Dark chocolate is associated with a number of health benefits related to the antioxidants it contains, including its effect on our mood. 

This article will break down a recent study by Shin et al. (2022) that explores “consumption of 85% cocoa dark chocolate improves mood in association with gut microbial changes in healthy adults: a randomised controlled trial” (1, 2).

What was the study investigating?

This study explored whether different types of dark chocolate (70% and 85% cocoa content) have a positive impact on gut bacteria that may lead to improvements in mood. 

This connection between gut bacteria and our mood is related to the 2-way connection between our brain and our gut which is called the gut-brain axis. 

Study methods

This study was a single-blind randomised controlled trial that included 48 men and women from Seoul National University (South Korea) in 2017.

A randomised controlled trial (RCT) is a study where participants are divided at random between treatment groups or a control group, the control group is used to compare the outcomes of the treatments or experiment. If participants don’t know which group they are in this is called a single-blind RCT. If neither participants nor the researchers know which groups participants are in (to reduce the risk of bias) this is called a double-blinded RCT. This type of study is considered the gold standard for clinical trials. 

Study participants:

• were healthy adults between 20 and 30 years old

• ate sweets like chocolate and cake less than twice per day

• had no history of diabetes or gastrointestinal tract surgery or disease

• had not used antibiotics within 3 months prior to the study or prebiotics / probiotics within 6 months of the study

• had low depression scores

Participants were randomly divided into one of 3 groups:

1. control group

2. group consuming 10g of 85% cocoa chocolate x 3 per day for 3 weeks

3. group consuming 10g of 70% cocoa chocolate x 3 per day for 3 weeks

The main outcomes measured at the beginning of the study and after 3 weeks were gut microbiota and mood. Gut microbiome was assessed by analysing the DNA of bacteria in stool samples and a technique called 16s rRNA gene sequencing. Mood was measured using psychological tools the Positive and Negative Affect Schedule and the Beck Depression Inventory.

Body composition (weight, muscle mass, fat mass, BMI, and percent body fat) and dietary intake using a 3-day food diary were also measured at these time points.

What did they find?

There were no significant differences between the groups at the beginning of the study in terms of body composition or nutritional intake. This makes comparisons between groups more accurate. Unsurprisingly in a study about eating chocolate, nobody dropped out and compliance with consuming the chocolate was high!

Eating dark chocolate was not found to significantly impact a positive mood state. However consuming 85% cocoa chocolate was significantly associated with a reduction in negative mood. 

Interestingly, the diversity of gut bacteria also significantly increased in the 85% dark chocolate group according to one of the two gut diversity calculations that the researchers used. Furthermore, levels of the gut bacteria Blautia obeum were significantly raised and Faecalibacterium prausnitzii levels were reduced in 85% dark chocolate group compared to the control group. The increased levels of Blautia obeum was found to be linked with lower negative mood scores, whereas Faecalibeactrium prausnitzii levels were not significantly linked with mood. 

The researchers also found that positive mood score was significantly associated with the ratio of two groups of gut bacteria: Firmicutes-to-Bacteroidetes (F/B). However, positive mood scores didn’t change in the 85% dark chocolate group where F/B ratio reduced (which may have been expected). But this reduction didn’t reach statistical significance (meaning it could be down to chance rather than the intervention).  

Negative mood was found to have a negative relationship with gut diversity (as measured by ‘operational taxonomic units’). This means that as the negative mood score increased, gut diversity decreased (i.e negative mood was linked with less gut diversity). 

What does this mean?

This study suggests that consuming 85% dark chocolate has a prebiotic effect by promoting better diversity and balance of the gut microbiome. It also found that this positive change in the gut may improve mood, as negative mood scores reduced when this dark chocolate was consumed and as gut diversity improved. 

Like all studies there are limitations to bear in mind with this study, such as:

• it only included 48 participants

• participants in the control group knew which group they were in as they weren’t given any chocolate to consume (although the researchers didn’t know who was in which group)

• mood scores are based on self-reported information (which isn’t always accurate even when robust questionnaires are used)

This isn’t the first study to have found dark chocolate to be associated with improvements in mood. For example, a large survey from the US in 2019 found a significantly lower risk of symptoms of depression in those who consumed higher amounts of dark chocolate (3).  Similarly, a study that involved consuming a dark chocolate drink for 30 days found significant improvements in feeling calm and content (4). 

One important difference with this study and previous research, is that participants consumed the chocolate under everyday conditions rather than in a more sterile lab environment. 

Other studies have also found that increased gut diversity is linked with better mental health (5, 6, 7). This may be related to the impact of gut bacteria on the immune system, inflammation and how the gut and brain communicate via the gut-brain axis (8). However, ongoing research is needed in this area, including larger and longer human studies. Similarly, more research is needed into gut-brain signalling after consuming dark chocolate. 

It was interesting that the gut bacteria Blautia obeum were raised in the 85% chocolate group and that was linked with lower negative mood scores, as this type of bacteria has been linked with better psychiatric health in other studies (9). This type of bacteria also produces a substance called butyrate that may impact brain function (10). 

The polyphenol level increases the darker the chocolate. Polyphenols are natural compounds found in plants that have a number of health-promoting benefits including acting as antioxidants. As polyphenols are known to be prebiotics (as they feed beneficial gut bacteria), it makes sense that the 70% dark chocolate did not have the same positive impact as the 85% dark chocolate in this study (11, 12). 

The previously mentioned chocolate drink study also only found mood improvements when the highest polyphenol version of the drink was consumed (4). Other research has also demonstrated that a higher intake of polyphenols is associated with lower depression scores (14, 15).  

Of course there may also be other factors at play as when it comes to the impact of chocolate on our mood. For example, the tastiness of chocolate and an emotional connection to this can be a significant factor. An interesting study from 2007 found that mood briefly improved after eating tasty chocolate, as compared with unsatisfying chocolate or nothing (16). 

Summary:

This study found that consuming 85% dark chocolate led to improved diversity of the gut microbiome as well as improvements in mood. 

These findings are in line with the evidence base in relation to the impact of dark chocolate on mood, how polyphenols promote gut diversity and how improved gut diversity is linked with improvements in mood. 

Although this is a fascinating area of research, it is still quite new so ongoing large and well-designed human studies are needed. 

It’s always important to highlight that although certain dietary factors are associated with improvements in mood, these shouldn’t be used in place of well-established mental health treatments. Always seek individual support if you are struggling with your mental health.