You'll need more food and water than you think.

[Highlander]

Remember that although an Army ration pack is 4000 cals, and the average squadie will eat around 3000 cals per day, that is the cals required per day for that exercise [ anything from a couple of days to about 3 weeks ]

If you were living permanently of less cals, your body would get used to that amount, providing it was not something stupidly low

If you consider how armies have fought month long campaigns on almost nothing, say for instance the German Army fighting in Russian cities,.. they fought for months on very meager rations,............... the body can get used to a great deal, and still function very well.

The biggest problem we would have where rationing is concerned is going to be getting used to a less cal diet, we are all used to eating to excess, far more than we need to, going down to only eating what our bodies need is going to be a shock for most of us