I'm not sure you can. If you're too subtle they won't see what you're getting at but if you're too blunt it'll hurt their feelings. The only possibility I can think of (which may well fail) is if you say that you'd like to 'get healthy' and would they like to join you?
Reporting to somebody - legal/ethical
Did you think you were poor growing up?
Govt announces Ukrainian style scheme to bring thousands more migrants to UK


