But I’ll agree that you might be able to quantitative compare prison A vs prison B if you can get a control prison and account for differences. Just a diet of 20% more carbs between A/B would be interesting.
I imagine someone has “done science” on prisons in other areas already. Way more ethical than the typical proposals I see for prisoners/prisons on the internet.
What sort of ethical issues are you thinking? I would have thought higher grade meals than you'd find in a standard prison would entice all the volunteers you could want. There's probably other groups were some good quality meals would be just as enticing, like college students.
A bigger practical issue in prisons would be things like the high prevalence of hepatitis and drug use along with all the confounding factors (like poverty) that lead them to end up in prison.