What is the study about? 

Recent studies have suggested that probiotics (i.e. ‘good bacteria’) may have beneficial effects on mood, anxiety and stress. Small clinical trials in people with depression have shown that probiotics can improve depressive symptoms when taken in addition to antidepressants for as short as eight weeks. However, it is not known how this effect occurs nor which probiotics are most beneficial. This study will aim to improve our understanding of the mechanisms underlying these effects by looking at how probiotics affect the gut, brain and the immune system in people with depression. The knowledge gained from this study will help us understand the involvement of the gut in mood disorders. 

What does participation involve? 

We are looking for 50 people currently experiencing depression to take part in an eight-week study looking at the effects of probiotics in depression. We will randomly assign 50% of participants to probiotics and the other 50% to placebo (a capsule with no active components) for eight weeks. Neither you nor the researchers will know which condition you have been assigned to. 

If you are interested in participating, we would first need to assess your eligibility by asking you a few questions over the phone (20 minutes). If eligible, we will invite you for a screening visit (approx. 1.5 hours) where we will perform questionnaire-based assessments to confirm your eligibility. If eligible, you will be asked to attend three further study visits (approx. two to three hours each) during which you will complete questionnaires and provide stool and blood samples. At the first and last visit there will also be an optional brain scanning session. 

You will be given an eight-week supply of probiotics/placebo and will be asked to take four capsules each day. The probiotic used in this study is called BioKult Advanced. 

You will be reimbursed £10 for the screening visit (regardless of whether you are eligible to participate or not) and £20 for each subsequent visit at the end of the study. 

Am I eligible to take part? 

To be considered for the study, you will need to meet the following eligibility criteria: 

• Aged 18 – 55 years; 
• Currently depressed and taking an antidepressant; 
• Generally in good health; 
• Not taking probiotics regularly; and 
• Not pregnant or breastfeeding. 

We will also be looking for 25 healthy volunteers (aged 18-55 years) with no psychiatric or serious medical illness to provide one stool and one blood sample and complete some questionnaires.