In the last decade, we've learned so much about the microbial organisms living in our gut. The bacteria in our guts play important roles across our body, including digestion, stress signaling, and immune function. Despite exciting findings in rats and mice, few of these findings have translated to humans thus far. 

Nonetheless, research on the dietary fibers that these microbes feed on is yielding exciting results. These fibers cannot be broken down by our enzymes. Only our gut bacteria can digest them. In the process, the bacteria generate byproducts that interact with our nervous systems. Past research has shown that these byproducts reduced perceived stress in men. Now, a new study finds that galacto-oligosaccharide (GOS) fibers also reduce perceived anxiety in healthy women.

Nicola Johnstone of the University of Surrey and her colleagues designed a double-blind, placebo-controlled trial. Johnstone recruited 48 women (ages 18-25) to provide poop samples and undergo psychological testing. The participants were assessed in the beginning of the experiment. Then, for four weeks, they ingested either a small packet containing GOS or a placebo. After, they participated again in the same psychological tests. 

At both time points, the researchers collected samples for microbiome analysis. They also conducted questionnaires to assess the participants' moods before and after the intervention, and measured their and attention through the psychological tests. During these four weeks, all the participants kept a food diary.

The participants who took GOS showed a small increase in the growth of the gut bacterium Bifidobacterium, which feeds on these fibers. When Johnstone and her team analyzed the impact of GOS on anxiety, they found that GOS reduced self-reported anxiety in women with high-baseline anxiety. Women that had a low anxiety score at baseline did not experience a further reduction even in the GOS group. GOS did not associate with any other psychological measures or changes.