A study in adult volunteers found that the consumption of probiotics in a randomised double-blind trial reduced measures of low mood and distress and urinary free cortisol which indicated a decreased stress level. Similarly, a 4-week course of a multispecies probiotic in healthy participants resulted in reduced responsiveness to sad mood. A neuroimaging investigation in female participants who had consumed a yoghurt containing probiotics over 4 weeks, revealed differential signals over brain regions involved in emotion processing and regulation. Recent reviews and meta-analyses of such studies on probiotic effects for improving mental health outcomes have found modest effects in reducing depressive symptoms, although there remains problematic between-study heterogeneity.
One potential drawback of using probiotics is that introducing an allochthonous probiotic species could disturb the cross-feeding microbiota population, which is particularly disruptive in participants with weakened immune systems. In the current study, we therefore used a different approach with prebiotics to support the beneficial bacteria that are already present in the participants’ gut, such as Bifidobacterium, which have been linked to emotional well-being.
Intake of a galacto-oligosaccharides (GOS) prebiotic over 3 weeks has also been shown to lower the secretion of the stress hormone cortisol and emotional processing in healthy adults in comparison to placebo. In the same study, participants exhibited decreased attentional vigilance to negative information in a dot-probe task. Given that anxious people routinely exhibit increased biases towards negative information, this suggests that GOS intake may be useful in modifying anxiety-related psychological mechanisms. Presently, reviews and meta-analyses on the efficacy of prebiotics for reducing anxiety symptomology are mixed, calling for further well controlled trials in human participants.
The main aim of the current study was to investigate whether GOS intake influences anxiety and mood measures in late adolescence and early adulthood in humans. Specifically, we used emotion regulation as a model for anxiety, as good emotion processing abilities in development are linked with various indices of well-being and mental health. It has been repeatedly shown that the transitional period from adolescence to adulthood represents an important developmental juncture for both the emergence of social anxiety and the development of emotion control abilities, which allow the individual to control their fear responses and anxiety. Given that adolescence also represents an important time point for fine-tuning the gut–brain axis, our study adopted a two-pronged approach which would allow us to shed light on this unique transitional period into early adulthood from both a mental health and gut microbiota angle. Here we compared the effects of a 4-week course of galacto-oligosaccharides (GOS) supplements compared to a placebo on the gut–microbiota and emotional behaviour and well-being. Our study aimed to replicate and extend the study by Schmidt et al. (2015) to a younger age range. Moreover, as in the original study, emotional behaviour was assessed with the attentional dot-probe task but we included a number of additional self-report measures of anxiety, depression/mood and emotion regulation abilities. In the current study we hypothesised that daily intake of GOS for 4 weeks would: (1) reduce self-reported levels of anxiety in the GOS group in comparison to the placebo group; (2) improve attentional bias towards positive emotional stimuli in the dot-probe task, and (3) stimulate the fecal abundance of potently beneficial gut bacteria (such as Bifidobacterium) in the GOS compared to the placebo group.
