system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:How can prison inmates gain so much muscle when their diet contains low amount of protein and lot of unhealthy food?
gpt
While there's no requirement that prison food tastes good or that it's especially healthy, most jurisdictions do require prisons to serve a required number of calories per day and a specific minimum nutritional need (carbs, proteins, vitamins, etc).  At least in most US states and most first world countries that's the case to some degree.  There's plenty of controversy in the US right now around prisons serving nearly inedible food or shirking those mandated requirements, but in general prisoners are required to be served a reasonable amount of food with specific dietary requirements.  

And of course, there can be extra food going around.  It used to be that the #1 item in prison black markets was cigarettes.  Nowadays I've read that it's instant ramen.  Which again goes back to "lots of prisons are serving bad food, so people get more when they can."  

But at the end of the day, so long as you're getting the required calories and some degree of protein, you can build some muscle.  And when you're in prison and there's not a lot to do, your daily exercise time probably becomes a big thing.  An hour or two of exercise a day is more than most people take, so a decent regimen over time combined with weight loss - again, as food is only meeting minimum requirements, it might not be easy to stay overweight - would make anyone start to look more muscly.