"Your BMI tells you whether your weight falls into a range that's optimal for health and longevity," says Morton Maxwell, M.D., co-director of the Obesity Center at the University of California at Los Angeles. The catch is, different authorities name different BMIs as optimal.
While the U.S. Government says a BMI of 30 and over equals obesity, Dr. Maxwell points out, "30 per cent is really quite high, while most every study in the medical literature shows that the ideal BMI for health and longevity is about 22 per cent."

For centuries the definition of fat has been up for grabs. A
hundred years ago, a man's bulging belly or a woman carrying 20 extra
pounds signified wealth, desirability and high social status. But if
today's rail-thin supermodels were transported back to those stodgy
times, their figures would doubtless be deemed undernourished and
under-sexy.

In more recent decades, insurance companies set general standards in
the United States for healthy height and weight. But their definitions
of overweight and obese have grown skinnier over the years.