There is an interesting study that was appeared online in Archives of disease in childhood called
The early stool patterns of young children with autistic spectrum disorder
In this study the authors looked at the stool patterns of a group of 13,971 children who were born in the between April 1991 and December 1992 in the Avon area of South West England. The data was obtained from Avon Longitudinal Study of Parents and Children (ALSPAC). This data contains, among other things, questionnaires that were filled out by the caregiver that asked a variety of health related questions.
The data that was analyzed in this study had to do with the frequency of stools, consistency and color of stools, history of diarrhoea, abdominal pain, and history of blood in stools from several discrete time points in the child's life (something like 4 weeks, 6 months, 18 months, 30 months, and 42 months although not all data points were available for all times).
In this study group they found 86 children with an autism spectrum diagnosis. There was complete data available for 76 of the children with autism and 12,905 of the control group.
The analysis of the data lead the authors to conclude that -
During the first 42 months of life ASD children had a stool pattern that was very similar to other children, apart from a slight increase in stool frequency at 30 and 42 months. There were no symptoms to support the hypothesis that ASD children had an enterocolitis.So this data is meant to contradict reports of GI issues being common in children with autism. Indeed if you read the introduction of the paper it appears the purpose of the paper is to rebut other studies done that show a relation.
After reading the study the data appears to be on the inconclusive side for the purpose of drawing any meaningful conclusions. But then again I am not as familiar with the symptoms of bowel disease nor that familiar with how these things are normally diagnosed.
The number of children with autism was on the smaller side (76) and the data is from parental reports over a long time period so I have to wonder at how accurate it is.
On the flip side it is does disprove the commonly held belief that all children with autism have bowel issues from a young age.
So who knows, just food for thought.