Gut bacteria have been found to control our emotional states, and those of mice. A new theory challenges the reasons this may have evolved. Kateryna Kon/Shutterstock

In recent years evidence has emerged showing the digestive system has far more influence on our minds than we previously realized. It's been suggested gut bacteria fond of particular nutrients can push us to eat what they prefer. We're also learning that the same intestinal microbes can influence mood. Now, a new theory proposes evolutionary reasons for why we evolved "that gut feeling".

A bacterium that thrives on a particular foodstuff might benefit by releasing chemicals that make a person crave that food. Consequently, dietary decisions may in part involve a war over whether we should nourish the fat fanciers or the fiber fanatics inhabiting our guts, although this is debated. It's less obvious how these bacteria benefit by changing our emotional states.

Yet Katerina Johnson and Professor Kevin Foster of the University of Oxford point to evidence suggesting some of our bacteria, particularly members of the Lactobacillus and Bifidobacterium genera, can induce sociability, anxiety, stress, or depression. As an example, when stressed mice get a transplant of certain Lactobacillus species, they increase oxytocin production and become more socially interactive.

Theories on how gut bacteria can affect our mental states involve many possibilities. The question of why is even trickier. Johnson & Foster/ Nature Reviews Microbiology

Increased sociability has been explained as a way bacteria can reach new intestines – bacteria stuck in the gut of a loner possibly have fewer opportunities to spread.

However, Foster is skeptical. “Any extra energetic cost invested by bacteria producing a neuroactive chemical to manipulate host behavior would make it very vulnerable to being outcompeted by other microbes not making this additional investment,” he said in a statement. “The conditions favoring manipulation appear rarely satisfied by the genetically diverse ecosystem of the mammalian microbiome.” Solo parasites like rabies or Toxoplasma gondii change behavior in ways that appear to their benefit. This would be harder in the competitive environment of the gut, where numerous species compete for resources.

In Nature Reviews Microbiology, Foster and Johnson propose mood-altering effects are the consequence of a more complex feedback process than “puppeteers manipulating our behavior", as Johnson put it. Instead, they argue in the paper, “Behavioral effects can readily arise as a by-product of natural selection on microorganisms to grow within the host and natural selection on hosts to depend upon [the organisms that inhabit them].”

Johnson and Foster propose certain gut bacteria produce molecules, such as short-chain fatty acids, as part of normal growth and division. Some of these have coincidental effects on our immune systems and brains. One example is the important neurotransmitter GABA, produced in abundance by Lactobacillus and Bifidobacterium. A host species that evolved with an inbuilt source of these molecules might become depressed when the gut bacteria changes.

Rather than microbes manipulating our minds, the theory goes, we've become dependent on waste products produced for their own benefit, and suffer in the absence.