In theory, a good money-making spin. However, when somebody is going to pay for an advisory service, they will rightly want to know your credentials.
Simply describing your "expertise" as "given good advice in the past" isn't going to convince them to reach for their debit / credit card. Particularly when you haven't had first hand experience or tried and tested (by yourself) experience.
If you had any qualifications in dietetics or experience working for a weight loss firm such as Slimming World or Slimfast, you would appeal to a wider audience.
Most people probably do know how to lose weight, they just can't change their ways.