"Starvation mode", exercise calories, dillema?

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That is so cute!

So your body requires a certain amount of calories just to function, this is called your Basal Metabolic Rate (BMR) and consists of energy needs for autonomic functions such as respiration, involuntary muscle contraction (like digestion and heart function), Central Nervous System activity...etc. Things you have little or no control over. These activities require about 60 to 75% of most people's calories. No matter what you do, you need this many calories to function, this is not a debate. The rest is all subject to variation, thermogenesis (the conversion of calories to heat), daily activity, and extra activities (exercises not occuring through normal daily activity) all add to the above total giving you your Total Daily Energy Expendature (TDEE).
Luckily for most people, they have a significant amount of calories stored in their body. Not only as fat, but also as protein and glycogen (and cholesterol to a smaller degree). Even fit and healthy folks with low body fat percent have a large amount of stored calories.
So what happens when you eat below what you need?

Part 3 long term underfeeding
This can begin anywhere from 10 days to six weeks depending on how large the deficit is. The body is fully in "panic mode" now. Storage of fat is a primary concern to the point where now both protein and carbohydrates coming in are shunted off in large amounts to fat storage. Muscle mass is critically low or starting to become critically low. If the person is keeping track, they will now see that their stamina and power is both significantly lower. And they will have large periods of the day where they are tired and/or lethargic, and could even exhibit "colds" and acheness very easily.
This point is where the brain is criticaly effected and organs can begin slowing down their efficiency. The long term health risks are now an issue. Some organs can shut down in parts, and sometimes these parts never start back up again. Compromised thought processes can dull perception and lead to balance and awareness issues. Sleeping becomes more difficult. It's a cycle that can end in chronic diseases and sometimes even death. I don't say this to scare folks, it's just the logical outcome (although would take quite a while unless in full blown starvation).


As to the finer points of recognizing how many calories are "enough" for you. That's easy enough to estimate (with some impressision I admit, but it'll get you in the ballpark). Just find your approximate BMR (there's a ton of sites out there that do this, go to webMD and put metabolism calculator in, you'll find their tool for TDEE) and multiply by 25% and you'll get close to your TDEE.