Heh, when I first got sick with a bout of mono (triggering this whole nasty thing), I initially lost nearly 25lbs. in that month that I did little else but lay in bed and sleep. I'm 6'1", and only weighed a meager 155lbs. before I got sick. Over the last several years my weight has just continued to slowly dwindle. I now weigh a buck twenty. I was underweight when I was "healthy" (largely due to being active and having a super high metabolism.)
Now my appetite hasn't exactly decreased a ton, but I just simply can't eat as much as I used to. I have to eat smaller meals and snacks throughout the day as digesting a big meal does not bode well with the low blood volume/POTS and really saps what little energy I have. Of course, I've also lost muscle mass due to a lack of activity, so that's part of it, but still. Being as tall as I am and weighing what I do is a bit scary. I've tried protein drinks, ensure, etc. to no avail.
I also try to stay away from wheat and dairy, and eat nuts, coat green leaf salads in extra virgin olive oil (which I always eat with grilled chicken mixed in), eggs every morning, etc. But alas, I can't gain a single pound for the life of me.
One of the (many) problems I have of trying to find a good diet to gain (or at least maintain) weight, is finding what, out of general guides and advice, is also okay for this illness! It just compounds the matters. Aside from the issues I was with the POTS, we have stay away from foods like can increase inflammation (I guess?) It's confusing. Nutrition and healthy eating is certainly not my forte (as when I was healthy I ate whenever and whatever I wanted.) Only very shortly before I became ill did I, ironically, start to be more conscious of what I ate (eliminating soda from diet completely, trying to eat more whole wheats, vegetables, etc.)