We know diets can shape the billions of bacteria that live in your gut. Now, research in rats suggests the gut bacteria may, in turn, affect brain function by changing the way genes are expressed in areas important for memory.
Scott Kanoski at the University of Southern California and his colleagues fed young rats a high-sugar diet while a separate group of rats were only fed standard chow.
Around a month later, the team tested the rats’ memory. Specifically, rats performed a task designed to measure their ability to tell whether or not they had seen an item before, in a specific context. This type of memory is thought to rely on a brain structure called the hippocampus.
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The rats on a high-sugar diet performed significantly worse than those that had been given healthy food. The two groups of rats also appeared to have differences in their gut microbiomes. An analysis of the animals’ faeces found that the guts of the sugar-fed rats had higher levels of several types of bacteria.
The levels of one of these types of bacteria seem to correspond to memory performance. Rats with higher levels of Parabacteroides bacteria performed worst on the memory task.
Gene expression
To find out if Parabacteroides bacteria might be influencing brain function, Kanoski’s team delivered the bacteria directly to the guts of a separate group of rats that had been fed a cocktail of antibiotics to kill off previous communities of bacteria. Rats given Parabacteroides performed worse in the same memory tasks compared with rats that had just been treated with antibiotics.
The hippocampi of the rats also appear to express genes differently. This brain region had a pattern of gene expression that suggested alterations in the way certain groups of neurons fire in both rats that were fed sugar and those treated with Parabacteroides.
There are several different ways gut bacteria might influence the brain, says Ted Dinan at University College Cork, Ireland. The vagus nerve, for example, provides a direct connection between the gut and brain. Preliminary evidence suggests that some types of gut bacteria can alleviate anxiety in mice – but only if the vagus nerve is intact. Gut bacteria also produce a variety of compounds that might affect brain function.
It is likely that other aspects of diet or gut microbiome diversity might influence other brain regions too, says Katerina Johnson at the University of Oxford. She says other studies suggest that antibiotics affect the prefrontal cortex – a brain region that plays a role in decision-making. “Every area of the brain is vulnerable, and we don’t know why.”
The hope is that research will eventually discover probiotics that can be used to improve brain functioning, particularly if they can help prevent or treat mental health conditions. One trial has found that , but “psychobiotics” are probably still a long way off, says Dinan.
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