Down in your large intestine live trillions and trillions of helpful bacteria and other microscopic creatures, collectively known as your gut microbiome. These critters help digest your food, keep you in good health, and — according to new research — influence your thoughts.
A study published in Psychosomatic Medicine: Journal of Behavioral Medicine has appeared to identify a link between the gut microbiome and human behaviour and emotion for the first time.
A University of California, Los Angeles (UCLA) research team profiled 40 women, taking MRI scans of their brains as they viewed images designed to provoke emotional responses, and also collecting poo samples from all of them (a gross but necessary step to map the gut microbiome).
More than three-quarters of the women had more of a bacterium called Bacteroides, and the remainder had more of a bacteria called Prevotella.
Bacteroides and Prevotella are the two "enterotypes" — a very broad way of categorising people based on the composition of their gut bacteria, which appears to be influenced primarily by your diet. (The Bacteroides type is linked to protein and animal fat; Provotella to carbs.)
Intriguingly, there appeared to be a link between the women's enterotype and the structure of the their brains, and how they reacted to the images.
The Bacteroides ladies' brains were thicker in the frontal cortex and insula, regions involved with complex information processing, and bigger in the hippocampus, which oversees memory processing.
All the Prevotella ladies (put ya hands up!) had stronger connections between the emotional, attentional and sensory regions of the brain, and less volume in the hippocampus and other areas.
The Prevotella group’s hippocampuses turned out to be less active while they viewed negative images, and they also rated higher levels of anxiety, distress and irritability after looking at such photos.
That suggests there's a connection between the gut and the brain, although it's not yet clear if gut bacteria influences brain development and activity, or if brain development and activity influences gut bacteria.
"Both possibilities, however, could lead to important changes in how one thinks about human emotions," concludes a UCLA statement about the research.
Past research out of UCLA determined that probiotics, aka the beneficial bacteria in the food you eat — in, say, a tub of yoghurt — impacts how your brain functions and responds to stimulus.
"Our findings indicate that some of the contents of yogurt may actually change the way our brain responds to the environment," said Dr Kirsten Tillisch, the lead author of the new UCLA study, in 2013.
"When we consider the implications of this work, the old sayings 'you are what you eat' and 'gut feelings' take on new meaning."