Inclusion Criteria:  Aged 5.00 years to 12.99 years. A confirmed diagnosis of ASD or Pervasive Developmental Disorders (PDD) including autistic disorder, Asperger's disorder (AS); PDD not otherwise specified (PDD-NOS); and atypical autism.  Exclusion Criteria:  Organic GI disorders such as inflammatory bowel disease, coeliac disease, eosinophilic disorders, or current infection of the GI tract. Bowel surgery or short bowel syndrome  Participants who suffer from the conditions listed below or who are taking any of the following medications or supplements:  antibiotics or antifungals in the last two months probiotic supplements in the last two months immunocompromised or severely ill genetic disorders (e.g. Fragile X Syndrome) chronic health conditions such as diabetes, heart disease or an eating disorder.