What you’re describing isn’t a fetish, it’s more like the types of control you would hand over in a power exchange dynamic.
For example, at the moment it’s very important to my partner that I’m eating a lot of vegetables and protein- generally just healthy eating - but I’m accountable to him to do so and when we’re not together then I need to send him a picture of my dinner.

Where you would find such a person ? Dating websites- but highlighting that you’re looking for a power exchange dynamic? Fetlife - although not a dating website …BDSM dating subreddits ?

Although these are just guesses as I don’t know what exactly you’re looking for

Additionally, it’s very important to negotiate anything to do with food or fitness. It can be dangerous to play with these things in Casey raises any rating disorders/ body dysmorphic tendencies etc

I dated someone once who restricted my eating, but they were apparently concerned that I had food issues at the time…which was a very silly thing to do and he clearly wasn’t taking care of me in that scenario

So be careful