Gary - I'm not a scientist but I would think what happens is when you initially start training your metabolism will continue to work at the same speed as before therefore you lose weight - after training for a few months your metabolism adjusts slightly to your new exercise levels and so you stop losing weight (although as you say keep gaining muscle). If you then stop exercising you would initially then go up to a weight probably above what you were before you started running for same reason.
I am really skinny 6ft, 10.5 stone - but when I upped miles recently I lost weight initially but this has now levelled out.
Exercise is just like dieting really - needs to be a whole life change otherwise its a bit of a waste of time from a weight perspective. (although great fun from a pain and personal suffering perspective!!
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Edited: 04/10/2012 at 09:41