And if ascetics were even a significant portion of the human population I would say that would be a useful measure. Instead their numbers are low enough that they are a statistical outlier along with those people that seem to eat as much as they want and not gain weight.
The fact is you can choose to starve yourself to death if you put your mind to it. I would counter that with 'why the hell would you want to'. Wasting a huge portion of my brain time to tell myself (oh yea, don't eat) seems like a massive waste of time and energy.
>to act on hunger requires conscious effort
Does it? I'd like to develop a test where I starve a large people for a little while. Then while I distract them with something I put some easy to eat like chips in their peripheral view and see how many of them realize they started consuming the chips. My ethicist says I can't do this unfortunately, but I imagine that a significant fraction of the group will not realize when they started eating them.