The CDC’s Website on the Body Mass Index
"Overweight may or may not be due to increases in body
fat. It may also be due to an increase in lean muscle.
For example, professional athletes may be very lean and
muscular, with very little body fat, yet they may weigh
more than others of the same height. While they may
qualify as ‘overweight’ due to their large muscle mass,
they are not necessarily ‘over fat,’ regardless of BMI."
"Adults who are overweight do not
necessarily have an excess of body fat."
- CDC web site, 2005