Zen, I wrote a long post in a sub-topic called Food and the Alaskan Experiment that you may find informative. We were discussing that short-lived reality TV show, which I thought was surprisingly interesting, for a short while, at least. (Gosh, was that discussion two years ago already???)

A quick recap: based on observing how poorly a lot of people felt on that TV show, and why that was, I postulated that going on some sort of ketogenic diet (e.g. Atkins) in the weeks before such an experience, if you knew it was coming, would probably give you a big edge once you were suddenly faced with a starvation diet.

Once you're into the starvation event, getting some protein on a consistent basis will let you function and efficiently draw on your fat stores (we carry many thousands and thousands of calories in our fat) for a long time and hopefully spare some lean tissue. Otherwise, if your diet is protein poor, your body is going to have to cannibalize even more of your lean tissue, and eventually, even organs, to access protein.

Anyway, just one theory of mine. Practicing fasting (whether for health or spiritual reasons) would probably be another way to condition your metabolism.