When I was a young and impressionable young woman, my mother convinced me that the "Fit for Life" regime was healthy.  I was miserable and starving on it.  The rest of the family rebelled because it was so restrictive that any decent, regular meal was literally off the menu.  I don't think she lasted much longer than I did.   There's no science in these diets, but they give people the sense that they are disciplined and oh, so superior to the silly person who brings a meatloaf sandwich for lunch.
Mmmmmm...meatloaf sandwich.
BTW, I have a BMI of 17.8, boundless energy, and get up three flights of stairs faster and with less gasping than students half my age.  And I do all that on...ordinary food. 
Darn.  Now I want a meatloaf sandwich.