Many U.S. prison facilities aim for 2,400–2,800 calories per day for men and 1,800–2,200 for women.
Despite this strict caloric budgeting, inmates are being released fatter and with more chronic diseases than they entered the penal system with.
Maybe calories aren't such a good measure of a healthy diet, especially when diets are high in refined carbs and added sugar?
A systematic review and meta-analysis found an average weight gain of about 0.43 lb per week. That's roughly 22 lb per year if sustained, though it often plateaus after 1–2 years.
Yes prison makes people sedentary but prisoners have the same caloric budget free Americans have, yet most Americans stay "imprisoned" in offices and homes and get very little exercise. The health results are similar for both groups.
Prison food proves the obvious: calories in vs. out is outdated. Diet quality is what actually matters the most.