A latest study has shown people with ADHD have significantly higher levels of the bifidobacterium bacteria in their gut than those without the condition. This makes researchers re-consider the effect of bifidobacterium, which is very common.
The researchers from University of Otago also found giving micronutrients to children with ADHD can enrich the bacteria types in the gut, and relieve the symptoms of ADHD. The published paper didn't specify what micronutrients they used for the study, however they're generally Vitamins and Minerals.