To be honest, I think most of the "on a VLCD" stuff is in our heads. Our body has all the energy it needs (and more!) from our excess bodyfat, and while burning it in Ketosis does not provide the more "instant" type of energy that a lot of f**d does, our bodies are fully capable of dealing with whatever we throw at them. Plus, once you start doing it your body very quickly gets used to the upped physical effort.
The hardest part is starting; that first few weeks can feel a bit gruelling, but it actually gets fun (and I never thought I'd say that about exercise!

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