Autism spectrum disorder has increased worldwide over the past few decades. Medical comorbidities are common in patients with ASD, with gastrointestinal (GI) diseases being especially common. Involvement of the gut-brain axis that communicates between the enteric and central nervous systems has been confirmed by several studies that have reported abnormal microbiota profiles in ASD subjects. In this review, a systematic database search was conducted. Six trials were included in the subgroup analysis that compared probiotic therapy with single or multiple strains to a placebo with respect to gastrointestinal severity index. The pooled effect size model revealed a slight decrease in gastrointestinal symptoms that favored probiotic supplementation (SMD = -0.189; 95% CI = -0.398 to 0.020; Z = -1.773, p = 0.07; I2 = 39.4%). There was a moderate level of heterogeneity among the included studies. Additionally, the effects of single-strain and multiple-strain probiotic formulations were compared to their respective placebo groups using subgroup analysis. The results demonstrated that single-strain probiotic medication reduced gastrointestinal symptoms when compared to the placebo group. However, there was no statistically significant difference between multiple-strain probiotic therapy and a placebo. To accurately establish the therapeutic effects of probiotics on ASD, more randomized, double-blind, placebo-controlled studies are required.
