It's still early days but I wanted to put this out there in case it helps someone else. I've had IBS-D for around 10 years and I know a lot of other people here have similar problems. For me it manifests as loose stools and urgency to go. I've removed dairy from my diet many years ago as I believe I'm sensitive to dairy proteins as it causes acne and nasal congestion for me. Unfortunately that didn't cure my digestion. I have now done 3 gluten free trials and each time my IBS symptoms completely stopped (in as little as 24-48 hours). I did have a celiac blood test once but it came back negative so I never thought of giving gluten free a try until now.
What does this mean for my POIS? I don't know yet. I do know that I have been feeling the need to sleep a lot more. This happened during all 3 trials. I've never been someone that can nap in the middle of the day, but since dropping gluten I'm napping once or twice a day. I don't think it's because of lower calories because I replaced the gluten carbs with other carbs like potatoes, rice, and sugary gluten free snacks. Perhaps the sleepiness is due to my body healing? I see some people on celiac forums say as much. Maybe it's due to withdrawals since wheat can have opioid like effects on the body.
Anyways I wanted to throw that out there in case anyone else wants to try. I'll update if it helps my POIS. I imagine years of loose stool and intestinal irritation could vitamin and nutrient deficiencies, neurotransmitter deficiencies, hormonal problems, or immune system problems. I'm trying to eat a healthy variety of meat, vegetables, fruit, fats and oils, now that I can properly digest them.