Actually this is a myth that is perpetuated by the media, diet companies, medical professions, industry.
Perhaps if they actually found the trigger that changes the set point that regulates the homeostasis of the hypothalamus and hence shape of your physique then none would be surprised to learn that diets have a yoyo effect. especially when your shape fluctuates in accordance to your metabolism which is the set point predetermined and that is the state the body tries to maintain. you can certainly change this by learning what toxins to eliminate from your body and learning which foods react with your metabolism.
ever wonder why some people eat truck loads and burn fast, high metabolism, aka set point does the work. same deal why some people gain weight faster, low metabolism, aka set point maintains the shape.
which comes back to endocrine health and then there is exercise. most people mistake that just a type of exercise shall remove weight. not when the metabolism isn't working. not when you only focus on aerobic or cardio or weights only. gotta incorporate them all.
not when some peoples dietary requirements clash with others and hence, the foods you consume either promote or help in your overall well being.
its a myth perpetuated by a very many people. and picking that myth up from supposedly trusted professionals, friends and family reinforces the subconscious mind.
Unfortunately for the poor girls a trigger happens that changes their psychological make up to think that anorexic is fat...
And for the voluptuous woman to think, curvy is fat...when this is, and men will tell you a zillion times over...mindblowingly hot!
And heavy women will think that its because of their diets and wonder why exercise and dieting don't work...