@Dr.Patient I lost about 10 kgs during the worst of my illness and I was skinny to start with. This improved after I worked on my gut. I had IBS after becoming ill with ME/CFS and developed many food allergies and intolerances. Also had gastric reflux and problems digesting food.
My diet was already relatively healthy, but I then changed my diet dramatically to cut out gluten, dairy, soy, FODMAPs and a lot of other problem foods and this has helped. My IBS is greatly improved and I have regained about 5 kgs (much needed).
It was tricky working out a healthy diet with these limitations, plus I am a skinny type 2 diabetic on meds and have high cholesterol due to genetics. I did my own research and also saw a dietician. Mostly it was trial and error.
One thing I noticed was I definately felt more fatigue and generally unwell if I didn't take in enough calories and protein. The ME/CFS specialist I see recently talked about the importance of sufficient protein for sufferers of this illness. You might want to consider alternative sources of protein which are more easily digestible. Some people use protein shakes. I have not tried these. When I am up to it I make fresh juices or smoothies. This is another way of getting nutrients in an easily digestible form.
It was difficult juggling all of my dietary needs and restrictions, but this is one area where I have improved.
Best wishes, Andy.