked to a person's body fat, and anyone with a BMI of 25 - that is someone, for example, who is 5-foot-4, 145 pounds or 5-foot-10, 174 pounds - should be considered overweight, the researchers determined.

Anyone with a BMI of 30 - such as 5-foot-6, 186 pounds, or 6-foot, 221 pounds - is obese.

By these measurements, 29 million more Americans are overweight than previously thought. That means a total of 97 million American adults - some 55 percent of the population - would be considered overweight or obese, the National Heart, Lung and Blood Institute said.

"I could never be what they say I need to be," one woman told CBS News Correspondent Dan Raviv.

Another government agency, the National Center for Health Statistics, previously has said a woman with a BMI of 27 or a man with a BMI of 28 was overweight. And dietary guidelines released by the federal government in 1996 defined anyone with a BMI of 26 or above as overweight.

That standard - a BMI of 26 - was used by outside researchers in a widely publicized report issued last week. Using that definition, about 54 percent of the U.S. population would be considered overweight.

But officials from the heart and lung institute lowered the threshold, concluding that health risks can begin with a BMI as low as 25. As BMI levels go up, blood pressure and total cholesterol levels also rise, increasing the risk of problems such as heart disease, diabetes, and some cancers, the researchers said after combing through thousands of health studies.

People who are overweight but have no other health risk factors should exercise and eat healthier to keep from gaining additional weight, the guidelines recommend.

People who are overweight and also have health risks such as high blood pressure or high cholesterol should try to actively lose weight, the study said.

Diet drugs should be reserved for people with BMIs of 30 or over, or for people with BMIs of 27 who have such other risk factors as diabetes or high blood pressure, the researchers said.

They also recommended that doctors first try to change patients' eating and exercise habits for six months before beginning any diet drug.