Yeah, the ethics would be tough, but essentially to really do nutrition research, you MUST control the food intake. Control for age, weight, calories and activity, and vary only the dietary makeup to varying degrees, etc.
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.
In the US, federally funded research institutions have to follow certain rules when doing human subjects research. When I was a researcher the training I took said prisoners generally could not be used as subjects just because they were convenient, and that the two situations where they could typically be studied were cases where prisons, crime, or something else that was inherently specific to prisoners was being studied and cases where some condition very highly over represented among prisoners was what was being studied (the example given was HIV).
I just don't know how ethical it would be to force a particular diet that might affect someone's life expectancy without their consent. I mean, I suppose you could get volunteers...
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.