Here's that info .... I tend to copy everything I find interesting and then (perhaps boringly I don't know) paste it where I think others might find it useful. However - what I must do is go back and find these things again so that I can credit the original posters with the info! This was by Pete someone .....
Toning exercise is better for your weight loss than cardio while on a VLCD.
I won't bore you with the science, but here's the basic idea;
you are existing on 500ish cals a day with the VLCD diet
your body requires around 2000(min) cals a day to survive
it therefore burns fat to get those extra cals
however
if you start doing lots of cardio, you up the required cals for your body to stay living
your body can't convert fat to energy quickly, it is a slow process compared to converting any other substance (including muscle tissues)
if you do too much cardio, your body will panic as it does not have the energy for it, and you risk pushing yourself into so-called "starvation mode", where you body will hang onto everything it has for use at a much more critical later date (i.e. it's better to let your body go wanting now and use the fats when you have been 'starving' for a week, then use it up now and let you starve moreso later on)
remember, your body doesn't *know* that you are trying to lose weight (and in the hunter-gatherer-survival sense, having fat is a good thing as you will be able to live longer without eating), all it does it convert one substance into another (food packs into energy, fat into energy) and if one of them gets in deficit or the demand placed on the body to perform is too high, then it will do the exact opposite.
So, in a nutshell, don't overdo the cardio, or you will probably see your fat loss slow down. light toning is fine, but again don't go in for the "muscle building" as on a VLCD you won't be getting enough protein to really build muscle up anyway (and by the end of the diet you will have most certainly have lost some muscle mass too)
Afterthought; muscle does NOT weigh more than fat. 1lb is 1lb, whether it is a pound of brick or a pound of feathers.
The difference is, 1lb of fat takes up more physical room than 1lb of muscle, as muscle cells are streamlined and fat cells are bulky.