Everyone feels down from time to time, and sometimes it seems to happen for no apparent reason.
Interestingly, research indicates that unexplained or persistent periods of low mood may be triggered by poor health. Some evidence even suggests that the bacteria living in your gut may be responsible.
Recently, a team of Iranian scientists examined the effects of supplementing with probiotics (healthy bacteria) on the symptoms of major depressive disorder.
Here is a detailed summary of their findings.
Depression is a mental state associated with low mood, loss of interest in life, hopelessness and a variety of negative feelings.
Major depressive disorder (MDD) is when depression becomes so severe that it starts to interfere with daily activities.
Researchers have estimated that around 20% of people may experience MDD at least once during their lifetime (1).
For this reason, some researchers believe that a regular intake of probiotics might help reduce depressive symptoms. This idea is supported by a few studies showing that probiotics may have favorable effects on mood.
Here are summaries of their findings over the years:
- 2007: Drinking a probiotic milk beverage for 3 weeks improved mood among those who initially had the lowest mood, while it didn’t affect others (7).
- 2009: Taking Lactobacillus casei significantly reduced self-reported anxiety in people with chronic fatigue syndrome (8).
- 2011: Supplementing with a probiotic formulation for 30 days reduced self-rated anxiety and depression in healthy people and anxiety-like behavior in rats (9).
- 2015: Eating probiotic yogurt or taking probiotic capsules for 6 weeks improved mental health in petrochemical workers (10).
However, until now, no studies have examined the effects of probiotics on the symptoms of major depressive disorder.
This study examined the effects of supplementing with probiotics on the symptoms of depression, as well as inflammation and oxidative stress.
The purpose of this 8-week randomized controlled trial was to examine the effectiveness of probiotics on treating the symptoms of major depressive disorder (MDD).
A total of 40 people (34 women and 6 men) with MDD were randomly assigned to receive one of two supplements:
- Probiotics: The participants took one probiotic capsule daily. Each capsule provided three types of probiotics: Lactobacillus acidophilus, L. casei and Bifidobacterium bifidum (6 billion CFU in total).
- Placebo: The participants took one starch capsule daily.
At the beginning and end of the study, the researchers measured the following:
- Depressive symptoms: Symptoms were assessed using the Beck Depression Inventory, a 21-question self-assessment of depressive symptoms.
- Inflammation: Inflammation was estimated by measuring high-sensitivity C-reactive protein (hs-CRP) in blood samples taken after a 12-hour fast.
- Insulin resistance: The sensitivity or resistance of cells to insulin was measured using the homeostasis model assessment of insulin resistance (HOMA-IR) and the quantitative insulin sensitivity check index.
- Glutathione: Antioxidant status was assessed by measuring glutathione, one of the body’s primary antioxidants.
Additionally, the participants filled out three dietary records (two week days and one weekend day).
Bottom Line: This was a randomized controlled trial examining the effects of probiotics on depressive symptoms, inflammation, insulin resistance and antioxidant status in people with major depressive disorder.
Finding 1: Probiotics Improved Depressive Symptoms
Taking probiotics significantly improved depressive symptoms, evaluated using the Beck Depression Inventory.
These findings are presented in the following chart:
This is the first study to examine the effects of probiotics on symptoms in people with major depressive disorder.
However, several previous studies support the current findings, showing that probiotics may improve mood (see background chapter).
Bottom Line: Supplementing with probiotics caused a significant improvement in depressive symptoms.
Finding 2: Probiotics Reduced Inflammation
Taking multi-strain probiotics for 8 weeks significantly reduced C-reactive protein (CRP), a marker of inflammation, as shown in the following chart.
These findings are supported by previous studies showing that probiotics may reduce CRP levels in pregnant women with type 2 diabetes, obese individuals and people with rheumatoid arthritis (11, 12, 13).
However, the present study is the first to report that probiotics reduce a marker of inflammation in people with major depressive disorder.
Bottom Line: Probiotic supplementation significantly improved C-reactive protein (CRP), a marker of systemic inflammation.
Finding 3: Probiotics Improved Insulin Sensitivity
Supplementing with probiotics for 8 weeks significantly improved insulin sensitivity and lowered insulin levels.
In contrast, changes in fasting blood sugar did not differ between groups. Also, probiotics did not significantly affect insulin sensitivity when using the quantitative insulin sensitivity check index.
As far as we know, no previous studies have examined the effects of probiotics on blood sugar and insulin sensitivity in people with major depressive disorder.
However, studies in people with type 2 diabetes support the present findings, suggesting that probiotics may improve blood sugar control. This is discussed in our previous research review.
Bottom Line: Taking probiotics for 8 weeks caused an improvement in insulin sensitivity and reduced insulin levels.
Finding 4: Probiotics Increased Glutathione Levels
Probiotic supplementation caused a slight increase in glutathione levels (a good effect), as shown below. In comparison, glutathione levels decreased significantly in the placebo group.
No changes were seen in total antioxidant capacity.
Low glutathione levels have previously been associated with major depressive disorder (14).
The effects of probiotics on glutathione levels in people with major depressive disorder have not been examined before.
Bottom Line: Probiotics improved levels of total glutathione, the body’s main antioxidant.
There are a few limitations to the interpretation of the findings.
First, since the supplement contained three types of probiotics, we do not know which of them is responsible for the benefits.
Second, it is unknown whether taking probiotics is of any use for those on an anti-depressive medication.
Summary and Real-Life Application
This study showed that supplementing with probiotics may improve depressive symptoms in people with major depressive disorder (MDD).
Although this is the first study to examine the effects of probiotics in people with MDD, previous studies suggest that probiotics may improve mood in other groups.
Regardless of whether you are depressed or not, taking probiotics appears to be an excellent way to improve many aspects of health.