In a recent study, 45 volunteers ages 18 to 45 took either a prebiotic (to nourish healthy gut bacteria) or placebo every day for three weeks. The researchers found that the volunteers who took the prebiotic paid less attention to negative information and more attention to positive information on a computer test. They also had lower levels of the stress hormone cortisol in their saliva than those who took a placebo.

Another study, published in the British Journal of Nutrition, investigated the effects of probiotics on mood and behavior in animals and humans. Rats were given probiotics for a two-week period, and human volunteers took either the probiotic or a placebo. The researchers assessed anxiety, depression, stress, and ability to cope with these symptoms.

Probiotics reduced anxiety levels in the animals and led to improvements in measures of depression, anger, hostility, and anxiety in the human study.

Findings linking the microbiota to mood were intriguing enough for Dr. Laura Steenbergen and her colleagues at Leiden University in the Netherlands to test the effects of probiotics on mood reactivity in human volunteers.

The scientists gave a mixture of probiotics to 20 healthy volunteers for a four-week period and compared their responses to controls who ingested inactive placebo. At the start of the study, the researchers used a depression-sensitivity scale to determine how volunteers reacted to sadness, which is a marker of depression.

The volunteers who ingested the probiotics showed significantly less cognitive reactivity to sad mood.

How the probiotics might have influenced mood reactivity wasn’t clear, but the scientists speculated that the bacteria may help to control inflammation and increase amounts of the amino acid tryptophan, which is an essential component of the mood-stabilizing neurotransmitter serotonin.