How can i slow down my small bowel transit time to gain weight (doctor says mine is going way to fast) ... he says the underlying reason is autonomic dysfunction.

Vinegar: 1/2 teaspoon of apple cider vinegar in 8 ounces of water two to three times a day should slow down transit and improve any reflux symptoms. Absorption and cellular health of the small bowel can be improved by taking 2-4 grams of glutamine daily.

Meds: There are several types of meds which can slow down GI transit time. Any med which causes lots of constipation would be a good choice. Tricyclic antidepressants and opioids are a good starting place.