Tommy Thompson is speaking out about new health care regulations demanding obesity ratings by 2014.
The federal government said this week that electronic health records will have to include Americans' height, weight and body mass index.
The former secretary of Health and Human Services said that using body mass as an indicator of obesity is an old and flawed science.
"And what I'm saying and what I'm representing is a new type of science called body composition. And body composition takes into consideration your body fat, and body fat really is a determining factor as to whether or not you're going to become obese, whether or not you're going to have hyper tension, whether or not you're going to have cardiovascular problems," Thompson said.
Thompson said body fat is the indicator of a worsening physical condition of a person, not the body mass.
He said he fears that many Americans will be misdiagnosed and classified as obese.