My understanding of ME/CFS is that it affects the digestive system. This includes hyper-acidity, waterbrash, nausea, retching, and vomiting. Waterbrash is the spontaneous flooding of the mouth with a clear, slightly salty fluid, which may be of sufficient quantity to require expectoration (spit). The source of the fluid appears to be the salivary glands. People sleep poorly, has unpleasant dreams, and develops an irritable disposition. There is flatulence, rumbling of the bowels, constipation may alternate with diarrhoea and a sense of weight or soreness over the abdomen.
Hyper-acidity slows the transit of certain foods and has the potential to cause digestive stasis. Unfortunately one of the main medicines given to ME patients is amitriptyline to regulate sleep. This drug stops food leaving the stomach in the evening and is a cause of biliary sludge.
To improve the digestion of food you can adopt a combination diet where carbs, protein, and acid fruit is not eaten together. You can google "Combination diet" and there are plenty of good books available.