Primarily, this has been stealing their body fat for various nefarious cannibalistic purposes, or cutting them up and selling their flesh as fried chicharrones.
Also, human feet have a tendency to become adipocere (a soap-like substance formed from body fat), which makes it hard for forensic scientists to find clues.
Individuals with anorexia nervosa and some athletes try to control body weight with laxatives, diet pills or diuretic drugs, although these generally have no impact on body fat.