Just thought I'd also say, I have periods when I am so thin I look quite revolting, and others when I just blow up and up. Usually thinness is the problem but lately I am balooning. I have absolutely no control over my weight. It has nothing to do with how much or what I eat, or my activity level. Some info that I know: Your hormonal balance affects accumulation of fat, and in women with CFS this is very unstable. I think this is why the men are usually the ones who get really thin. With CFS your thyroid can be low periodically. Mine has varied notably from day to day, something which most doctors swear is impossible. You can also develop peripheral thyroid resistance, so your thyroid measures normal but your body tissues do not respond to it (same concept as insulin resistance.) Your liver helps burn up fat by creating ketones and when it is in a very ill state it cannot do this, The fat stays and you have no energy. Failure to mobilise fat for energy in this way is what many people call "slow metabolism".