"These are the same hormones that have been implicated in chronic illnesses like obesity and asthma," said researcher, Dr Lisa Christian.
"A toddler's temperament gives us a good idea of how they react to stress.
"This information combined with an analysis of their gut microbiome could ultimately help us identify opportunities to prevent chronic health issues earlier."
Study co-author Dr Michael Bailey added: "There is definitely communication between bacteria in the gut and the brain, but we don't know which one starts the conversation.
"Maybe kids who are more outgoing have fewer stress hormones impacting their gut than shy kids.
"Or maybe the bacteria in their gut are helping mitigate the production of stress hormones when the child encounters something new.
"It could be a combination of both."
For the study, which was published in the journal Brain, Behaviour and Immunity, researchers asked the mothers of 77 toddlers to assess their child's behaviour using a questionnaire which measured 18 different traits.
Scientists then analysed the types of bacteria, and their quantities, in the children's stool samples.
Overall, the associations of temperament with the gut microbiome were stronger in the boys than the girls, the researchers said.
The average gastrointestinal tract contains 400-500 different species of bacteria.
Previous research has found that people suffering from anxiety may simply need to eat more "healthy" bacteria.
Some scientists think there may be a link between our digestive tract microbes and disorders such as anxiety, schizophrenia and autism.
They are beginning to recognise the power of healthy gut bacteria, especially seeing that the average adult carries up to five pounds of bacteria - trillions of microbes - in their digestive tract.
Probiotics are commonly consumed as part of fermented foods with specially added active live cultures, such as in yogurt, soy yogurt or as dietary supplements.
- Daily Mail
