I don't know if this is relevent to POIS or not but I'm just throwing it out there for information purposes. If you have trouble digesting carbs, leaky gut, allergies, bloating and IBS symptoms, it could all be caused by SIBO. It can cause nutritional deficiencies and stop you absorbing vitamins properly as well as causing a leaky gut. Starches, carbs, fibrous foods and vegetables will cause bloating or brainfog if you have SIBO.
http://en.wikipedia.org/wiki/Small_intestinal_bacterial_overgrowthI'm currently waiting to get tested and hopefully rule it out.