Prison food may not look like it but it is loaded with all the nutrients to survive.
They don't become body builders in prison. They came in n look lean because all that work out n poor nutrition will do that to the body.
If your full time work is to work out, what do you think your output be?
They get lean n strong but ask them to run a mile, they will die.
Food in prison still has to pass specs or they will cry cruel n unusual punishment.