I don’t know how one can maintain, let alone build, muscle as we age on one meal per day. Muscle loss is a real thing.
I’d think if one were sendentary, maybe 50-60 grams per day is what I would think as a bare minimum. And while you can eat 50-60 grams in one meal, I’d posit that’s not what I’d call a “standard meal.”
All I’ve got to say is to be careful with just one meal a day. Our bodies need various nutrients, vitamins, minerals, protein, fiber, etc. every day.