No. It would be considered rude/unacceptable in most situations for a photographer to ask a model to go on a specific diet for a shoot, and I’ve never had such a request.
I’m expected to look as I do in my portfolio images, which is the result of my current diet & shoot prep routine. If a photographer prefers a model that looks different than me (thinner, more muscular, etc.) they can simply contact another model with the look they desire.