I know the 'set point' theories are debated, but many people certainly find that they tend to hang around a certain weight, or hit weight loss plateaus. This can be due to a lot of things...
What about something like testosterone levels? If someone has low-normal testosterone levels, and they supplement to bring them up to high-normal, what happens in the body to down-regulate endogenous production and thus require higher dosages to maintain the same effect?
(If in fact that happens, which I think it does)