from "Rust Out" by Dr. Royce Bailey, page 204

Keeping weight off is more difficult than losing it in the first place.  The human body is genetically programmed to keep fat consistent over long periods.  In patients who lose weight, body metabolism (the rate at which calories are burned) slows by 10-15%, in order to compensate for the missing pounds.

Reference:
R. Leibel M.D., Rockefeller U., N.Y.