Could probiotic yogurt be changing your behavior?

Mice are less anxious when they eat a diet that includes the kind of probiotic yogurt you can buy in the health food sections of most stores. The yogurt includes Lactobacillus rhamnosus, a bacterium that lives in the guts of humans and many other animals, and is believed to contribute to healthy digestion. It also apparently affects the brains of mice, making them calmer.

But how can bacteria reach the brain from the gut? According to researcher John Cryan, the vagus nerve provides a direct line of communication between the gut and the brain — this a rare nerve that stretches from your lower body to your brain via your stomach rather than via your spine. Mice whose vagus nerves had been severed got no mood benefits from the probiotic bacteria.

Discover's Ed Yong writes:

The fact that doses of gut bacteria can change adult behaviour has important implications. John Cryan, who led the study, says, "It is highly plausible that probiotic agents in the future could be used to treat mood and anxiety disorders." After all, his study has shown that these bacteria play around with the same brain chemicals in the brain that antidepressants and anti-anxiety drugs do.

This doesn't mean that eating a lot of yoghurt will sort out a bout of depression. The idea of using gut bacteria to treat disorders isn't far-fetched . . . [researcher Javier] Bravo still doesn't know how the bacteria use the vagus nerve to influence the brain, only that they do. Nor does he know how long the effects on GABA and behaviour would last, or how the microbes affect other chemical signals within the brain. And critically, all of these experiments have been done in mice. No one knows if the same thing applies to humans.

"We have no reason to expect that the same would not apply to humans," says Cryan. "However, such clinical studies need to be carried out." However, he cautions that not all probiotics do the same thing and he warns against overinterpreting the results of the study.