I cant answer the slimfast 1 as i dont work for them.
All i can do is guess, lol!
Having shake for brekki almost same amount of cals you'd have for brekki in cereal for example, only it contains alot of vits etc you need. They are designed to fill you up, make feel fuller for longer. Which am guessing shrinks ya stomach..? lol
Anyways, it works & its affordable!