Don't forget you are not actually surviving on 500 calories, you are using your fat to provide the other 1500, or whatever you use each day.
Yes good point. What I meant about reserves for exercise was that there isn't much readily available energy in my body without burning fat which isn't instant. So when I swim my body can't seem to get me energy quick enough and I am knackered !So I have to take it slowly and do exercise in small chunks.
I don't know if this is true for everyone but I heard that when you eat there are instant reserves for exercise but on a VLCD it takes time for fat to burn etc to get the energy to you.