> And how hard is it to make such controlled studies on prison populations (where both sun and food intake is also a known value)?

Very hard. Not impossible, but even something as low risk as a dietary study with Vitamin D supplements would come under very heavy scrutiny and likely be rejected.

Prisoners are considered a vulnerable group with diminished autonomy so there is no such thing as “voluntary as it's for people outside“ in prison. Full stop. The Nuremberg Code, Belmont Report, and Declaration of Helsinki all explicitly spell that out.

Even ignoring the obvious “ethical issues” that have been settled since the Nazis, they’re also legally protected. See 45 CFR 46 Subpart C [1]. Even if the experiment got past the academic ERBs, the HHS ones will likely shut it down.

[1] https://www.hhs.gov/ohrp/regulations-and-policy/regulations/...