My feeling is that if there is a nationwide food shortage/cut off, people would get by for a few days on normal home supplies, in a few more days people would start getting hungry, a few more days after that people would be desperate and start breaking into places in search of food, but they would be weak. A few days after that, the only people with food would be scattered hoarders, and if they are also armed, would be able to defend themselves. Roving gangs would be hungry, because they would have been in the group who didn't hoard, and had been hungry for quite a while, so their ability to find significant, un-armed, places for raiding would be limited, and I think even they would get starved out, unless they found some lucky cache of food that was unprotected. This would leave only the hoarders, with defense, left. But the real question is, who, other than the real hard-core hoarders, would have enough food to last more than a month or two? (wild game and wild plants would be the only real food source unless you had a self-sufficient farm)