That's actually incorrect. It's an exaggeration to say that people can survive six months. It is possible in rare circumstances of very low core temperature to survive three months. We know that from a couple of survival cases.

The reality is many people will die after three weeks. If one could sit and do literally nothing, then the minimal loss of calories would result in a mental stupor. Your body uses up glycogen in the liver and muscles first. Still you need water in sufficient amounts to maintain biochemical processes. You pee out a lot of water (though the body attempts to conserve water in the large intestines). You pee because you eat proteins and they degrade the amines to form urea.

Water is very heavy to carry, and I'll bet that 90% of you live far from a water source. Since it's Winter in my hemisphere, there's little rainfall. It's impossible to collect enough to survive until the Spring rains. That means digging a well in Winter, or carrying it far distances for most.

As glycogen is used up, people get very weak. The body begins liberating muscle tissue through a process called gluconeogenesis to produce adequate sugar for to maintain consciousness. If insufficient, then people pass out. You've probably heard of diabetic ketoacidosis. Diabetics Type 1 don't have insulin so they can't convert fat to blood sugar and they eventually die.

As people lose muscle tissue, they can't rely upon that tissue to do as much work. This leads to inefficent hunting, trapping, fishing, gathering firewood and water. The whole starvation phenomena accelerates.

Most people will give up and do less and less work even after a week or so. They'll make mistakes that will cause dehydration to set in. Think about it...most people don't drink enough water. It causes mental confusion under workload and people can collapse easily.

Because of all that, it's my belief that if you could merely have sufficient food and water for two months, that most of the sociopaths would die, along with a huge percentage of the population. It's grim, but if in an isolated protected area, then those who survive could potentially live on.

The problem is that if they run out of food and can't find food, they'll die too. Much of the food in any kind (pets, livestock, warehouses, stealing, etc) will have been used up. This is why you need six months.

That sounds expensive, but really isn't. It won't be food that you'll like, but it would keep you alive. The main issue is having enough water. That's a real concern. This is why you need to check your average rainfall, think of storage capacity, collection methods like dew collectors, how's the bedrock on your land, what's the water table, how to make a biosand filter, etc.