Not to mention that we didn't evolve to handle the kinds of foods most people in developed countries eat. Natural selection, in the common use of the term, was long over by the time the average person could expect to have sugar added to everything. We never had a need to develop any real regulatory mechanisms to recognize an excess of it. And that's not even touching on the effects that long term use of various medications will have on someone's hunger or tastes. Which is a pretty important point given that a huge percentage of the population are on medications that they'll be taking for the rest of their life. Whether we're talking birth control or psych meds.  The further removed a body is from a natural state, whether biological or environmental, the less reliable instincts become.