system
Given the following post, respond as if you're commenting on Reddit:
human
Why are longitudinal studies done on the general public and not inmates?

I was reading about the observation of diets and the outcomes after 3, 6, and 12 months and one of the concerns was a self-reporting bias with people and simply forgetting what they ate. Thought about which environments a person has their diet and exercise almost 100% controlled and is almost guaranteed to be in the same place after 12 months.

If they were fed above the state or country dietary minimum for prisoners, wouldn't they make a better sample?
gpt
Science dealing with humans has a number of ethical rules, some of which have been made into law. One of those is that you can't experiment on prisoners simply because they are convenient- you can only study them if you are specifically doing a study of the prison experience or something like that.  When you want to do a study it must pass a human subjects ethics board review (IRB) and they would never allow a study such as the one you describe. 

In order for scientists to effectively do research on humans we need their trust, and experimenting on prisoners is not a great way to gain that.  Apart from that is one of the very basic ethics rules, which is that participation in science experiments must be voluntary, and prisoners cannot be said to be giving voluntary consent because of their situation.