According to recent research in animals, maternal gut bacteria may influence the development of autism by altering the immune system in pregnant mothers, (Catherine R. Lammert, Elizabeth L. Frost, Ashley C. Boite, Matt J. Paysour, Mariah E. Shaw, Calli E. Bellinger, Thaddeus K. Weigel et. al., "Cutting Edge: Critical Roles for Microbiota-Mediated Regulation of the Immune System in a Prenatal Immune Activation Model of Autism," Journal of Immunology 201, no. 3 (2018): 845-850).
In another recent study, artificial sweeteners have been found to be toxic on gut bacteria (Dorin Harpaz, Loo Pin Yeo, Francesca Cecchini, Trish H. P. Koon, Ariel Kushmaro, Alfred I. Y. Tok, Robert S. Marks et. al., "Measuring Artificial Sweeteners Toxicity Using a Bioluminescent Bacterial Panel," Molecules 23, no. 10 (2018): 2454).
Diet for many reasons in addition to the microbiome is an integral part of health. If there was one compulsive behavior that was good for my brother, it was his need to eat cantaloupe every day with dinner. I remember during a long day of residency training my brother called me at work rather anxious, there was no cantaloupe in the house. He was staying with me for a few weeks while my parents took a vacation. Even though I was exhausted after the ten to eleven hour day, I knew if I did not pick up a cantaloupe on the way home from work, it would have been much worse if I came home without one. However at his new residence, no matter how often my mother and I prompted the staff, there was often no cantaloupe there for my brother. Now that that routine is broken, he refuses to eat any fruit there at all.